What's Eating the Universe?: And Other Cosmic Questions 9780226816326

Citation preview

What’s the Universe? W H AT ’S Eating E AT I N G T HE U NIVERSE?

What’s W H Eating AT ’S the Universe? E AT I N G and Other Cosmic Questions

the

pau l dav ie s

UNIVERSE? and Other Cosmic Questions

E N L A NE I E S PA U LA L LDAV an imprint of

The University of Chicago Press

The University of Chicago Press, Chicago 60637 AL L EN L AN E © 2021 by Paul Davies UK | USA | Canada | Ireland | Australia The author has asserted his moral rights. India | New Zealand | South Africa All rights reserved. No part of this book may be used or reproduced in any manner whatsoever written permission, except the case whose of brief Allen Lane is part ofwithout the Penguin Random House group of in companies quotations addresses in criticalcan articles andatreviews. For more information, contact the be found global.penguinrandomhouse.com University of Chicago Press, 1427 E. 60th St., Chicago, IL 60637. Published 2021 Printed in the United States of America 30 29 28 27 26 25 24 23 22 21   1 2 3 4 5 ISBN-13: 978-0-226-81629-6 (cloth) ISBN-13: 978-0-226-81632-6 (e-book) DOI: https://doi.org/10.7208/chicago/9780226816326.001.0001 First published 2021 001

First published by Allen Lane, an imprint of Copyright © Paul Davies, 2021 Penguin Random House UK, 2021. The moral right of the author has been asserted

Cover image: Cosmic microwave background, as observed by thepublishers Planck spacecraft. and the Planck Collaboration The author and are grateful©toESA the following for permission to reproduce images: Figures 1, 4, 6, 7 and 11 – photographs courtesy of NASA; Figure 9 – designed and cut by the Cardozo Kindersley Workshop, Cambridge; Figure 13 – copyright ©Data Westminster Abbey. Library of Congress Cataloging-in-Publication Set in 12.5/15pt Garamond MT Std

Names: Davies,Typeset P. C. W., author. by Jouve (UK), Milton Keynes Printed andeating bound the in Great Britain: and by Clays Ltd, Elcograf S.p.A. / Title: What’s universe? other cosmic questions Paul Davies. The authorized representative in the EEA is Penguin Random House Ireland, Description: Chicago : The University of Chicago Press, 2021. | Morrison Chambers, 32 Nassau Street, Dublin d02 yh68 Includes index. LCCN 2021004010 | ISBN 9780226816296 (cloth) | AIdentifiers: CIP catalogue record for this book is available from the British Library ISBN 9780226816326 (ebook) isbn: 978–0–241–45985– 0 Subjects: LCSH: Cosmology—Popular works. Classification: LCC QB982 .D383 2021 | DDC 523.1—dc23 www.greenpenguin.co.uk LC record available at https://lccn.loc.gov/2021004010 Penguin Random House is committed to a ∞ This paper meets the requirements of ANSI/NISO Z39.48-1992 sustainable future for our business, our readers (Permanence of Paper). and our planet. This book is made from Forest Stewardship Council® certified paper.

Contents Preface

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1. Journey from the Edge of Time

1

2. The Search for the Key to the Universe

4

3. Why is It Dark at Night?

10

4. The Big Bang

15

5. Where is the Centre of the Universe?

24

6. Why the Cosmos is Actually Fairly Simple

28

7. What is the Speed of Space?

31

8. What is the Shape of Space?

37

9. Explaining the Cosmic Big Fix

46

10. Most of Our Universe is Missing

49

11. What is Dark Energy?

53

12. Where Does Matter Come From?

59

13. Gravity Conquers All

68

14. Warped Time and Black Holes

72

15. Is Time Travel Possible?

78

16. What is the Source of Time’s Puzzling Arrow?

83

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17. The Black Hole Paradox

88

18. A Theory of Everything?

93

19. Fossils from the Cosmic Dawn

104

20. Can the Universe Come from Nothing?

111

21. How Many Universes Are There?

117

22. The Goldilocks Enigma

123

23. What’s Eating the Universe?

127

24. Is the Universe Actually a Botched Job?

131

25. Are We Alone?

133

26. Is ET in Our Backyard?

136

27. Why Am I Living Now ?

141

28. The Fate of Our Universe

147

29. Is There a Meaning to It All?

155

30. What’s New on the Cosmic Horizon?

160

Index

167

Preface What’s Eating the Universe? is a scientific detective story. It explains how age-old cosmic puzzles have recently been solved, while startling new discoveries are upending our understanding of physical reality. To fully grasp the big picture, we must take a journey from the very edge of time itself, through our own epoch, into the infinite future. This is cosmology, the study of the origin, evolution and fate of the entire universe. It weaves together the large and the small, the vastness of space with the innermost recesses of subatomic matter – a human endeavour of breathtaking audacity, tackling fields of inquiry that for millennia lay exclusively in the provinces of religion and philosophy. There may be those who feel that, when the bright light of scientific inquiry is shone into the hidden workings of nature, it strips away the romance of the unknown. I have always believed, however, that the deeper we probe, the more beautiful and awe-inspiring the physical world appears. Nature demystified is nature revealed in all its glorious subtlety and elegance. It is good that we should know. In the chapters that follow, we will explore humankind’s cosmological discoveries in detail, and examine the fundamental philosophical questions that flow from them  –  questions such as why the universe exists at all, why it has the form it does, why the laws of nature are as they are, and how a system of mindless, purposeless

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particles brought forth conscious, thinking beings who can make some sort of sense of their world. I have been preoccupied by these big questions of existence all my life, and I am privileged to have been able to work on some of them professionally in my career as a theoretical physicist, cosmologist and astrobiologist.  While my own contributions have been modest, I have rubbed shoulders with some of the giants of physics and astronomy, women and men of sparkling intellect and insatiable curiosity, whose infectious zeal for understanding the world from the perspective of science has been a constant source of inspiration. I have lived through one of the most exciting times in the entire scientific enterprise, when many big questions were transformed from dreamy theorizing to hard-won discovery. As so often in science, one question was answered only to raise a dozen others. And so it is today. Much has been achieved but much remains shrouded in mystery. I conceived of this book as a concise summary of our current state of understanding. So many people have helped me with my research over the years that they are too numerous to list here, but I should like to acknowledge Cecilia Lunardini and Rich Lebed of Arizona State University, who clarified some aspects of particle physics for me, Charley Lineweaver of the Australian National University for his assistance with the characteristics of cosmological horizons, Glenn Starkman for his inspirational lecture about the oddities in the cosmic microwave background radiation, and Simon Mitton for drawing my attention to some historical inaccuracies. Lucy Hawking and Christopher McKay provided valuable help with the figures. Special thanks must go to my wife, Pauline, for her viii

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encouragement and critical reading of the text. Her communication skills far exceed mine, and much of the credit for the end product must go to her editorial refinements. Finally, I should like to thank my editor at Penguin, Chloe Currens, for her suggestions, comments and guidance, delivered in an unfailingly cheerful tone. Paul Davies Phoenix, March 2021

1. Journey from the Edge of Time On 14 January 1990, the world’s press featured a mottled redand-blue image purporting to show nothing less than the birth of the universe. ‘It was like looking at the face of God,’ proclaimed the project’s lead scientist, George Smoot. In the words of Stephen Hawking, the picture represented ‘one of the greatest scientific discoveries of the century, if not all time’. The subject of these superlatives was a colour-coded heat map of the sky produced by a satellite called COBE, for Cosmic Background Explorer. COBE had been tasked with surveying the fading afterglow of the Big Bang, a sea of microwaves that suffuses space and travels to us largely undisturbed

Figure 1. All-sky map of the Big Bang afterglow obtained by the satellite COBE.

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from a time when the cosmos was a tiny fraction of its current age. The amorphous-looking splodges decorating the image indicated slightly hotter and cooler regions of the universe. Etched into this kaleidoscopic pattern were important clues about the birth pangs of the cosmos a split-second after its origin, at the very edge of time itself. COBE ushered in a golden age of cosmology. In the three decades since, the field has been transformed from a speculative backwater to a precision science. Paradoxically, we now understand the history of the universe in its overall outline better than we understand the history of our own planet. Yet, to borrow from Churchill, this is not the end of cosmology. It is not even the beginning of the end. But it is, perhaps, the end of the beginning. Cosmology might seem like a rarefied discipline, but in many indirect ways it touches everyone. We all have a need to know why the world is as it is and how we came to exist. Throughout history, societies have sought to address this need by producing creation myths: accounts which weren’t explanations in the scientific sense but stories intended to place human beings in the context of a grander scheme. When cosmology emerged as a scholarly discipline among the ancient Greek philosophers two and a half millennia ago, it was given a name that derived from the same root as ‘cosmetic’, meaning beautiful, whole and complete; standing in opposition to chaos. The word implied that there was such a thing as ‘a universe’, a coherent and organized entity that can be understood by human reason. Further progress, however, had to await the scientific age two thousand years later, which unleashed a stream of dazzling discoveries. When in 1543 Copernicus declared the Earth goes around the sun, he 2

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shattered the anthropocentric model of the cosmos that had prevailed for centuries. To be sure, the immediate effect on daily life was minor; there were no riots, no wars, no economic disruptions. Yet, over time, the knowledge that we are not located at the centre of the universe fundamentally transformed the context of all human existence. The impact was felt not only in science, but in religion, sociology and economics too. Today we are poised to undergo a shift in perspective even more disruptive than that initiated by Copernicus. Future generations will look back at our era and envy those privileged to witness it first-hand. But beneath the catalogue of discovery lies a profound mystery. For some reason, on an unexceptional planet orbiting a run-of-the-mill star, a species of organism evolved that managed to work out how the world is put together. That surely tells us something deeply significant about our place in the natural order. But what?

2. The Search for the Key to the Universe It’s impossible to look up at the night sky and not be struck by the grandeur and beauty of the vista –  the sweeping arc of the Milky Way, the myriad twinkling stars, the insistent, steadfast brightness of the planets. The sheer vastness and complexity are overwhelming. For millennia after millennia, our ancestors observed the same sky, and struggled to make sense of what they saw. What was the key to understanding it? How did the cosmos come to exist? What was the place of human beings in the grand scheme of things? For many ancient societies, making sense of the heavens was not merely a philosophical or spiritual quest; it was also a practical necessity. Knowing the movement of heavenly objects was critical for human well-being: not just for navigation, but seasonal migration, crop growing and timekeeping. Our distant ancestors’ preoccupation with the cycles of the sun, moon and planets is obvious from the megalithic structures they built, some deliberately designed to align with astronomical events –  events often imbued with divine significance and marked by elaborate ceremonies. The sky was regarded as the realm of supernatural agents. In some cultures, the sun, moon and planets themselves were treated as gods. But the regularities evident in the movement of astronomical bodies suggested a very different concept of the heavens, not as the playground of the gods but a mechanism, 4

the search for the key to the universe

an elaborate system of moving parts. Once this notion became established, precision measurements were crucial to determine how the mechanism was organized and regulated. Arithmetic and geometry were now essential skills. Astronomers became powerful and important figures in society, alongside priests and emperors. Their careful measurements and analysis gradually revealed order and harmony, number and form, in the celestial activity. Many theoretical models were constructed over the centuries. A well-known distillation of earlier ideas was compiled by the second-century ce Greek astronomer Claudius Ptolemaeus. Ptolemaic cosmology featured a complicated system of nested spheres, turning around the Earth at different speeds.

Venus

Moon Mercury

Saturn

EARTH Sun Mars

Jupiter

Figure 2. The universe according to Ptolemy, with Earth at the centre.

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The mechanistic models of the universe were on the right track, but the theological dimension was never eliminated entirely. There was ever the vexatious issue of origins. How did the great cosmic contraption come to exist in the first place? Was there a Prime Mover who set the complex mechanism in motion? A supernatural Creator who conjured order out of chaos? A god who made the universe from nothing? No attempt was made in these early models to link the motion of astronomical objects with the motion of material bodies on Earth. Heaven and Earth, each filled with movement, remained separate domains. This view persisted through much of the Medieval period, until the seventeenth century. Then, suddenly, humankind’s understanding of the universe was transformed. A small band of visionary ‘natural philosophers’ came to realize that the key to the universe was not to be found in divine agency, nor in the geometry of the cosmic architecture itself. Rather, it resides in laws of nature that transcend the physical world and occupy an abstract plane, invisible to the senses but nevertheless within the grasp of human reason. Number and form, beloved of the ancient philosophers, are manifested not just in specific physical objects and systems, but interwoven into the very laws of nature themselves, forming a mosaic of subtle patterns encrypted in a kind of cosmic code. It was a stunning conceptual pivot, marking a transition from mere description of the world to explanation. The quantum leap in comprehension accompanying this transition was poetically expressed by Galileo in 1632: ‘The great book of nature is written in the language of mathematics,’ without which, ‘one wanders in vain through a dark labyrinth.’ The key to the universe, said Galileo, lay with mathematical decryption, a sentiment 6

the search for the key to the universe

echoed three centuries later by the astronomer Sir James Jeans, who declared ‘The universe appears to have been designed by a pure mathematician!’ Galileo himself began the task of unlocking nature’s hidden mathematical order, but it was a generation later, especially with the work of Isaac Newton and Gottfried Leibniz, that everything came together. Theirs wasn’t a grand and highly organized cultural enterprise, as scientific research is today. The founders of what we now call science were more like members of an exclusive cult, almost a secret society, unconventionally religious, fractious and egotistical, still steeped in the mystical traditions of antiquity. Galileo pioneered the use of the newly invented telescope to study the heavens. This enabled more accurate measurements to be made of the movement of planets and the shapes of their orbits. Like Galileo, Newton set his sights on the solar system, seeking to uncover mathematical laws of motion that applied equally on Earth and in space, and which could be tested by observation and measurement. To achieve this, he needed to find the correct mathematical key, but it was nowhere to be found in the great corpus of Greek arithmetic and geometry, nor in their Medieval refinements. So, he fashioned it himself, calling it the theory of fluxions  –  what is today termed differential calculus. Starting in his early twenties, and no doubt assisted by the need to self-isolate at home in Lincolnshire during the great plague of 1665–6, Newton applied fluxions to the laws of motion, and discovered how gravity, that enigmatic force that reaches across space, weakens in a precise arithmetical way, in inverse proportion to the square of the distance between objects. Suddenly, humanity possessed a new window on the 7

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heavens. The swooping parabolas of comets, the graceful ellipses of planets, the scalloped gyrations of the moon – all the delicate tracery of celestial orbits – fell into place, linked to each other by the immutable logic of fixed mathematical relationships. I’ll never forget the thrill I felt as a student when I first applied Newton’s laws to the motion of a planet going around the sun and out popped the formula for an ellipse! It was like magic. Imagine the sense of awe that Newton himself must have experienced when his own handcrafted equations produced the very geometrical patterns so assiduously catalogued by astronomers from years of observations. Spectacular though such an advance was, Newton had a grander vision. Having explained the solar system, he set about applying his law of gravity to the entire cosmos. Since Galileo had turned his telescope on the Milky Way it was obvious that the universe is teeming with stars. But how were they arranged? Were they clustered in a huge but finite cloud, or scattered endlessly through infinite space? Newton envisaged the cosmos as a gigantic clockwork, with gravity shaping its structure, literally holding it together, a universal force of attraction tugging tenaciously on every object in space. Where gravity is opposed by motion along a curved path, as it is for the Earth going around the sun, stability is attained: our planet is pulled by the sun but doesn’t fall into it. But what about the universe as a whole? With nothing to prop it up, why, wondered Newton, doesn’t the entire assemblage of stars fall together into one great mass? The solution he proposed was that the universe must be infinite. With no boundary and no centre of gravity, the cosmos lacks any privileged place to collapse towards. A given 8

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star would be pulled equally in all directions, the forces balancing out: ‘by their contrary attractions [they] destroy their mutual actions’ was how he quaintly expressed it. Clever though this dodge may have been, it was something of an intellectual sleight of hand. The delicate equilibrium isn’t in fact stable, as Newton himself acknowledged: ‘For I reccon this as hard as to make not one needle only but an infinite number of them  . . . stand accurately poised upon their points.’ Newton’s infinite cosmos, it appeared, would teeter on the brink of collapse. Yet as a religious man (albeit in his own eccentric way), Newton did not baulk at invoking the hand of God to sustain His creation when necessary. And there the matter rested. It was to be another two centuries before the correct solution was found, but through the lens of history we can now see that the answer was hiding in plain sight – right above our heads.

3. Why is It Dark at Night? Unless you live in the Arctic, night follows day as reliably as, well, night follows day. Most people never spare it a thought. But it turns out that the familiar darkness of the night sky tells us something of (literally) cosmic profundity. The fact that the sun has set is only part of the night-time story. There is also the matter of the stars. We don’t normally notice the illumination from starlight because it’s so feeble. That’s simply because the stars are extremely far away. Take Sirius, the brightest star in the sky. It’s actually intrinsically about twenty-five times brighter than the sun, but the way it appears to us, the sun outshines it by a factor of 13 billion. Sirius is 80 trillion kilometres away, whereas the sun is only 150 million kilometres away. Put the sun next to Sirius and it would be dim by comparison. The brightness of a glowing object diminishes with distance in a precise manner: a star seen from twice as far away would appear one quarter as bright, from three times as far it is one ninth as bright, and so on. The farther out in space you look, the dimmer the stars appear. But set against that is the fact that at a greater distance there are more stars, and the growth in stellar population compensates for the dimming effect of distance. It’s estimated there are 400 billion stars in our galaxy alone, and there are billions of galaxies within the scope of modern telescopes. So why don’t we notice the combined light from all these glowing sources? 10

why is it dark at night?

The same basic conundrum troubled a little-known Swiss astronomer, Jean-Philippe Loys de Cheseaux, in the middle of the eighteenth century. It occurred to de Cheseaux that if, as Newton proposed, the universe is boundless, with stars scattered throughout space to infinity, then the night sky should be ablaze with starlight  –  and the Earth fried to a crisp. The same conclusion was drawn by the German astronomer Heinrich Olbers in 1823 and became dignified with the term ‘Olbers’ paradox’. The paradox would go away if the stars were distributed out to some particular finite distance, with nothing but a

Figure 3. Olbers’ paradox. If the universe is infinitely populated with stars, every line of sight ought to intersect a star eventually, so there would be no dark parts in the night sky.

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dark void beyond. The total amount of starlight might then not add up to much. While that’s true, it runs straight into the problem Newton fretted over: if the number of stars is finite, then what’s to stop them all from falling into the middle to form a huge messy agglomeration? So, it’s between the proverbial rock and a hard place: either the universe will collapse, or the sky should be a permanent incinerator. However, there’s another escape route from Olbers’ paradox. Even if the stars do indeed populate space without limit, could they nevertheless be limited in time ? Today it seems obvious that stars can’t go on shining for all eternity. Whatever their source of power, sooner or later they will run out of fuel and fade. This was not so obvious in the nineteenth century, though; nobody knew what made the stars shine. In fact, it wasn’t until the 1940s that astrophysicists pieced the story together. Granted that the stars can’t shine for ever, they are certainly shining now, so if there really are an infinite number of them why doesn’t their accumulated light turn night into day? To answer that one, we have to go back to 1676, and a landmark observation made by Ole Rømer, a Danish astronomer who studied the motion of Jupiter’s moon Io, discovered by Galileo. Io goes around the giant planet as regularly as hands go around a clock. Rømer spotted that the Jovian clock seemed to be running slow when Jupiter was on the far side of the sun from Earth. The mismatch could be explained by the fact that light would take a few hours to travel from Jupiter to Earth. Depending on where Jupiter and Earth were positioned in their orbits, the journey time would vary. Rømer knew the sizes of the planets’ orbits, so he used that information to work out the speed of light 12

why is it dark at night?

based on the careful timing of Io’s movements. The answer he gave was 214,000 kilometres per second. Considering the crude methods then available, that’s impressively close to the actual value of 299,792.458 kilometres per second. The speed of light is so fast it might as well be infinite for everyday purposes. But in astronomy it makes a big difference. The light from Sirius, for example, takes 8.6 years to get here, so when you see Sirius in the sky, you are seeing it as it was 8.6 years ago. If it exploded today, we wouldn’t know about it for nearly a decade. The farther away the star is, the deeper in the past we see it. The Andromeda galaxy, for example, faintly visible to the naked eye as a fuzzy patch of light, contains stars seen by us today as they were two and a half million years ago. The distance light travels in a year, known as a ‘light year’, is a convenient unit in astronomy: one light year is about 10 trillion kilometres. Back to Olbers’ paradox, then. The finite speed of light transforms the entire argument. Suppose a particular star has been shining for a billion years. If it’s more than a billion light years away we wouldn’t be able to see it anyway, because its first light would not have arrived on Earth yet. So even if the universe is infinite as Newton suggested, we could still see only a limited number of stars –  those whose light has had time to reach us. The region beyond that would appear dark. Just in the last decade or so, the Hubble Space Telescope has actually been able to penetrate almost as far as the dark zone, beyond which there are no stars. It’s over 13 billion light years away and encloses a volume of space around Earth that encompasses about a trillion trillion stars. I just did a back-of-the-envelope calculation to estimate how much 13

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the combined light of all those stars adds up to, and it’s about the same in total as the light cast on Earth by Jupiter but spread evenly across the sky. No wonder we don’t notice it. At the end of 2020, astronomers announced that they had actually managed to measure the integrated starlight of the cosmos, using the New Horizons spacecraft which is zooming away from us beyond the orbit of Pluto (where it is very dark). The night sky turns out to be less than a ten-billionth as bright as the sun, but apparently twice as bright as predicted, for reasons as yet unclear. It’s remarkable that it took so long for astronomers to put two and two together in this way. At any time since the seventeenth century they could have drawn the obvious conclusion: the sky is dark at night because the universe cannot always have been the way it is now. Something very different – or perhaps nothing at all – must have preceded it.

4. The Big Bang In Flagstaff, Arizona, there is a famous observatory built in 1894 by a rich businessman, Percival Lowell. His plan was to use the telescopes to look for Martians. In the latter half of the nineteenth century this wasn’t considered totally off the wall. Scientists openly discussed the possibility that Mars was inhabited, and astronomers searched eagerly for signs of life on the red planet. In 1877, an Italian astronomer, Giovanni Schiaparelli, said he could see straight lines, or ‘channels’, on the planet’s surface, and this stoked much speculation about ‘canals of Mars’. Mars fever was brilliantly captured in H. G. Wells’ 1898 science-fiction story The War of the Worlds. Fixated by the notion of Martian engineers, Lowell embarked on his own observations, producing elaborate maps of what turned out to be an entirely fanciful network of canals. While Lowell pursued his quixotic quest, rather more conventional astronomy was also being conducted at his observatory. By the late nineteenth century, telescopes had advanced to the point where they could probe far beyond the confines of our Milky Way galaxy. A big issue of the day concerned the large-scale organization of the cosmos. In particular, what were all those nebulae  –  wispy patches of light  –  seen scattered across the sky? Were they giant gas clouds located within our galaxy, or were they entire galaxies in their own right, too far away for the individual stars to be discerned? 15

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In 1909, a Lowell Observatory astronomer named Vesto Slipher set about examining the quality of the light from the mysterious nebulae. The only telescope he had was a relatively modest 24-inch instrument, so this was slow, painstaking, repetitive work. With no fancy electronic gizmos of the sort astronomers have today, all observations had to be done by hand and eye, often with on-the-fly improvisations to coax the best out of the equipment. Slipher analysed the faint nebulous glows with a device called a spectroscope, designed to split light into its constituent colours. He laboured away night after night, often in freezing conditions, recording the results on film. In those days, an astronomer’s lot was not a happy one. Yet the compulsion that drives the Sliphers of this world is that dedicated toil in some small corner of a subject can unexpectedly strike gold. And that’s just what happened at the Lowell Observatory. By 1912 Slipher had assembled enough data to conclude that most nebulae were measurably redder in colour than the Milky Way. Why was that? An explanation was immediately apparent. When a light source is receding at great speed, the emitted light waves are stretched, shifting the wavelength towards the red end of the colour spectrum. Slipher therefore concluded that most nebulae are rushing away from us. With hindsight, we can see that 1912 marked the true birth of modern cosmology. But there was no fanfare, no press conference, just a careful technical paper buried in the Observatory’s bulletin. It took several years and many more observations before Slipher’s discovery gained prominence, eventually coming to the attention of a certain Edwin Hubble, a lawyer turned astronomer, rarely seen without a pipe. In 1924, Hubble used the big 100-inch telescope at Mount 16

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Wilson in California, then the world’s largest, to measure the distance to the Andromeda nebula by managing to detect individual stars, and in so doing proved that Andromeda is in fact another entire galaxy like the Milky Way. Hubble went on to estimate the distances to another twenty-three galaxies. Then he combined his results with Slipher’s red-shift measurements and glimpsed the outline of something systematic: the farther away the galaxy was located, the redder its light and the faster its recession. It seemed to be in proportion. The simplest interpretation of this pattern was that the universe was gradually expanding, growing larger over a timescale of billions of years (see ‘Hubble wars’, below). Hubble duly announced his results to the world in the New York Times on 23 November 1924. It was undoubtedly one the most momentous discoveries of the twentieth century, and plaudits were soon showered upon the pipe-smoking astronomer, leaving Vesto Slipher as the unsung hero of the expanding universe. With the realization that the universe isn’t just there –  an unchanging collection of glowing oddments  –  but is a dynamic system, evolving with time, a host of questions arose about its trajectory. Where had this system come from and where was it heading? What determines how fast the universe is expanding? Could the expansion rate change with time? When did it begin, and will it go on for ever? Two implications of the expansion were immediately obvious. First, if the universe is getting bigger, it must previously have been smaller and denser. Second, the effect of gravity – a universal attractive force  –  would act like a brake on the expansion, slowing the recession of the galaxies as they pulled on each other. Because of this deceleration, the expansion 17

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Hubble wars The rate of expansion of the universe is expressed as a number, known as the Hubble constant, or H. The man himself assigned H a value of 500. (In the peculiar system of units that astronomers prefer, that means a galaxy about 3.3 million light years away is receding on average at 500 kilometres per second.) Given H, and factoring in the braking effect of gravity, you can work out the age of the universe. Hubble’s original value put the universe at only about 2 billion years old – less than half the age of Earth! Astronomers lifted their game and produced estimates of H that progressively pushed back the inferred age, but they were divided into two warring factions. One touted a value of H of 180, the other 55, using the same methods and each insisting that the errors in measurement were far too small to close the gap. The discrepancy was an important issue because the smaller the number, the greater the age of the universe. In the 1980s, data from the Hubble Space Telescope put paid to the angst. H finally came out at 73 – a nice compromise – making the universe 13.8 billion years old by current estimates. But recently a new discrepancy has surfaced. Measurements of H using cosmic microwave background (CMB) data yield a value of only 67, implying an age for the universe of well over 14 billion years. Does this suggest something seriously amiss with our understanding of basic cosmology? Time will tell.

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must have been faster in the past.  Hubble’s observations weren’t extensive or accurate enough to detect any change in the rate of expansion over the few million years that they encompassed. However, the braking effect of gravity was easy enough to study theoretically, and as early as 1921, the Russian astronomer Alexander Friedman had calculated precisely how the expansion rate would gradually slow, but his deliberations were mostly ignored. Astronomers were flirting with one of the most far-reaching discoveries in the history of science, but such was the tentative nature of these early results that no leading scientist was prepared to take the plunge and state the obvious implication. In the event, it fell to a young Belgian priest and theoretical physicist, Abbé Georges Lemaître, to come out and claim explicitly in 1927 that the universe we now observe must have begun billions of years ago as a ‘cosmic egg’, a state of enormous density that expanded with explosive force. This was the precursor of the modern ‘big bang’ theory. You might have expected that a declaration of such profound importance would be a scientific, not to mention theological, sensation. Yet again, however, the response was muted. Hubble himself doubted Lemaître’s conclusion. Albert Einstein, by then the world’s greatest authority on gravitation and cosmology, was equally dismissive. ‘Your calculations are correct,’ he wrote to Lemaître, ‘but your physics is atrocious.’ Einstein had also shrugged Friedman’s earlier theoretical effort aside, and indeed he only accepted that the universe was actually expanding after visiting Hubble in California in 1931. After that, he did a U-turn and backed Lemaître’s work. In spite of this illustrious endorsement, speculation about the origin of the universe wasn’t taken 19

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very seriously in the 1930s; indeed, cosmology was hardly even a recognized subject. Happily, the theoretical work of Friedman and Lemaître wasn’t forgotten, though it took another two decades before it was revivified by George Gamow, a defector from the Soviet Union working in the United States. Gamow wasn’t an astronomer; he was a nuclear physicist.  It was he who explained the type of radioactivity known as alpha decay. Gamow reasoned that the young, highly compressed universe must have been hot enough to permit nuclear reactions. Consequently, it would have glowed like a furnace. Which raised a fascinating possibility. Might a fading remnant of

Figure 4. The antenna that first detected the hiss of microwaves emanating from the birth of the universe.

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Did the big bang really happen? Not all astronomers accepted the link between the cosmic expansion and an explosive origin. Ironically, the familiar moniker ‘big bang’ was initially coined by the British astronomer Fred Hoyle in 1949, while dismissing the idea. Hoyle thought Lemaître’s model of a universe exploding into being was nonsense, and developed a completely different interpretation of Hubble’s observations, called the ‘steady state’ theory. The basic idea is that as the universe expands and the galaxies move apart, so new matter is continually created, gradually aggregating into fresh galaxies to fill the ever-growing gaps. As a result, on a very large scale the universe would look much the same for ever – a mix of old and new galaxies sustained by a process of continual replenishment. There would be no beginning and no end, and no hot, dense primordial state. Hoyle fought fiercely for his theory, marshalling a band of loyal supporters. For about twenty years the two theories rivalled each other for support. But then the knockout blow came. The discovery of the CMB had no credible explanation within the steady state model, and support for it rapidly dwindled. I went to work with Hoyle in Cambridge at this critical juncture. Here was this world-famous astronomer and public celebrity, known for his science-fiction novels as well as his research, strangely isolated, casting around for some way to rescue the essence of his theory, perhaps by treating the big bang as an interlude rather than an absolute origin. He gave up on the idea that particles of matter were continually created in a thin soup throughout space in favour of concentrated

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what’s eating the universe? ‘creation centres’. In the 1960s, highly compact objects called quasars were discovered that fling out huge quantities of energetic material. Hoyle believed these energetic sources were cosmic spigots, pouring brand new matter into the universe. But the simplicity of the original steady state concept was lost, and in 1972 Hoyle resigned his Chair at Cambridge in disillusionment and became something of a recluse, tucked away in a remote cottage in Cumbria, where he filled his days fell-walking and sniping at the scientific establishment.

that primordial heat still pervade the universe today, forming a cosmic background of microwave radiation? Gamow was on the right track. In 1964, two scientists working on satellite communications at the Bell Laboratory  in New Jersey accidentally came across this remnant heat, bathing the universe at a temperature of about 2.7 degrees above absolute zero (absolute zero is about −273oC). It showed up as an annoying hiss in their receiver (see Figure 4). All attempts to explain it away as a defect in the equipment (including pigeon droppings in the antenna) failed, and the only explanation left was that the disturbance came from outer space. This was the big bang’s smoking gun. Suddenly, cosmology was propelled into the scientific mainstream, and began to attract some of the brightest minds in physics and mathematics – the likes of Roger Penrose and Stephen Hawking. The origin of the universe in a big bang was at last taken seriously and became the focus of intense theoretical analysis. Astronomers began to clamour for better observations of the cosmic microwave background –  today 22

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referred to simply as the CMB –  confident that it contained crucial clues about the early universe. Because of atmospheric absorption, a decent view of the CMB needs a satellite, and that’s what they got. In November 1989 NASA launched COBE, and, with it, cosmology’s golden age.

5. Where is the Centre of the Universe? It’s all very well to say the universe is expanding, but what does that mean? Expanding into what? Coming from where? When I first learned that the galaxies are rushing away from us it seemed to me that Earth must be close to cosmic ground zero – the point in space where the big bang happened. On the other hand, I knew it was crazy to believe we were at the centre of the universe. To properly understand the nature of the cosmic expansion, you have to think about it in a totally different way. Forget all those TV animations depicting the big bang as the explosion of a glowing lump, with fragments flung out into the surrounding space. As far as we can tell, the universe has no centre. The impression that we are close to it is an illusion. In reality, every galaxy is moving away from all the others.* The view from any given galaxy would be roughly the same; there is nothing special about our own location. A loose analogy is with a dance class where the students * Or, rather, clusters of galaxies are moving away from other clusters. Within clusters, galaxies mill about and occasionally bump into each other. The local group of galaxies includes Andromeda, which is actually flying towards us at 110 km per second (its light is blue-shifted, and was first measured by our unsung hero Vesto Slipher, no less) and will eventually smash into the Milky Way. This awesome encounter won’t happen for a few billion years, however, and even when it does it is unlikely to result in stars actually colliding.

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form a ring holding hands while the teacher explains the steps. The teacher tells all the students to step back five paces. As a result, the ring expands, and every student is now farther from every other student. But there is no special, privileged student that the others are fleeing from. Of course, the galaxies aren’t arranged in a ring, but the same basic idea is at work. Actually, it is better to stop thinking about galaxies moving through space altogether. Instead, imagine the space between the galaxies to be swelling. This image is not just an aid to visualization: space really is growing bigger: every day, a hundred billion billion cubic light years of additional space appears within the observable universe. It takes a while to get used to the idea that empty space  –  a void  –  can stretch or swell, but it’s true: space is provably elastic. In fact, not only can space stretch, it can buckle, twist and shudder too. The cosmic expansion is literally ‘putting space’ between the galaxies, and as a result they get farther and farther apart. This new space doesn’t have to ‘come from anywhere’ or expand into anything. It is space. Armed with this new insight, we can confront the question ‘Where did the big bang happen?’ The answer is everywhere. The CMB – the afterglow of the big bang – doesn’t have an epicentre; it isn’t a stream of radiation coming from a specific point in space. The entire universe is immersed in microwaves, as if it were entombed in a gigantic oven. Observing the CMB, you are witnessing the birth of the universe at every point in the sky. The stretching of space also gives a useful alternative way to think about the red shift. Light waves from a distant galaxy have to travel through expanding space to reach us. As 25

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the intervening space stretches, so the transiting light waves get stretched with it, meaning their wavelength increases and their frequency falls: blue becomes red. For the oldest observable galaxies, the wavelengths are eleven times longer when they arrive on Earth than when they set out from the source. The farther out into the universe we peer, the bigger the red shift becomes. The stretching factor –  the ratio of the wavelength observed on Earth to what it was when emitted by the source  –  escalates with distance, until it shifts the received light well beyond the visible region of the spectrum into the infrared, microwave and radio regions, longer and longer, without limit. Theoretically, the stretching factor would become infinite at a certain critical distance. In a simple model, that distance corresponds to how far light can have travelled since the big bang. Obviously, we couldn’t see any farther than that, because light hasn’t had time to reach us yet during the age of the universe. There is, therefore, a horizon in space – a light horizon – restricting our view. And because nothing can go faster than light there’s no way to know for sure what lies beyond. No instrument, however powerful, can enlighten us. But just as a terrestrial horizon doesn’t mark the edge of the world  –  what lies beyond is much the same – so the cosmic horizon isn’t ‘the edge of the universe’. There is no edge. The horizon is simply the boundary of our visible cosmic patch. The universe may very well be infinitely extended in space, but if it is finite in time we can’t look to see. I should mention that, in practice, the horizon is shrouded by the fiery afterbirth of the big bang. The red shift stretching factor of the CMB radiation coming from that epoch is about 1,000. 26

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One final point. If a galaxy is, say, 12 billion light years away, we are seeing where it was located 12 billion years ago –  so one can ask: where is it now? Because of the expansion of the universe it will be many billions of light years farther away today than it was then. For example, the horizon is estimated to now be about 47 billion light years away. It’s important to remember that our telescopes don’t provide a snapshot of the universe as it is today. Instead, the amalgamated images form a compilation, a historical time sequence cascaded together. It’s rather like cutting a movie into frames, stacking the frames into an ordered pile, and looking down through them to see the whole stack at once.

6. Why the Cosmos is Actually Fairly Simple Browsing in the public library as a teenager, I picked up a little book purporting to contain nothing less than the equations of the universe. I confess I was sceptical. Could the great story of the cosmos be captured in just a few lines of mathematics? Was it really that simple? The book, by Dennis Sciama, was called The Unity of the Universe, and it left a deep impression on me. The title says it all: there is a coherence and unity on the largest scale of size that lets one distil the basic story of the universe into three or four equations. Why do I say that the universe is simple? Mustn’t it be the most complicated thing we can think about? Well, yes and no. If you tried to describe everything that exists in individual detail, then of course it would be incomprehensibly complex. But just as one can recount the history of, say, the Vietnam War without mentioning the exploits of every given soldier, so we can determine the basic thrust of cosmic history without getting too bogged down in local details. Nevertheless, even this broad-brush narrative wouldn’t be possible without some very specific features. The most important of these is the uniformity of nature. As far as we can tell, identical laws of physics apply on the far side of the universe as they do in our cosmic neighbourhood. Why this should be so nobody knows, and it may not even be a proper scientific question anyway, but it is a critical reason why we can even discuss ‘the universe’ as a single entity and 28

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construct a narrative for the system as a whole. If the laws differed from place to place or time to time, there would be no common scheme of things. But that is not all. Even with universal laws, there are countless ways the universe could be arranged. Imagine an orchestra scheduled to perform a concert featuring, say, Beethoven’s Fifth Symphony. It would make perfect sense to attribute this musical performance to ‘the orchestra’, even though orchestras are made up of many individual musicians. So long as their playing is ‘orchestrated’ the result is harmonious. If each musician played from a different score without regard to the others, the upshot would be a horrible cacophony. It would then be pointless to say the orchestra was performing anything specific; indeed, it would be pointless to even speak of ‘the orchestra’ as opposed to an assemblage of independent musicians. The universe is organized in an analogous way. Rather than a lot of complicated parts behaving independently, there is orchestration and harmony and co-ordination. It shows up most obviously in the way the universe is observed to expand at the same rate everywhere. Furthermore, the number of galaxies in a similarly sized patch of sky is the same in every direction, as is the temperature of the CMB. Evidently the big bang went ‘bang’ in a remarkably ordered fashion. Contrast that with explosions on Earth. If a leaking gas main blows up a house, fragments are scattered haphazardly far and wide. It is a mess. There is a third crucial simplifying feature. According to folklore, in 48 bce the great library of Alexandria was destroyed by fire, one of the great disasters to befall human civilization. Priceless knowledge was lost as books were 29

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transformed into piles of ash in a tragic demonstration of the destructive power of heat. As a rule, the hotter the fire, the more it consumes. Modern incinerators can break down the most stable chemicals. The interior of the sun is so hot that even atoms can’t exist; they are smashed into nuclei and electrons. The big bang was hotter still. At one second after the beginning, it was ten billion degrees; at one microsecond, ten trillion degrees; at a picosecond, ten thousand trillion degrees. Hot = simple is one reason cosmologists can talk so confidently about the early universe. It really is true that a handful of simple equations – basic algebra and calculus – accurately captures most of what happened in the primordial phase. A new-born universe in which everything is mushed together and broken down to its basic constituents is far easier to describe theoretically than, say, the Earth.

7. What is the Speed of Space? Many people are fearful of flying, so take-off is a stressful time for them. I suffer from the opposite problem: I’m so stressed about missing the flight that by the time it leaves I’ve usually fallen asleep from nervous exhaustion. I often wake up half an hour later with no idea whether the plane is still stuck on the runway or halfway to Los Angeles. The thing is, you can’t tell from within the plane whether it’s moving or not. Of course, if it banks or dives you’d know it right away, but steady motion can’t be felt. It was Galileo who first made this explicit, pointing out that uniform speed can be measured only relative to something else. My car speedometer, for example, measures the speed of the car relative to the road. The plane’s speed is measured relative to the ground, or the air. Though physicists long ago accepted that uniform motion is only relative, some of them still wondered how fast Earth moves through space itself as our planet swings around the sun and glides across the galaxy. We don’t feel this motion, but we are travelling through space, which means that from our frame of reference, space is passing through us, all the time, unnoticed. Well, how fast is it going? How many litres of space per second slide through my body with ethereal stealth? After Galileo, it was recognized that no material device could measure the uniform speed of space. We can’t dip a metaphorical finger into the void around us to detect 31

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the slipstream as space sweeps by. But what about nonmaterial methods? By the late nineteenth century, a new possibility had presented itself. Perhaps light could be used to measure how fast Earth is travelling through space? At that time, physicists imagined space as filled with a sort of ghostly jelly-like substance referred to as ‘the ether’, and they envisaged light waves as vibrations in the ether travelling at a definite, fixed speed – the speed of light. Physicists invented various optical devices to try and measure the speed of the Earth through the ether. After a few years of effort, the result they obtained was, precisely, zero. Apparently, the Earth wasn’t moving through space (i.e. the ether) at all! This made no sense. There was a flat-out contradiction between Galileo’s relativity of uniform motion and the theory of light travelling at a fixed speed. The clash called for a fundamental re-evaluation of the nature of space, time and motion, and the properties of light. The challenge was taken up by Einstein, culminating in the first phase of his famed theory of relativity, published in 1905. Einstein abolished the ether, declared that the speed of light was an absolute fixed number however the observer who measures it is moving, and reaffirmed Galileo’s position that uniform motion is always relative to some other material thing. It is not only impossible, he said, but meaningless to talk about the speed of something relative to space itself. All that’s well and good, but what about non-uniform motion, i.e. acceleration? We have no trouble detecting that. If my in-flight coffee spills because the plane hits turbulence I don’t need to look out of the window to tell that our uniform motion through the air has been disturbed. Acceleration is a change in velocity, and we can literally feel it happening. If 32

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you slam on the brakes in a car, you get pitched forward. If you yank the steering wheel and take a tight corner, you lurch against the door. Both are examples of acceleration, the first being a change of speed, the second a change of direction. A steady rate of rotation represents a constant acceleration, and Newton used a simple example to illustrate its properties. He pointed out that water in a spinning bucket creeps up the sides; we normally attribute this to ‘centrifugal force’. The question, ‘Is the bucket spinning or not?’ is immediately answered by seeing whether the surface of the water is flat. It’s not necessary to look at, say, the ground, to tell, which suggested to Newton that rotation, and accelerated motion generally, need not be gauged relative to other material systems, but is absolute. Sometimes this was expressed by saying that acceleration is relative to absolute space itself. Not everybody agreed. Right up to the twentieth century some scientists argued that even rotation must be understood as relative. Relative to what? Physicists used to say relative to the ‘fixed stars’. Of course, the stars aren’t really fixed, they are just so far away that we don’t notice their movement: relative to the distant matter in the universe is a better description. To visualize this proposal, imagine riding with your eyes closed on one of those horrid fairground contraptions where you pay good money to be hurled around in circles in great discomfort at high speed. Open your eyes, look up. What do you see? You see the stars whizzing round. When the stars stop, the ride has stopped, and you can get off. This basic observation led some people, most notably the Austrian engineer and philosopher Ernst Mach (the same Mach whose eponymous numbers are used to describe 33

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aircraft speed), to attribute the feeling of rotation – and acceleration more generally – to the far-flung stars. When you are pressed against the side of a whirling fairground seat, or feel your ‘stomach left behind’ when a lift suddenly descends, it’s because the stars are pulling on you, said Mach. It was a beguiling theory and became known as Mach’s principle. It The most beautiful theory of all time General relativity has been hailed as humanity’s finest intellectual accomplishment, at once a scientific theory and an art form of breathtaking elegance. It entwines space, time, matter and force in an ingenious system of equations. Unlike many theories of physics, which are built in stages from the bottom up, general relativity springs whole from a few grand overarching principles, such as requiring that all physical properties must be independent of the co-ordinates used to describe them. Geometrical forms are sculpted from space itself by the arrangement of matter and energy. There is no actual force of gravity as Newton proposed, only restless, evolving geometry through which matter churns and light glides. The blandness of Newton’s immutable void is replaced by a concept of space as a vibrant, dynamic entity, stretching and shrinking, twisting and curving, pulsating and convulsing, and vibrating in energy-transporting undulations that ripple out across the universe at the speed of light (see ‘Gravitational waves’, p. 41). And general relativity works brilliantly. More than a century after Einstein presented his masterpiece, not a single observation has contradicted it.

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had many adherents, including Einstein, who warmed to the idea that all motion was relative, not just uniform motion. He  hoped Mach’s principle could be incorporated into a more general version of his theory of relativity, which he worked on in the early part of the twentieth century (see ‘The most beautiful theory of all time’, opposite). His thinking was that the gravity of all the distant stars and galaxies would combine to produce locally detectable effects like centrifugal force. Sadly, it didn’t work. Einstein’s general theory of relativity predicts that a planet spinning in otherwise empty space will bulge round the equator (as the Earth does) on account of its rotation, even in the absence of any ‘fixed stars’ with which to gauge its motion. Einstein’s theory is our best current understanding of gravity, so when it comes to rotation, it rather vindicates Newton’s view that it is absolute and not relative. And although we cannot meaningfully speak of the speed of space, the acceleration of an object in empty space still makes sense. That isn’t quite the end of the matter, though. A few physicists and cosmologists have continued to tinker with subtler formulations of Mach’s principle, and the subject is not completely settled. Indeed, many of my colleagues toggle back and forth between thinking of space as some sort of stuff (as did Newton) one day and as ‘nothing there’ the next. The lure of Mach’s principle is that it links everyday human experiences (like the feeling of spinning) with the organization of the universe, a sentiment captured most beautifully in the poet Francis Thompson’s lines:

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what’s eating the universe? All things by immortal power, Near or far, Hiddenly To each other linkèd are, That thou canst not stir a flower Without troubling of a star.

8. What is the Shape of Space? Hotel rooms sometimes come with those tiny fish-eye lenses in the door so if somebody knocks you can peer through the little hole and see a face with a huge nose and tiny ears. Similar effects are produced with fairground mirrors that are rather unflattering for the figure. Distorting shapes with lenses and mirrors is trickery, but it turns out that gravity does it for real. That is, gravity really does warp space. The first spacewarp to be observed was in 1919 when the British astronomer Sir Arthur Eddington led an expedition to West Africa to study an eclipse. To cast their horoscopes, astrologers follow the movement of the sun as it migrates through the constellations of the zodiac, moving against the background of stars. From time to time the sun’s movement brings it into close alignment with a star. When that happens it’s of interest to astronomers too. Eddington wanted to know how the sun’s gravity might affect the light from such a star. The trouble is, you can’t see stars in the daytime except during a solar eclipse. Then the moon blots out the sun’s glare for a few minutes, the sky darkens and the stars become visible. Astronomers know the positions of the stars very accurately when the sun isn’t in the way, and Eddington’s plan was to compare those recorded positions with their positions during the eclipse, to see if they were shifted. And they were. The stars were indeed very slightly askew (see

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Figure 5). It’s as if there is a gigantic fish-eye lens in the middle of the solar system. Four years previously, Albert Einstein had used his general theory of relativity to predict that the sun’s gravity would warp space and shift the apparent positions of the stars. The very idea that gravity can ‘bend starlight’ caught the public’s imagination, and Eddington’s confirmation of the prediction elevated Einstein to international fame. Curiously, the man himself took it all in his stride. He was so convinced

apparent position real position

Eddington

Figure 5. The bending of light. During an eclipse, stars can be seen near the sun’s location in the sky. Einstein predicted that the sun’s spacewarp should slightly displace the position of the stars, and Eddington confirmed this in 1919. Rather than thinking of the star beam as bent when it passes close to the sun, it’s better to regard light as taking the shortest, straightest path it can through curved space.

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that his brainchild, general relativity, was right, that when he was told about Eddington’s measurement he seemed unimpressed. Suppose Eddington had obtained a contradictory outcome, Einstein was asked by an assistant. ‘Then I would feel sorry for the good Lord,’ he replied. ‘The theory is correct anyway.’ Since Eddington’s pioneering expedition, gravitational lensing has matured into an entire branch of astronomy. Dramatic spacewarps occur when one galaxy eclipses another, occasionally producing a complete ring of light (see Figure 6). Black holes produce enormous spacewarps, leading to spectacular effects on surrounding light sources. Early on, Einstein realized that the warping or curving of space would not only occur around massive objects; it would also affect the shape of the universe as a whole. In 1917, he

Figure 6. Gravitational lens in which the spacewarp of a foreground galaxy images a more distant object to produce a ring of light.

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presented his idea of what he thought the overall cosmic geometry might be. This was pre-Hubble and Einstein assumed the universe was static. He ran into the same conundrum as Newton had, two hundred and fifty years earlier: why didn’t the cosmos collapse under its own unsupported weight? The difference was that Einstein came at the problem armed with a totally new theory of gravitation. Perhaps general relativity could provide a means to shore up the universe? He soon found that it could, but only by adding an extra term to his prized gravitational field equations. This he did reluctantly, feeling that the additional term was a fudge, compromising the elegance and simplicity of the original equations. But it seemed to do the job. The fudge factor described a new kind of repulsive force, or antigravity, with the strange property that it would become stronger with distance, whereas normal gravity grows weaker. Einstein picked the strength of the new force to match the combined gravity of all the matter in the universe, to produce a static equilibrium. A distinctive feature of Einstein’s static universe was its shape: space curves in on itself in such a way that it closes up into a finite volume, but without having any edge or boundary. To understand what this means, imagine that a light beam deflected slightly by the sun goes on to pass close to another star, getting deflected a bit more, then another, and another . . . Is it possible for the light to be bent right around in a closed loop? Einstein’s model universe showed that it was. The entire universe could turn light beams into circles in every direction at once. You could look out into space and, with a big enough telescope, you’d see the back of your own head! A closed space is difficult to picture in three dimensions, 40

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Gravitational waves Imagine that by some magic the sun were to disappear at noon. We would not know about it until just after 12:08 when the sunshine abruptly stopped. According to Newton’s theory, in which gravity acts instantaneously across space, Earth’s orbit would cease curving at precisely noon. Our planet would sense the sun had gone even if we couldn’t. But in Einstein’s theory nothing can go faster than light, including changes in gravity. If the sun abruptly dematerialized, the resulting gravitational disturbance would ripple outwards at a finite speed. In 1918 Einstein predicted that ripples of space can indeed exist, and they travel at the speed of light. These gravitational waves will be produced whenever there is a cataclysm, such as the collision of two black holes (see Figure 7). A century passed before tiny vibrations of space from a black-hole merger were finally picked up as disturbances in delicately arranged laser beams. These ultra-sensitive systems are now being used as gravitational telescopes, opening up an entirely new window on the universe and paving the way to study the details of the most violent cosmic events.

but easy in two. The spherical surface of the Earth is finite but has no edge or boundary. Einstein’s universe is an analogous concept, but with one dimension extra  –  forming a threedimensional shape called a hypersphere. Although it’s hard to visualize, the amazing artwork of M. C. Escher attempts to portray the geometry of curved space in aesthetically striking woodcuts. Whether you can imagine it or not, a 41

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Figure 7. Gravitational waves produced by two black holes spiralling together. These ripples in the geometry of space spread out at the speed of light and have been detected on Earth with ultra-sensitive laser devices called interferometers.

hypersphere makes perfect sense and can be described mathematically. It’s often asked what’s ‘outside’ the hypersphere. The answer is nothing: there is no outside. All observers are located in the space itself, so any discussion of a bigger space ‘containing’ the hypersphere is pointless. If space really is closed up in this manner, would we not notice it? Well, not if it’s big enough. Einstein was able to appeal to general relativity to estimate the total volume of his loopy universe. It all depends on the amount of matter there is: the more matter, the stronger the gravitational grip and 42

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the smaller the universe. Using the average density of matter estimated by astronomers, Einstein calculated that his static closed universe would have a total volume of a few thousand billion billion cubic light years, which I figured out to be equivalent to about a trillion trillion trillion trillion trillion trillion gallons, give or take a few pints. Intriguing though it was, Einstein’s closed static universe had a rather brief shelf life: within a few years, astronomers had accepted that the universe was expanding – a point that would take Einstein several years to concede. The poignancy of this episode is that the static equilibrium that Einstein strove to capture in his fudged equations is actually unstable; as with Newton’s infinite universe, it’s like trying to balance needles on their tips. Had Einstein acknowledged this he would surely have predicted that the universe could not possibly be static, that it must either collapse or expand. When he eventually learned, to his chagrin, about the expansion of the universe, he abandoned the fudge factor as ‘his greatest blunder’. As we shall see, there is a double irony here: decades later, astronomers would find that Einstein’s fudge factor was needed after all, in a somewhat different context. Though Einstein invented the idea of a closed universe for the wrong reason, the concept of a hypersphere also works for an expanding universe. But in that case, there is another possibility, which is negative curvature –  space curving ‘out’ instead of ‘in’ (see Figure 8). According to general relativity, the shape of an expanding space depends on how much matter is present: a high-density universe curves space positively (i.e. inwards) and a low-density universe curves it negatively (outwards). In between these two geometries is a space with zero curvature, meaning that the rules of 43

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Figure 8. Curved space depicted in two dimensions. Einstein’s ‘hypersphere’ universe – closed with no boundary – is represented by the top picture. Beneath it is a negatively curved space, and at the bottom a flat space.

geometry are the standard ones you learn at school. It arises if the universe has a certain critical density, equivalent to about six hydrogen atoms per cubic metre on average. So there are three possibilities. For many decades, astronomers, Hubble included, tried to measure which of these three was the right one. It’s easy enough in principle: you just have to estimate how many galaxies there are out to greater and greater distances. If space has zero curvature, then the number of galaxies should increase like the cube of the distance. If space is curved, that 44

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relationship changes. The observations are not that easy to do, and in practice, the best estimate for the curvature of space has come from measuring the splodges in the CMB. When this is done the answer comes out as zero –  the special, intermediate case. Cosmologists often express this result by saying that space is ‘flat’. They don’t mean it’s like a pancake. They just mean that the geometry of space (on average, over a cosmological scale) obeys the same rules of geometry as Euclid assembled over two millennia ago –  the rules you use on a whiteboard or a flat sheet of paper –  and not the rules you would apply, for example, to the curved surface of the Earth. All of which leaves us with a big question: why ?

9. Explaining the Cosmic Big Fix A trained marksman can hit a bullseye from a thousand metres, but if most of us took a potshot we would be well wide of the mark. The same laws of physics are at work, but the marksman has a sharper aim. To hit a bullseye a thousand metres away requires precision and skill. But hitting the same target a thousand light years away defies imagination. Yet that’s how precisely the cosmos seems to have been set up at the beginning. Let me illustrate. If the universe began with a little bang, it would have quickly fallen back on itself under the weight of all its material. The bigger the bang, the longer it could endure before collapsing. For the universe to last for billions of years, the bang had to be exactly big enough given the gravitational pull of all the cosmological material. And an exact match is precisely what the flatness of space indicates, according to general relativity. But that’s not all. The afterglow of the early universe comes to us from about 380,000 years after the big bang. At that time, light can have travelled no more than 380,000 light years in any direction, which, viewed from our perspective 13.8 billion years later, covers a region of the sky only about 1o across. But the CMB is uniform across the whole sky –  even regions on opposite sides of the universe have the same temperature. If no physical process can operate faster than light, why are these regions, which have seemingly never been in contact or able to influence each other, equally hot? There was nowhere near 46

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enough time for heat to flow between these regions to even out the temperature. It’s another of those balanced pins problems: to achieve such an exquisite degree of orchestration, the initial conditions at the big bang would have to be arranged with nuanced perfection by the Great Cosmic Marksman. It looks like a fix. Scientists, being instinctively wary of Cosmic Marksmen or any other form of Fixer, have long sought to find a physical mechanism to explain the apparently contrived nature of the universe. In the 1970s, cosmologists looked to the kitchen for inspiration. If you pour hot water willy-nilly on gravy powder it forms a lumpy, inhomogeneous mess. But if you pour carefully and stir vigorously, the gravy is homogenized by frictional effects. Perhaps the universe exploded into being in a mess, then smoothed itself out by friction? Detailed calculations showed that friction helps, but not much. No Cosmic Spoon is big enough or fast enough to do the job. That was pretty much the way things stood in the 1970s. But then along came a completely new theory in 1980. Suppose the initial explosion was indeed shambolic, but a split second later the universe abruptly leapt in size by a huge amount, as if it had taken a sudden deep breath. As a result, all the irregularities would be quickly smoothed out, just as inflating a balloon removes all the crinkles. No mixing would be necessary. To distinguish this sudden dramatic burst from common-or-garden expansion the former was dubbed inflation. Inflate for long enough and space will flatten out in the same way that the curvature of a balloon decreases with size. The inflationary universe scenario explains a lot, and soon became adopted as the party line among professional cosmologists, a status it retains to this day, but it is not without 47

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its problems. Chief among these is deciding what drove the frenetic inflationary burst. A huge pulse of antigravity would explain it, like the fudge factor that Einstein invented, but enormously stronger, and transitory, operating for just a brief moment. Cosmologists have invoked an ‘inflaton’ field to do the job, but nobody is sure what it is – it’s just a madeup answer beyond the reach of known physics. There’s also an exit problem. How did inflation – runaway accelerating expansion  –  slow down and morph into the traditional hot big bang picture in which the explosively expanding universe slams on the brakes and decelerates? The theory says the energy pent up in the inflaton field was dumped into the universe as heat, the remnant of which we see today in the CMB. But getting the exit phase to work properly is hard, and some of the suggestions look almost as much of a fix as simply positing an ultra-smooth big bang from the outset. Cosmologists are all agreed on this point: inflation enables a universe to balloon into existence without the need for any input energy. Alan Guth, the originator of the inflationary theory, described the universe as ‘the ultimate free lunch’. Over time, it became popular to say that, with inflation, the universe can come into being from nothing, and therefore its origin is fully explained. But this exaggerated claim rests on obfuscation around the word ‘nothing’. As we shall see in Chapter 20, there is a great deal else required to give a complete account of cosmic existence.

10. Most of Our Universe is Missing Twinkle, twinkle little star, How I wonder what you are.

Every schoolchild learns this ditty, and for the greater part of human history nobody knew the answer. In 1835, the French philosopher Auguste Comte claimed we would never know what stars are made of because they are so far away. But even before he said this, astronomers were on the case. In 1814 Joseph Fraunhoffer had found dark lines slashing across the spectrum of sunlight  –  the rainbow of colours made by a prism. The lines resemble supermarket barcodes. And barcodes are pretty much what they are, each pattern made by a specific chemical element: one barcode for carbon, another for oxygen, and so on. Because astronomers see these distinctive patterns everywhere, we know the whole universe is made of more or less the same material. Stars are composed mostly of hydrogen and helium, the two main elements that were coughed out of the big bang. The heavier elements we observe across the universe –  like carbon, oxygen, nitrogen and iron  –  were produced by nuclear reactions inside stars, built up step by step starting with hydrogen, in a form of serial alchemy. The sun, the star on our doorstep, is the easiest for us to study, and we now know that its core is a gigantic thermonuclear bomb going off at 100 billion megatons every second. The reason we’re 49

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not blown to smithereens is that the explosion is smothered by the weight of half a million kilometres of overlying gas. This controlled nuclear explosion continues day after day, year after year, in an astonishingly steady outpouring of energy. It’s an arresting thought that a mere 1 per cent reduction in solar output would plunge Earth into an ice age and make global warming a distant memory. The lifetime of a star depends on its mass. Massive stars live fast and die young, gobbling up their resources greedily. The first stars were like this – superstars of a hundred solar masses or more, burning out in a few million years, then exploding, lacing the surrounding space with their chemical products. When our solar system formed four and a half billion years ago, it scooped up a potpourri of this stellar detritus, which is why astronomers are fond of saying our bodies are made of stardust. More prosaically, we are made of nuclear ash. Working out the life cycle of stars and the origin of the elements was a decades-long research project linking astronomy to chemistry and life. Just when it seemed scientists had finally explained the complete inventory of cosmic material, things began to go wrong. It now seems that all the chemical elements, from hydrogen to uranium, make up only a tiny fraction of the mass of the universe. The rest is Something Else. Astronomers can tell this mystery matter exists by following a method that dates back three and a half centuries. When Newton discovered the law of gravity, only five other planets were known. That changed on 13 March 1781, when William Herschel, a German musician turned British amateur astronomer, inadvertently discovered a sixth planet – Uranus – using 50

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a home-made telescope. The discovery of Uranus was a milestone in the history of astronomy, as it was the first planet to be discovered with the aid of man-made technology, rather than observed by the naked eye. But by the middle of the nineteenth century it was apparent that something wasn’t quite right. The movement of Herschel’s planet across the sky was being disturbed by the gravitational force of an unseen object. Astronomers scrambled to calculate where in the sky the interfering object might be located. On the night of 23–24 September 1846, Johann Gottfried Galle of the Berlin Observatory found it where predicted; it was the planet we now call Neptune. Detectives use the dictum ‘follow the money’ to track down shadowy criminals. Astronomers follow the luminous material to trace shadowy matter. Neptune was the first in a long list of unseen, unseeable, or just plain dim entities that were discovered from the way they tug on their more visible neighbours. And there’s a lot of dark matter out there: in fact, it outweighs visible matter by a factor of five. The Milky Way is embedded in a vast blob of the stuff, and there’s even more lurking between the galaxies. There’s no agreement on what the majority of the unseen material is. Apart from the gravity it exerts, there seems to be no obvious detectable effect when dark matter runs into normal matter. It’s almost as if it’s ghost matter, gliding through solid objects without a murmur. A clue to what dark matter might be comes from particles called neutrinos. Made deep inside the sun as a by-product of the nuclear reactions there, neutrinos are so impervious to matter they fly straight out, unimpeded, from the solar core (unlike photons, which slowly meander their way to the 51

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surface). And if neutrinos hit Earth, they go right through that, too. On the face of it, neutrinos look the part for dark matter, but because they have an extremely low mass they can explain no more than a tiny fraction of it. Maybe dark matter is mostly made of highly penetrating particles like neutrinos, but much more massive? To search for these hypothetical WIMPS (for Weakly Interacting Massive Particles), physicists have built specialized detectors and put them deep underground to shield them from the disrupting effects of cosmic rays, which can trigger false positives in the equipment. So far, however, the researchers have drawn a blank. But, as Mr Micawber liked to say, no doubt ‘Something will turn up’ – sooner or later. Although dark matter greatly outweighs normal matter, it doesn’t in fact make up most of the mass of the universe. Added together, normal and dark matter represent only about a third of the total. As to what the rest is, the mystery deepens . . .

11. What is Dark Energy? The ancient structure known as Stonehenge is one of Britain’s foremost cultural sites, and a familiar landmark to tourists from all over the world. Volumes have been written about its astronomical and religious significance, but there has also been considerable puzzlement over how it came to be constructed, around four and a half thousand years ago, by Stone Age people without the benefit of modern technology. How were those massive blocks transported and erected? In particular, how were the huge capstones raised? Levitation would do the job nicely, and it’s a favourite explanation among devotees of the paranormal – not just for Stonehenge but the pyramids of Egypt, the walls of Machu Picchu and UFO propulsion. There is no doubt that the notion of levitation exercises a strange fascination for people. It has been part of our folklore for centuries, from magic carpets to alien abductions, and flying, floating, or levitation dreams are commonplace. There must be something deep in the human psyche that makes us find the idea compelling. The problem with actual levitation is that it flies in the face of our best understanding of the law of gravity, both in Newton’s original theory and Einstein’s general relativity. But there was always a loophole in the latter. As I explained earlier, Einstein fudged a type of antigravity  –  levitation, indeed!  –  to prop up the universe. But this cosmic force

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wouldn’t have been much use to the Stonehenge work gang, lightening a 10-ton block by a paltry 10−32 grams. The fudge factor in general relativity has swung in and out of fashion over the decades. When I was a student it was quipped that Einstein’s antigravity was repulsive, in both the physical and psychological sense. All that flipped, however, in the late 1990s when astronomers discovered that the expansion of the universe is in fact speeding up. Previously it was assumed that the expansion rate must slow as the weight of the universe puts a drag on it. Now it seems that antigravity not only exists but actually has the upper hand: a cosmic accelerator that has overwhelmed the brake. Antigravity is dubbed ‘dark energy’, partly because it is invisible, but also because its source is somewhat mysterious. (Don’t confuse dark energy with dark matter or the cosmic Dark Age –  they are totally different things. ‘Dark’ seems to be a favourite word among scientists.) The dark energy antigravity that is gripping the universe today is a faint remnant of inflation –  that brief super-fast burst of expansion at the birth of the universe explained in Chapter 9. It may or may not derive from the same basic physical mechanism. If dark energy turns out to be nothing more than Einstein’s antigravity, the source of the force is then simplicity itself – it’s the energy of empty space. Which sounds baffling at first: why should space possess energy if it’s empty? There’s a fundamental reason why, which has to do with that other gem of twentieth-century physics: quantum mechanics. In the 1930s, it dawned on physicists that there is no such thing as true emptiness. Even in a region of space devoid of all permanent residents (atoms, subatomic particles, 54

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E = mc 2 Even non-scientists know this famous equation, which links energy E, mass m and the speed of light c. It follows from the rule that nothing can travel faster than light, a key prediction of Einstein’s theory of relativity. What happens if you try to break the light barrier, for example by using a huge particle accelerator machine, like the famous Large Hadron Collider (LHC) near Geneva  –  a 27-kilometre ring-shaped underground tube in which counter-rotating high-energy proton beams are brought into head-on collision? Couldn’t one simply make the circulating particles move faster and faster until they exceed the speed of light? Actually, no. When the collider starts up, most of the energy fed to the protons does indeed increase their speed. But as the speed of light is approached, the particles become progressively more massive (i.e. heavier). The machine has to work harder and harder to gain additional speed. It’s a case of diminishing returns. At full power, almost all the input energy goes into making extra mass and very little into extra speed. According to the theory of relativity, no amount of input energy could push the particles through the light barrier. The best the LHC can do is reach 99.9999 per cent of the speed of light. This illustration shows that energy can be manifested as both speed and mass; the formula E = mc 2 quantifies how much mass a given amount of energy represents. And it works backwards: mass is a source of energy. Because c (the speed of light) is such a big number, very little mass is worth a lot of energy. For example, just one gram of mass, if fully converted into electricity, will power the average middle-class household for several years.

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photons . . . ) there remains an irrepressible traffic of temporary residents, particles that flit in and out of existence for the briefest duration, bubbling up from the void into halfreality only to dissolve away again almost immediately. But like the grin of the Cheshire cat, these ‘virtual particles’ leave behind subtle physical traces. Experiments confirm that the quantum vacuum really does seethe with energy, and where goes energy, there goes mass (see ‘E = mc 2’, above). For those interested in the specific numbers, a billion cubic kilometres of empty space contains about seven micrograms of dark mass-energy. Doesn’t seem like much, but it all adds up – there’s a lot of space out there. I have dodged around a potentially confusing aspect of dark energy: why does ‘dark mass’, or vacuum mass, produce antigravity? Surely mass is mass, and it gravitates? For Newton, mass was the sole source of gravity: the greater the mass, the stronger the pull. But in general relativity, mass (= energy) is just one of several physical properties that generate gravity. Another is pressure. This is a bit counterintuitive because we normally think of pressure as a pushing force. And it’s true that the mechanical effect of pressure is just that. But pressure also produces a gravitational pull of attraction, which for everyday gases like air is far, far less than the mechanical push. With heat radiation, though, pressure actually rivals energy in its physical action. We rarely notice the pressure of radiation, but it’s there, all right: the pressure of sunlight helps produce the tails of comets. The extra contribution from pressure gives radiation the edge over ordinary gas when it comes to gravitating power, and for a few hundred thousand years after the big bang, when the

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heat was so intense, it was radiation that dominated the gravitational pull on the universe – that is, the braking effect on the rate of expansion – outweighing everything else. When it comes to empty space, you might think that a void should have a pressure of precisely zero atmospheres, but it doesn’t. It’s very slightly less, by about a hundred trillionth of an atmosphere in fact. To sum it up in a phrase: space sucks. If pressure = gravity, then suction = antigravity. It turns out that the antigravity of dark suction beats the gravity of dark energy by a factor of three, so antigravity wins. The upshot is that space repels itself. Even if the universe were nothing but empty space, it would expand, swelling without limit, doubling its own volume every few billion years. This, by the way, is the secret of the free-lunch inflationary universe I discussed in Chapter 9. Self-repelling space is a cosmic perpetual motion machine, yielding as much universe as you like with no need for any input energy. All it needs is a trigger, and away you go. Although the account I have given may very well be the correct explanation for the accelerating universe, nobody knows how to calculate the actual quantity of quantum energy that is surging through space to produce the minuscule but all-important negative pressure. The numbers I quoted above come from astronomical observations, not theory. A straightforward calculation using quantum physics yields absurd answers. Some calculations estimate dark energy to be 120 powers of 10 too big! In reality, you can cook up any answer you like. At cosmology conferences, scientists place ‘the dark energy problem’ high on the list of cosmic mysteries to be solved, and routinely intone that it is

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one of the toughest challenges to theoretical physics ever. Personally, I think their hyperbole is somewhat overblown, and physicists will get to the bottom of it soon. But whatever the answer turns out to be, it will, alas, be unlikely to provide us with a magic carpet.

12. Where Does Matter Come From? If you visit Westminster Abbey in London, and go to Newton’s tomb, you will see a small plaque in the floor nearby with an equation inscribed on it. The plaque commemorates Paul Dirac, one of the twentieth century’s most brilliant scientists. Take a look at Figure 9, and marvel at how simple and compact the formula is, even if the symbols mean nothing to you. This little equation provides an entrée to a wonderland of mathematics and physics, and to a deeper layer of physical reality that you would never suspect existed, even though it is all around us. Published in 1929, Dirac’s equation merges the two great triumphs of twentieth-century physics –  the theory of relativity, and quantum mechanics. It was crafted for the very practical purpose of describing how electrons behave when moving at near the speed of light, as they do, for example, in radioactive emissions or when orbiting heavy atoms like uranium. But the equation is also a work of sublime artistry: these sparse symbols embed novel geometrical structures that lead to an entirely new way of describing space, time and matter (see ‘Spin’, p. 66). When you invent an equation the first thing you try to do is solve it. In the case of the Dirac equation the solutions correspond to various possible states of the electron, for example in orbit around an atom or just moving freely with a certain energy. When Dirac attempted this, he was puzzled 59

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Figure 9. Plaque in Westminster Abbey commemorating Paul Dirac, featuring his famous equation.

by the fact that for every choice of energy there were two solutions to his equation. Only one was needed to describe the electron. What was the other one for? What was this strikingly elegant equation trying to say? Dirac made a wild guess: his equation was telling us (well, him) of the existence of a type of anti-world, in which for every state of an electron there is a corresponding state of an ‘anti-electron’–  a particle with the same mass as the electron but with the opposite electric charge (positive instead of negative). But no such particle was known in 1929. Yet Dirac was right. In 1932, anti-electrons (usually referred to as positrons) were discovered by the American physicist Carl Anderson. Dirac was awarded a Nobel prize the following year. 60

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Soon after the discovery of the positron it became clear that other anti-particles must exist: anti-protons, antineutrons, anti-neutrinos, etc. Today, the anti-particles of all known particles of matter have been discovered and, in some cases, harvested and used in experiments. Collectively, these anti-particles are called antimatter. The existence of antimatter presents cosmologists with an oddity. Why is the beautiful mathematical symmetry implicit in Dirac’s equation not reflected in the make-up of the physical world? You and I and everything we encounter in daily life are made of matter. Where is all the antimatter? I should say that it’s a good thing there’s so little of it: if matter runs into antimatter then both vanish in a shower of gamma rays. Dirac wondered whether the universe might actually contain equal amounts of matter and antimatter, but they somehow became sorted into widely separated regions where they wouldn’t often collide. He thought there might be entire anti-stars. As one couldn’t tell by looking whether a star was made of matter or antimatter this wasn’t such a crazy suggestion. However, some mixing of material is always occurring in the galaxy and astronomers don’t see any of the telltale gamma rays that occur when matter and antimatter collide. If there is antimatter out there, it can’t be more than a smidgeon. The absence of antimatter is part of the wider problem of the presence of matter. All those protons, neutrons and electrons that came out of the big bang  –  a hundred trillion trillion trillion trillion tons of it within the purview of our instruments  –  how did it come to exist in the first place? Making matter as such isn’t actually a mystery; it happens all the time. For example, cosmic rays  –  not really ‘rays’ but 61

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high-energy particles, mostly protons  –  continually pepper the Earth from space. They are hurled our way at nearly the speed of light from powerful sources far across the galaxy. When they slam into the air above us they create whole showers of new particles: one proton may produce dozens of electrons as well as exotica like muons, pions, kaons . . . It was in fact in cosmic ray showers that Anderson first found the positron. Switch on a Geiger counter and much of the background click-click-click will be from freshly minted muons raining down from on high. Controlled matter creation occurs in particle accelerator machines like the LHC. Crucially, however, when processes like these create new particles, they are always accompanied by the right complement of anti-particles. The essential transformation taking place when new particles appear is the conversion of kinetic energy into matter. All that is needed is enough energy to pay for the masses of the created particles and anti-particles (see ‘E = mc2’, p. 55). Since there was plenty of heat energy in the big bang, the appearance of matter at that time is no surprise. But that still leaves us with the problem of the missing antimatter. It seems that something must have broken the matter– antimatter symmetry in that first fierce flash of cosmic existence, and the study of precisely what has been a nagging problem in physics for two generations. Whatever the mechanism was, it didn’t need to totally favour matter, resulting in zero antimatter. It would be enough for the created particles of matter to very slightly outnumber their antimatter counterparts by, say, one part in a billion. Then, as the universe cooled, all the antimatter and most of the matter would have embarked on an orgy of mutual destruction, leaving 62

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only a small residue of unpaired matter particles –  a cosmic afterthought, one might say, yet enough to build a universe. If this is correct, then the legions of gamma ray photons resulting from this wholesale annihilation, greatly red-shifted, are what makes up the CMB we detect today. The (slightly flawed) symmetry between matter and antimatter in the laws of physics is just one of several fundamental symmetries that physicists have focused on in the decades since Dirac’s discovery. Another is mirror symmetry. In a mirror, a right hand reflects into a left hand. You might suppose nature wouldn’t favour left over right, but in 1956 physicists were shocked to learn that it does. The radioactive process known as beta decay, in which a neutron vanishes to be replaced by a proton, an electron and an anti-neutrino, is distinctively lopsided between left and right (see Figure 10). If you sneakily played a movie of beta decay in a mirror, a physicist could spot the deception. A third symmetry that crops up is time reversal. Just as you can play a movie in a mirror, so you can play a movie backwards. The question then arises of whether, in particle physics, for every process that occurs, the laws of physics permit the reverse process. For example, when an electron meets a positron they annihilate one another and make two gamma ray photons. Can two photons create an electron– positron pair? Yes, they certainly can, but will pair creation always occur with precisely the same likelihood as pair annihilation? If nature preferred one direction of time over the other, there might be a small difference in the rate of the forward and reversed processes. In the 1960s it was discovered that, sure enough, there are some examples of particle transformations where this is the case (though not the one just cited). 63

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Figure 10. Nuclear processes called beta decay break the symmetry between left- and right-handedness. The figure shows a spinning atomic nucleus emitting an electron towards the ‘south’. Seen in a mirror, the spin direction is reversed so the electron appears to emanate towards the ‘north’. An experiment performed in 1956 with radioactive cobalt found a marked asymmetry in the direction of emissions between ‘north’ and ‘south’ – on average, the electrons preferred to ‘go south’ – implying a difference between the process and its mirror image.

That is, subatomic processes occur that do violate timereversal symmetry, by a tiny amount. A wily physicist could watch a movie of the process and deduce this without having been told that it was being played backwards. What about a movie of a particle process played both in a mirror and backwards? Would our clever physicist spot that 64

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deception? Well, maybe not, because it turns out that reversing left–right and the direction of time together is equivalent to reversing the electric charge, which occurs in the switch between matter and antimatter (for example, between a negatively charged electron and a positively charged positron). Likewise, switching all the charges has the same effect as reflecting in a mirror and reversing the direction of time. In 1967 the Russian dissident and theoretical physicist Andrei Sakharov pointed out that if there were fundamental particle processes that violated the combined charge reversal and left–right symmetries, then the way would be open to explaining why the big bang coughed out an excess of matter. Although this line of inquiry seems on the right track, nobody has yet pulled it all together to account for the actual degree of matter–antimatter asymmetry. It’s possible, however, that we are on the verge of a breakthrough on this puzzle. A new clue was announced in April 2020 from an experiment in Japan, in which beams of neutrinos and anti-neutrinos were shot through the Earth from a particle accelerator in Tokai, on the east coast, to an underground neutrino detector located at Kamioka, near the west coast. During the brief duration that the beam traversed 295 kilometres of solid rock, the particles underwent subtle changes. The degree of change differed slightly between neutrinos and anti-neutrinos, hinting at a symmetry violation that could be big enough to explain the mystery of the missing cosmic antimatter. All of which leaves us with an interesting philosophical question. Why are there any asymmetries in the laws of nature? The universe would be altogether simpler if everything was 65

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Spin Now here’s something to make your head spin. Dirac’s equation predicts that electrons spin like tiny tops. All electrons do it at exactly the same rate, unceasingly: it’s intrinsic to their nature. But the equation also says there’s something really weird about intrinsic spin compared to, say, a gyroscope or the spinning Earth. If you hover over the North Pole and look down, you see the Earth spinning anticlockwise. If a cosmic giant tipped the planet upside down before your eyes, you would now find yourself looking down on the South Pole and Earth would be turning clockwise. Roll the planet round another half-turn and all would be restored. Nothing confusing about that. Electrons, however, behave quite differently when rotated through one complete revolution (360o): they don’t come back to where they started. You have to roll them right around twice (i.e. through two complete revolutions, or 720o) for them to return to their starting state. Physicists can actually test this property using magnetic fields to rotate electrons. It’s as if these point-like particles possess a double view of the world. Just as some people are colour-blind and miss the full splendour of the world about us, so humans are ‘geometry-blind’ and can see only half of reality.

balanced, although there would be no stars or planets, or beings to admire the simplicity. But it’s possible one could have one’s cake (or rather one’s universe) and eat it too, if the observed imbalance in our universe is compensated for elsewhere. For example, maybe there’s a mirror universe 66

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where all things are transposed, a universe made of antimatter with the arrow of time reversed, relative to us. An Anti-World. Where might this Anti-World be? There are many possibilities: through a black hole or wormhole in space, interpenetrating our world but invisible to us, a parallel world . . . Or perhaps the big bang is a symmetry point, with time’s arrow pointing away from it in both directions –  an idea suggested by Sakharov himself. There’s no evidence for any of these speculations, but they do have a certain attractiveness, especially for those scientists who think that nature must be both simple and elegant. Symmetry is undeniably a factor in beauty, as any cosmetician will tell you. But in cosmology too? Well, beauty is in the eye of the beholder, so perhaps a little asymmetry also has aesthetic appeal.

13. Gravity Conquers All In 1930, a twenty-year-old Indian student named Subrahmanyan Chandrasekhar was sailing from what was then Madras (now Chennai) to England to study astrophysics at Cambridge University. To occupy his time during the long voyage, he began toying with the equations describing the stability of stars. From those untutored jottings came a discovery so shocking that it almost wrecked the young student’s career. Astronomers of the day had only a sketchy understanding of what makes stars tick. They knew that a star is a ball of hot gas engaging in a mammoth balancing act. The gas tries to expand into the vacuum of the surrounding space, but gravity holds it back. In stars like our sun, an equilibrium is achieved, so long as the gas stays hot, which requires it to consume fuel. But what happens when the fuel runs out? It seems that gravity would inevitably prevail, making the star contract; the smaller the radius, the fiercer the gravitational grip would become. Astronomers had long been familiar with tiny stars known as white dwarfs, which contain a mass comparable to the sun but squashed into a volume the size of a planet. These burned-out stars are so compressed that their atoms are squeezed cheek by jowl. It was once thought that the laws of quantum physics would prevent any further compression, but from his shipboard deliberations Chandrasekhar concluded otherwise. His equations suggested that if a star has a big enough mass, 68

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the crushing effect of its immense gravity would cause the atomic electrons to rattle around so fast they would approach the speed of light. Far from the star stuff getting more rigid, the effect would be to make it even more compressible, presaging a cataclysmic implosion. In the absence of any other factor, the ball of matter would plunge down its own gravitational well and disappear into what mathematicians call a singularity – a point of infinite density and geometrical curvature representing a boundary or edge of space and time. Chandrasekhar was able to calculate the critical mass above which this instability would set in. The answer he obtained was 1.44 solar masses, now known as the Chandrasekhar limit. On reaching England, he proudly announced his result, only to find he was dismissed as a young upstart peddling frivolous nonsense. In the 1930s, the gravitational collapse of a star was considered too outlandish to take seriously. The most distinguished astronomer of the day, Sir Arthur Eddington (he of the bent light beams), publicly ridiculed Chandrasekhar during an infamous encounter at the Royal Astronomical Society, declaring that there should be a law of nature ‘to prevent a star from behaving in this absurd way!’ It was a devastating put-down for a brilliant aspiring scientist. Chandrasekhar was so stung by Eddington’s derision, he decided to leave Britain and settle in the United States, where he followed a highly distinguished career until his death in 1995. Eddington was no fool, yet on this subject he was wrong. If a burned-out star has a mass exceeding Chandrasekhar’s limit, it does indeed undergo a spectacular convulsion, collapsing and exploding at the same time: the core implodes in a tiny fraction of a second, while the rest of the star gets 69

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blasted into space in what is known as a supernova. An explosion of this type was witnessed by Chinese astronomers in 1054. One possible fate of the core is to form a neutron star, in which the very atoms are crushed by gravity into a ball of neutrons about the size of a city. Neutron stars were discovered in the late 1960s (see the next chapter) and today many are known and intensively studied. There is one where the 1054 supernova was located, and the shattered remains of the rest of the stricken star now form a cloud of gas known as the Crab nebula (see Figure 11).

Figure 11. The Crab nebula, showing the detritus from a star seen to explode in 1054. A neutron star (not visible in the picture) lies near the centre, a relic of the original star’s core that imploded under its own weight.

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More massive stars end their days by totally collapsing to black holes. Although it took decades for the concept of a black hole to be fully understood and accepted, the basic idea was implicit in general relativity almost from the start. A paper published by Karl Schwarzschild in 1916 contained the fundamental concept. But for decades it was dismissed as a mathematical artefact with no physical meaning. Even Einstein would have none of it. It took the youthful genius of Chandrasekhar to prove that such an object could exist and form from the transformation of a dying star. And in 1972, the first black hole – a stellar remnant in the constellation of Cygnus – was discovered by astronomers. The subject of black holes is now an entire branch of astronomy in its own right, and we have to take seriously what Eddington and Einstein refused to confront: that some stars can undergo total gravitational collapse. When this happens, the effect on the imploding stellar material is violent in the extreme. But as we shall see, the effect on time is even more violent.

14. Warped Time and Black Holes Many is the occasion that my wife has been so engrossed in shopping that she returns, late and flustered, with a throwaway remark like ‘Sorry, the time absolutely flew by!’ Whereas I, hanging about waiting for her with nothing to do, think she has been gone ages. Yet the actual interval of time was exactly the same for both of us: the perceived discrepancy is all in the mind. But what if the duration between the same two events – ‘wife enters shop, wife emerges from shop’ – really was different for the two of us, not just psychologically but as measured by an accurate clock? The idea may seem bizarre, but that is exactly what is implied by the theory of relativity. Not only can space be warped, but time can too. Warped time is one of those dazzling topics that makes physics such a thrill to study. As a teenager, I liked to bewilder my friends and family with fun facts about clocks getting out of kilter and twins ending up with different ages. Mostly they didn’t believe a word of it. And in fact, I still get contacted by sceptics who flatly refuse to accept that time can be warped. They tie themselves up in all sorts of logical knots attempting to refute it. Time is one of those things we feel inside ourselves in a visceral way, and people can react strongly when told that this most primitive of experiences isn’t solidly rooted in reality. When I was a young lecturer at King’s College London in the 1970s, I worked close to the offices of the journal Nature. 72

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They got so many papers submitted by lay scientists trying to pick holes in the time-warping predictions of the theory of relativity that they had a special section to deal with them, and I used to go along every couple of weeks to sift through the submissions and frame polite responses. Mostly this did the trick, but there were a few tiresomely persistent correspondents, and some who even threatened legal action if their arguments were rejected. Warped time was already a decades-old idea when I dealt with these submissions. As long ago as 1915 Einstein predicted that gravity slows time, and he gave a formula for how much. For example, a clock at the summit of Mount Everest should tick 0.0000000001 per cent faster than Big Ben, meaning it would gain about a second every 33,000 years. It happens because the top of Everest is farther from the centre of the Earth than London, so gravity is slightly weaker there. Though extremely small, the effect is real enough: it shows up clearly in the Global Positioning System, which uses highly accurate timing on board orbiting satellites. For decades, gravitational time warps were largely ignored by astrophysicists as being too small to be of interest. But all that changed dramatically in 1967 with the discovery of the first neutron star by a Cambridge postgraduate student, Jocelyn Bell. In the glorious tradition of impoverished British science, Bell had hooked up a radio receiver to a lot of, in effect, chicken wire, strung out between poles in a field, forming a cheap and cheerful radio telescope. One day she noticed what she described as ‘a little bit of fuzz’ in the printed trace of the signal. It turned out to be a spinning neutron star emitting regular radio pulses. At the time, Bell and her PhD supervisor, Antony Hewish, wondered whether 73

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the pulses might be a signal from an alien civilization, so they dubbed the source LGM, for Little Green Men, and kept a lid on it pending further evaluation. Had it been an alien message, Bell would have been immortalized for an entirely different reason. In the event, she and Hewish soon found another pulsing radio source, and it became clear they were dealing not with ET, but a new class of astronomical object. On the surface of a neutron star, gravity is so fierce that a hypothetical clock would be slowed by about 30 per cent relative to Big Ben. Because neutron stars are held together so tightly by gravity, they can spin up to a thousand times a second without coming apart. As they rotate, they emit slender beams of radio waves that paint the galaxy like a lighthouse beam. From Earth, the beams are detected as brief repeating radio pulses, arriving with extraordinary regularity; pulsars are actually the most accurate clocks in the universe. It was these staccato pulses that Bell first noticed. When, a few years later, a pair of neutron stars was found locked in close orbit, the system became a physicist’s dream for testing general relativity. As the stars cavort around each other, spacetime buckles and writhes, and the pulsar clock faithfully tracks the shifting time warps. For a ball of matter with a given mass of material, the strength of gravity at the surface rises as the radius is reduced. If, by some magic, Earth were squeezed to half its radius, clocks at the surface would be slowed by an additional 0.0000001 per cent. Go on shrinking and the warp factor would rise faster and faster until, if Earth were squashed to the size of a large pea, it would become infinite. At this point the escape velocity from the Earth’s surface would reach the speed of light, so no light could escape. The pea-sized Earth 74

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would now be a black hole. As long ago as 1783, an English clergyman named John Michell deduced there could be objects in space with gravity strong enough to prevent the outflow of light. That was long before general relativity, but it turns out that Newton’s theory of gravity yields the same formula: light can’t escape when a ball of matter is confined to less than a certain specific tiny radius. And black holes certainly are tiny by astronomical standards: a 10 solar mass one, for example, has a radius of only about 30 kilometres. Because light provides an absolute speed limit, no information about the interior of a black hole can get out. It is surrounded by a one-way imaginary membrane called an event horizon, so named because we cannot know from outside what events happen inside. Black holes swallow everything and give nothing back. That means the identity of anything that falls in is lost. Whether formed from matter or antimatter or green cheese, black holes all look the same from the outside. So much for ‘black’; what about ‘hole’? Inside the event horizon, even outwardly directed light gets pulled inwards, towards the centre. Light tries to get away but is dragged back by the enormous gravity. Matter can’t go faster than light, so no material in the universe is stiff enough to remain at a fixed radius when light itself is being hauled inwards. Therefore, the material of the object that imploded also has to go on shrinking, perhaps to a point of infinite density – a singularity –  leaving behind empty space. The surroundings are thus both black (when viewed from afar) and empty; hence a ‘black hole’ in space. People often wonder what it would be like to fall into a black hole. Would time freeze? The answer is no. Time is 75

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relative. It stands still only in relation to a distant clock. For you, the in-falling observer, nothing weird happens to time, or to a travelling clock you take along for the ride. But if you compare your clock readings with, say, those back on Earth, you would certainly notice a huge and escalating discrepancy as you plunge to oblivion. Actually, the inferred time warp would be the least of your problems. The intense and varying gravity would rip you to shreds before you even entered the black hole, an effect often described as ‘spaghettification’. Many black holes are the collapsed remnants of stellar cores, as Chandrasekhar foretold, but there are also supermassive black holes at the centres of galaxies; nobody is sure how these formed. The Milky Way, for example, contains a black hole with a mass equivalent to about 2 million solar masses, while the galaxy M87 has a monster equivalent to 6.4 billion suns. These awesome objects have notoriously voracious appetites and will vacuum up anything  –  stars, gas, dust, other black holes – that venture too close. But they are messy eaters. The ingestion events are so turbulent and violent that a huge amount of energy gets released as the tormented material churns about and swirls down the hole. Some of this energy is swallowed by the black hole itself, but a large fraction spews out into space, often blasting surrounding material into narrow high-energy jets. In March 2020, the largest such explosion ever recorded was detected coming from a giant cluster of galaxies about 390 million light years away in the constellation of Ophiuchus. The energy of that single momentary burst was equivalent to a hundred billion times the energy output of the sun over its entire lifetime. Violent outbursts like these make black holes 76

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look like explosive sources of matter rather than sinks, which explains why Fred Hoyle thought they represented the creation of new matter (see ‘Did the big bang really happen?’, pp. 21–2). But far from facilitating the birth of matter, they spell its death.

15. Is Time Travel Possible? It’s not often you come across yourself in a novel, but that’s what happened to me when I read Gregory Benford’s awardwinning book Timescape. Greg, whose day job is astrophysics, came to visit me at King’s College London in 1976 while researching the novel, to talk about the nature of time and what distinguishes past from future. The story describes a world in 1998 sliding towards eco-doom, and a band of heroic scientists (including one Paul Davies, still labouring away at King’s College) cook up a scheme to send messages back in time to 1962, warning of the coming catastrophe and urging humanity to take steps to avert it. Signalling through time is challenging enough, but what about actually travelling through time? If you don’t like living now, is it possible to go to somewhen else? It’s a tantalizing idea. What child hasn’t longed for their next birthday, or Christmas, or a summer holiday? When you are young and bored it seems like ages to wait for future treats. In my own childhood, I used to fantasize about speeding up the flow of time or, even better, leaping over the intervening weeks. I did this too when something unpleasant loomed, like a visit to the dentist. Wouldn’t it be great, I mused, to press a button and hop to the day after the appointment? It sounds fanciful, but according to the theory of relativity, such a thing is not impossible. One way to travel through time I’ve already mentioned  –  exploiting the effects of 78

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gravity, which slows time. If you were to take a vacation somewhere with a big gravitational field (close to a black hole, say) you could twiddle your thumbs for a few moments while Earth-time fast-forwarded to your desired future date. Admittedly, hanging out next to a black hole is not a very practicable suggestion, but the theory of relativity provides another way to leap into the future, and it’s much easier. Not only gravity, but speed, has the effect of slowing time, as Einstein noted in his original 1905 paper. You can reach the future quicker simply by travelling fast. Unfortunately, that means very fast. To give you a feel for the numbers, on a longhaul flight, a clock on an aeroplane typically changes by a few nanoseconds relative to an identical clock left at the airport, which, in terms of human experience, means the passengers age (very slightly) less. Nanoseconds hardly make for a Doctor Who-style adventure. Far bigger time warps require extremely high speeds: the closer the speed gets to that of light, the bigger the warp factor. When the LHC propels protons to 99.9999 per cent of the speed of light, it generates a time warp so big that one second of proton time corresponds to about two hours in the lab. Cosmic ray protons are even more out of temporal rhythm with Earth. The highest energy cosmic ray so far discovered was found at the Pierre Auger Observatory  –  an array of particle detectors scattered over the pampas in Argentina. With an energy 2 million times greater than anything produced at the LHC, it was dubbed the ‘Oh-My-God’ particle because its energy is off the scale. It was moving so close to the speed of light that in its own reference frame, it could theoretically have crossed the galaxy in a few minutes, compared to the 100,000 years we on Earth would estimate 79

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that it took. Going that quickly, a person could fast-forward to Earth year 3000 in a mere 2 seconds! I’m ignoring the cost, the dangers and the effects of starting and stopping, but the basic physics is sound enough. Unfortunately, it’s not possible to use warp speed to go to the future and then come back again to now by the same means, because that would entail going both forward and backward in time. And the physics of visiting the past turns out to be far more problematic than going to the future. It is certainly philosophically problematic because of the wellknown paradoxes it entails, the most famous being the so-called ‘grandfather paradox’. What happens if a time traveller goes back a few decades and murders his own grandfather as a youngster? Then the time traveller would never be born. How, then, could the murder happen? But, if it didn’t happen, the time traveller would be born, and so could have carried out the murder. Either way, the narrative is inconsistent. This is called a causal loop paradox. By intermingling past and future events, the normal causal chain – that effects follow causes – is messed up, and chaos looms. Of course, it is those very paradoxes that make time-travel stories such compelling science fiction, ever since H.  G. Wells blazed the trail in the 1890s with his book The Time Machine. Though authors often fail to resolve the paradoxes, it is possible to have consistent causal loops. For example, imagine a story in which a time traveller visits the past and prevents a murder, and the rescued individual goes on to become the time traveller’s grandfather. That narrative hangs together well enough, but it implies that the time traveller cannot be free to kill his grandfather, which some people find absurd. Yet the laws of physics restrict our freedom in 80

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many ways we accept unthinkingly: I’m not free to walk upside down on the ceiling, for example, however much I may wish to do so. Even if there are ways around the philosophical paradoxes, there remains the question of whether travel into the past is in fact physically possible. And this is where things get a bit murky. When Einstein formulated his theory of relativity, he accepted that it predicted strange time-warping effects but he hoped it wouldn’t permit a person or an object to loop back into the past. In fact, merely signalling the past, as in Benford’s Timescape, unleashes causal paradoxes, because if someone is informed of the future they can act to change it. But Einstein was unable to rule out such things because nothing in his theory of relativity said it couldn’t be done. The nagging doubt lay like an embarrassing secret in the foundations of general relativity for some decades. By then, Einstein was living in Princeton and working at the Institute for Advanced Study. At this stage of his career he was a living legend, but his research was drifting and he sometimes said the only reason he went to work was to walk home with Kurt Gödel, a logician also employed at the Institute. Gödel worked on foundational aspects of mathematics, but he also took the trouble to learn Einstein’s general theory of relativity, and in 1948 he produced a solution of Einstein’s equations describing a universe that did indeed allow observers to travel into their own past.  Although physically possible, Gödel’s example was peculiar inasmuch as it needed the whole universe to be rotating. At that time astronomers lacked the means to rule out a slowly spinning universe, but we now know that if it was rotating, even at a snail’s pace, it would show up in the cosmic microwave background. 81

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Unrealistic though it was, Gödel’s solution served to demonstrate that general relativity does indeed allow causalitybusting time loops in principle. Over the years, other scenarios have been discovered that permit backward-in-time travel, the most well-known of which is the wormhole in space. Wormholes are like tunnels or stargates linking distant places to form a shortcut – similar to a black hole, but possessing a way out as well as a way in. Both black holes and wormholes have powerful gravitational fields that warp time, but whereas a black hole is a one-way journey to nowhere, an astronaut could traverse a wormhole and come out in another part of the galaxy, a scenario imaginatively depicted in the sciencefiction movie Contact. If the intrepid astronaut then dashed home the conventional way – by crossing the space between the stars  –  she could complete a loop in space that would also be a loop in time, thus becoming a temponaut astronaut. Although black holes are common, nobody has ever detected a wormhole and it could be they don’t exist, or, more relevantly, cannot exist. Even though general relativity doesn’t outlaw wormholes or time travel, quantum physics might, for example by creating an instability in the quantum vacuum (see p.56). Stephen Hawking once proposed what he called a Chronology Protection Hypothesis, stating that, in practice, time loops could never form – ‘making the world safe for historians,’ he quipped. To date, however, nobody has proved chronology protection, and the disturbing possibility of time loops remains on the ‘maybe’ list.

16. What is the Source of Time’s Puzzling Arrow? Our fascination with time-travel stories stems largely from a natural desire we all have to undo the wrongs of the past so we can make better lives for ourselves and our fellow humans in the present. However, in the absence of time travel we have to confront the reality that the past is a done deal: a car can’t be un-crashed, a failed exam remains a fail, a life lost cannot be recovered. But here’s the curious thing: the future remains open, and we retain the power to shape it. What, then, is the source of this asymmetry between past and future, this ‘arrow of time’? I first became interested in the subject after a lecture given by Fred Hoyle at the Royal Society in London in 1968 on why radio broadcasts are heard after they are transmitted and not before. (In truth, it was a bit more scientific than that, but the asymmetry of radio waves captures the gist of it.) Later I wrote a whole book on time asymmetry, intending it to be the last word on the subject. That was in the mid-1970s and was the reason why Greg Benford came to see me. All these decades later, somewhat shockingly, there remains widespread confusion – and many more books – about the arrow of time. Boiled down to its essentials, the issue with time’s arrow is this. Imagine taking a movie of an everyday incident and playing it in reverse to an audience. Everybody laughs because it looks so preposterous. People walking backwards, rivers 83

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flowing uphill, sandcastles washed into shape by retreating waves. But in physics a laugh test isn’t enough. What exactly gives the game away? Here’s a clue. Open a new pack of cards. The manufacturers arrange them in numerical order by suit. Shuffle the cards. The sequence will now be jumbled. If a magician shuffled a pack of jumbled cards and gave them to you in numerical order, you’d know you were being duped. While it’s not impossible for jumbled cards to be randomly shuffled into order, it’s exceedingly improbable. The arrow of time here is clear: random disruption turns order into disorder. Scientists zeroed in on this basic property in the middle of the nineteenth century. The Austrian physicist Ludwig Boltzmann considered how gas molecules rush around randomly, banging into each other and spreading heat energy around. He analysed this natural shuffling mathematically and identified something called entropy, a precisely defined quantity that measures the degree of disorder in the gas. Then he used Newton’s laws of mechanics, plus an averaging assumption, to prove that entropy would never go down. The rise and rise of entropy is one expression of the so-called second law of thermodynamics, perhaps the most inclusive law in all of physics. Because what’s true of gases is true of everything: all systems have a natural tendency to grow messier, to degenerate and decay (see Figure 12). Readers with teenage children will understand: for ‘system’ read ‘bedroom’. And, of course, children know that when it comes to bedrooms they are much easier to mess up than tidy up. Understanding the cosmic implications of all this, the British physicist Lord Kelvin delivered a lecture in 1852 famous for what has to be the most depressing prediction in the history of science. The 84

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entire universe, claimed Kelvin, is dying, slowly choking on its own entropy. Gradually, inexorably, cosmos is turning into chaos. If the relentless march of disorder defines the arrow of time, then the universe must have been more ordered in the past. And indeed it was. As I have been at pains to point out, the universe that emerged from the big bang was astonishingly, bafflingly, extremely highly ordered. Had it been exactly ordered, the arrow of time would have stalled, because perfection persists. It would be a case of blandness for ever: gravity would have nothing to get its teeth into. But, of course, the nascent universe wasn’t 100 per cent perfect. There were those ever so slight, wispy splodges found by COBE, a mere 0.001 per cent variation in temperature, far below what human senses would register. The splodges betray a minute departure from orderly perfection, a palette of almost imperceptible density perturbations in the primordial plasma. Gravity set to work on the splodges: the over-dense regions pulled more strongly, drawing in the surrounding

Figure 12. An everyday kitchen phenomenon. It’s not hard to tell which picture was taken earlier and which later.

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material and amplifying the density contrast, generating large-scale complexity – clusters of galaxies, churning clouds of gas and meandering stars. Clumping is gravity’s gift to the cosmos. Without clumping we wouldn’t be here, for gravity sculpts the element-forming stars, the habitable planets, the orderly structure of the solar system. But, like a petulant god, the creator is also the destroyer. Gravity is remorseless, tirelessly awaiting its next victim, as Chandrasekhar foretold. The sun’s life-sustaining stability is the result of a temporary hang-up, a multi-billion-year quiescent interlude in which fuel is squandered to combat the crushing effects of gravity, a titanic tussle that gravity will ultimately win. Being irreversible, gravitational clumping, like the flow of heat from hot to cold, represents a type of entropy increase. The end point of gravity’s tendency to pull things together is the black hole. Paradoxically, black holes are at one and the same time perfectly ordered and perfectly disordered. Their geometry is extremely simple and symmetric. On the other hand, they swallow and obliterate everything that comes their way. Black holes seemingly eat all forms of order, erasing its memory. If Earth and its human cargo were sucked into a black hole, the only record of our erstwhile existence would be the increase in the black hole’s mass. The fire of Alexandria at least left the ashes of the books. If the big bang had coughed out black holes instead of smooth gas, there would have been no cosmic order, no life, no library of Alexandria. Thus it is that gravity, the incubator and annihilator of habitable order, is also the source of time’s pervasive arrow. The time asymmetry that distinguishes yesterday from tomorrow, memories from anticipation, birth from death, 86

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can be traced back to the birth of the universe itself and, specifically, to its extraordinary degree of primordial smoothness. But where did that smoothness come from in the first place? Do we just accept it as an unexplained initial condition? A Big Fix? One possible explanation is an appeal to the tiny violation of time-reversal symmetry in certain hard-to-discern particle processes touched on in Chapter 12. Did the very particles of the universe themselves come with their own inbuilt arrow, which somehow projected itself on to the entire cosmos in the torrid aftermath of the big bang? Maybe, but in my view not very likely. Far and away the most popular explanation for the smooth start to the universe is the inflationary scenario that we looked at in Chapter 9. A burst of antigravity-propelled expansion in the first split second creates precisely that almost, though not quite perfect, uniformity. But that’s still not the end of the trail, because the universe has to get itself into an inflationary state at the outset. How did that come about? The scientific community is still very far from reaching a consensus on these thorny issues. All that can be said for certain is that one of the most fundamental properties of the physical world – that tomorrow is different from yesterday – still lacks a full explanation, and it lies high on my own list of essential, unanswered big questions.

17. The Black Hole Paradox During my career, I must have attended thousands of lectures, and very few of them stand out as historic. But one such occurred in January 1975 at the Rutherford Appleton Laboratory near Oxford. The lecture was given by Stephen Hawking, and it has been dramatically portrayed in films and documentaries. I have a vivid memory of the event, sitting near the back of the lecture theatre, straining to follow Hawking’s distorted speech and trying to make sense of the mathematics. At that time, Hawking was a familiar figure in the theoretical physics community, but he was far from being a public celebrity. All that changed, however, with this lecture, for it was on that occasion that he explained how quantum effects fundamentally alter the nature of black holes. In a nutshell, according to Hawking, black holes are not in fact totally black but glow with radiant heat, as a result of which they slowly shrink, and finally explode out of existence in a shower of subatomic debris. It was a sensational claim. I have to admit that I was initially very sceptical of Hawking’s prediction. I had myself been working on the quantum theory of black holes, using the celebrated solution to Einstein’s gravitational field equations discovered by Karl Schwarzschild in 1916 – the one that, all along, contained a description of a black hole. My own work indicated the existence of a cloud of quantum energy around the event 88

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Figure 13. Headstone in Westminster Abbey commemorating Stephen Hawking, featuring his formula for the temperature of a black hole.

horizon, but because Schwarzschild’s solution describes an eternally existing black hole, it is symmetric in time: nothing distinguishes past from future, because nothing changes. As a result, the cloud of energy around the black hole is static; it does not flow away from the hole in the form of radiation in the manner that Hawking suggested. I put in some arduous work to get to the bottom of this discrepancy. The key to Hawking’s result lies in the brief phase at the outset, when a ball of matter (e.g. a star) undergoes a convulsion and collapses to produce the black hole. The death of the star – abrupt, catastrophic, near-instantaneous – marks the birth of the black hole, but a subtle record of the star’s sudden demise lingers on, engraved into the quantum vacuum, disrupting it to produce a continuous flux of energy that lasts for as long as it takes for the black hole itself to die. And black holes take an unconscionable time a-dying: an object born in a 89

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microsecond might take a billion trillion trillion trillion trillion trillion years to evaporate away completely. With the benefit of hindsight, all that was there in Hawking’s lecture if I hadn’t been too stupid to see it. What finally convinced me that Hawking was right was a simple calculation I made to describe an observer who accelerates through empty space, in the absence of any gravitational field. Because acceleration mimics a gravitational force, constant acceleration represents an analogue of a black hole, but with simpler mathematics. What I found was striking: an accelerating observer would perceive a bath of heat around herself, closely analogous to the Hawking radiation around a black hole. That result was confirmed the following year in an elegant calculation made by William Unruh, who showed that a particle detector accelerating through a quantum vacuum would indeed absorb energy as if immersed in heat radiation. Hawking’s lecture had an energizing effect on the physics community. Previously, only a handful of physicists were interested in quantum effects induced by gravitational fields, which were generally deemed to be too small to make much difference to anything, but it would quickly become one of the most intensively studied topics in theoretical physics. It’s true that ‘the Hawking effect’, as the evaporation of black holes soon became known, is exceedingly weak. But the real significance of his result lay in its unexpected combination of simplicity and depth: the equation for the temperature of a black hole, like Dirac’s formula for the electron, contained very few symbols but nevertheless opened a whole new window on reality. And, as with Dirac, Hawking’s equation may be found in Westminster Abbey (see Figure 13). 90

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Although nobody has ever detected radiation from a black hole, the conceptual ramifications of the prediction soon proved both vexing and profound. One immediate puzzle concerned the source of heat energy, which must ultimately come from the mass (= energy) of the black hole. Since nothing can get out of a black hole, the quanta of heat radiation have to be created outside the event horizon. But then how does their energy derive from the interior of the hole? It seemed paradoxical. I set to work on this problem with colleagues William Unruh and Stephen Fulling, and we soon figured out what was happening. No energy emanates from inside the black hole itself –  it can’t, because of the one-way nature of the event horizon. What happens is that negative energy flows into the black hole. Negative energy? In quantum physics energy can indeed be less than zero. When the rules of quantum physics are applied to totally empty space, the resulting ‘quantum vacuum’ is a ferment of activity, with half-real ‘virtual’ particles flitting in and out of existence (see Chapter 11). Well, all those ghostly comings and goings get upset by the proximity of the black hole, because the virtual particles are obliged to flit about in a space that is geometrically warped by gravity. And the warping affects their energy, making it less in total than it would be with no spacewarp. Hence negative energy, which streams across the event horizon, conveying negative mass into the belly of the beast. When you do the sums, the books balance nicely: the ingoing negative energy causes the black hole to lose mass and shrink, which exactly pays for the energy of the Hawking radiation that flows away into the depths of the universe. The energy paradox was relatively easy to solve. But all 91

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along there was a tougher riddle lurking, one that Hawking himself wrestled with until the very end. He first discussed it with me in a Boston hotel room in 1978, where we had travelled to an astrophysics conference. The Hawking effect predicts that a black hole should gradually get smaller and smaller, eventually disappearing completely. That raises the question of what happened to all the stuff that fell into the hole in the first place. Is it permanently lost? Or is it a case of what goes in must come out? Can an echo of its identity somehow end up mingled in the accumulated heat radiation, but scrambled beyond recognition? Or does new physics come into play at the event horizon to change the whole scenario? Basically, it all boils down to the nature of information  –  in this case the information concerning the identity of the material that fell into the black hole in the first place. Quantum mechanics says information is never lost; general relativity says it is. Quantum black holes require both theories, so what gives when a black hole shrinks to a vanishing speck? Although his black-hole work addressed a well-defined branch of astrophysics, Hawking also tackled some questions of a more general philosophical and even theological nature. He liked to profess his strict atheism, but in his writing he often displayed what Einstein called a ‘cosmic religious feeling’. From within his own scientific framework, Hawking celebrated the beauty and ingenuity of a universe that he felt compelled to understand in all its rational majesty. And central to that vision was a belief that, however baffling and complex the universe may seem, beneath it all lies a harmonious mathematical unity  –  perhaps even a theory of everything. 92

18. A Theory of Everything? In 1980, Hawking was appointed to the Lucasian Chair of Mathematics at the University of Cambridge, a position previously held by Newton and Dirac, and on 29 April he delivered his inaugural lecture: ‘Is the end in sight for theoretical physics?’ Hawking didn’t mean that physics might peter out for lack of money or talent; rather, that it would succeed so brilliantly that the only thing left would be to pick over the minor details. It wasn’t the first time such a prediction had been made. In 1894, the American physicist Albert Michelson had proclaimed: ‘The more important fundamental laws and facts of physical science have all been discovered, and these are now so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote.’ Ironically, seven years earlier, Michelson himself had performed the crucial experiment to measure the speed of the Earth through space (see p. 32), the zero result of which precipitated the crisis in physics that led to Einstein’s theory of relativity. However, Hawking’s grand vision far surpassed that of Michelson and others, for it heralded not merely a completion of physics, but its unification into nothing less than a Theory of Everything: ‘A theory of the physical interactions that would describe all possible observations,’ is the way he put it. This capstone project might, he ventured, be attained by the end of the twentieth century. Alas, forty years on we seem to be very far 93

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from the final, all-embracing theory that he envisaged. If anything, the obstacles to unification have multiplied. As Mark Twain might have expressed it, rumours of the death of theoretical physics turned out to be greatly exaggerated. The quest for a unified theory of the universe dates back over two thousand years and has its origins in an ideological confrontation between ancient Greek philosophers concerning the nature of permanence and change. ‘No man steps in the same river twice,’ wrote Heraclitus of Ephesus, a member of the school that maintained the world was in a continual state of flux. An opposing school, led by Zeno of Elea, argued that change was in fact paradoxical, for how could something transform into that which it was not? A third school was born of a scholarly compromise. Known as Atomism, it was represented by Democritus and Leucippus. They suggested that although matter is very complex, it is ultimately composed of simple indivisible building blocks, which they called atoms (a-tom meaning ‘no cut’). The universe, declared the Atomists, consisted of just two things: atoms and the void. All forms of matter could be explained by different arrangements of atoms, and all change by the movement of atoms. Though rooted purely in philosophical deliberations, Atomism is a basically correct account of the structure and behaviour of matter. But Atomism’s influence extended well beyond a theory of form and transformation, for it suggested that the best way to understand a physical object or system is to smash it apart and see what it’s made of, a viewpoint known as reductionism. And that’s precisely what particle physicists have spent the past few decades doing. Reductionism is a powerful methodology, but, as we shall see, it falls short of giving a complete account of the world. 94

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What we now call atoms are actually misnamed: they aren’t the unbreakable particles that the Greek philosophers had in mind, but are composite bodies consisting of a nucleus orbited by electrons. The nucleus is made of protons and neutrons, which are in turn composed of two varieties of smaller particles called quarks, three apiece. This is not a bottomless pit of complexity. Normal matter is mostly explained by the particles I’ve just mentioned, plus the neutrino. Oddly, however, nature chose to triplicate the inventory –  there exist two entire additional sets of quarks and neutrinos and two heavier versions of electrons, known as the muon and the tau. And now you have to double this whole list to include all the anti-particles. Apart from the neutrinos, the particles in these additional families are very short-lived and don’t produce many noticeable effects. They aren’t components of ordinary atoms, but subatomic fragments that fly off and quickly disappear. The entire field of particle physics is confusingly jargonridden. Particle properties go far beyond spin and electric charge. There’s isospin and hypercharge and colour and strangeness, and on and on. Listen to particle physicists talking animatedly in a pub and it’s reminiscent of a bunch of insurance brokers or accountants – incomprehensible verbiage. But the plethora of attributes physicists assign to their beloved particles isn’t just a bookkeeping exercise; it reflects the elaborate mathematical organization of the subatomic world. To a physicist, the particle zoo is a wonderland. These little particles all have, if not quite their own personalities, at least distinctive properties and behaviour – and charm. (As a matter of fact, one of the properties of quarks is actually known, whimsically, as its ‘charm’.) There are light ones and 95

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heavy ones, long-lived and short-lived, charged and uncharged; some of them decay in half a dozen different ways, producing rich cascades of progeny, each with their own intrinsically fascinating qualities. Physicists often have their favourite particle (neutral pion, tau neutrino, sigma minus . . . ) just as a zoologist might feel especially passionate about the naked mole-rat or three-toed sloth. The pinnacle of particle fever comes with the discovery of a new one, which was common fifty years ago but is now a rarity. A complete understanding of matter  –  a theory of everything – requires not just a catalogue of the particles that comprise the universe, but a theory of the fundamental forces that act between them. Physicists have been able to identify four: gravity, electromagnetism and two nuclear forces, called simply weak and strong. Two of these four –  electromagnetism and the weak nuclear force –  turn out to be locked in a mathematical embrace, forming a hybrid ‘electroweak’ force in which the properties of one force intimately link to those of the other. A lynchpin of this linkage is the existence of a new type of particle, known as the Higgs boson. First predicted by Peter Higgs and others in the 1960s, the Higgs boson was finally produced by the LHC in 2012. This was the last definitive discovery of a fundamental particle. As a result, the four fundamental forces have now convincingly been whittled down to three. The way the forces operate in the microscopic realm, where quantum effects prevail, is by deploying yet more particles, which act like messengers conveying the force from one particle of matter to another. The photon is responsible for the electromagnetic force, while the strong force requires a whole set of particles called gluons, so 96

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Figure 14. Unifying physics. Science is ultimately about finding unexpected connections between things. Newton discovered a few basic laws that link together the orbits of planets and comets to the motion of bodies on Earth. Einstein linked space and time, mass and energy and gravitation and geometry. James Clerk Maxwell in the nineteenth century found a connection between electricity and magnetism, which was extended in the 1960s to include the weak nuclear force. Could everything be connected, deep down? That is the dream of a final unified theory.

named because they glue the quarks together. The weak force particles are simply given uninspiring letters: W and Z. Gravity is transmitted by the graviton, doing for gravity what photons do for electromagnetism. The graviton is far and away my own favourite particle, because nobody has ever detected one. Gravity is such a weak force that there’s 97

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no hope of seeing it at work between individual subatomic particles. That makes the search for evidence of gravitons a tantalizing challenge. Quantum theory says gravitons have to be there. But where to look? Together, this rather long list of particles –  the electron and neutrino (times three), two sorts of quarks (also times three), the photon, W and Z, eight species of gluons, and the (celebrated) Higgs boson –  make up what is known as the Standard Model of particle physics (see Table 1). It has been enormously successful in describing the subatomic world, but nobody thinks it’s the last word. For a start, the electroweak and strong forces haven’t been unified, so the Standard Model is not a fully integrated theory  –  more cohabitation than marriage. And some really basic things are left out: the poor old graviton, for a start. Also, the Standard Model has little to say about why neutrinos have a non-zero, albeit tiny, mass, or what produced the matter–antimatter asymmetry in the hot early universe. It’s also inconclusive about the nature of dark matter. These shortcomings mean that when the status of the Standard Model gets discussed, it’s generally in the context of what lies beyond it.* In his lecture, Hawking envisaged a complete unification, in which all the particles and forces are amalgamated into a single, beautiful, mathematical scheme – perhaps even a formula that would fit on a T-shirt. There have been quite a few imaginative candidates for this Last Word theory over the years, but none that has really nailed it. Full unification needs * In April 2021, it was announced that the magnetic field of the muon – the electron’s heavier cousin – seems to deviate by a tiny amount from the Standard Model prediction, again hinting at new physics.

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Leptons

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Table 1. The fundamental particles in the Standard Model of particle physics The four particles in the left-hand column go to make up normal matter. They consist of two quarks, u and d, the electron e and the neutrino that pairs with it, labelled νe. The next two columns are replications of this scheme with heavier, unstable particles. The column on the right shows the particles that convey the forces: the gluons, collectively represented by the symbol g, the photon γ and the two weak force particles W and Z. Also shown is the Higgs boson, H. The six quarks can combine together in many ways to produce composite particles, of which the proton and neutron are the most familiar.

equations that combine particles of matter with the particles that convey the forces. And therein lies a huge problem. Matter particles all possess the weird ‘double-take’ property called intrinsic spin (explained on p. 66), while the force particles don’t. Amalgamating them in a unified theory is like 99

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mixing chalk and cheese. An elegant solution is available, however, care of an esoteric branch of mathematics called supersymmetry; physicists have long been attracted to its economy and beauty. It undeniably does the technical job, but it predicts a whole slew of new and exotic particles which haven’t been found. When the LHC was built, it was thought these supersymmetric particles would show up first, but so far there’s no sign of them. Maybe it’s because the LHC, powerful though it is, still lacks the punch to make them. Or perhaps nature doesn’t share our sense of mathematical elegance. The most ambitious attempt at complete unification is called string theory (or M theory), and the physics community has become rather fixated by it. The basic idea is that the world is indeed built from fundamental, indecomposable entities as the Greek Atomists maintained, but they are not particles. Rather, they are tiny loops of string. And I mean tiny  –  about twenty powers of ten smaller than a proton, placing them completely beyond the reach of experiment. The strings have tension, so they vibrate like plucked musical instruments. Different patterns of vibration represent different particles and forces when viewed on the relatively large scale of experimental particle physics. So far, string theory ticks all the right boxes, and is compelling because it includes both supersymmetry and gravitation in the mix. To get the scheme to work, though, the strings have to inhabit more than three space dimensions. For example, one version uses nine. The reason we don’t see the extra six dimensions is because they are rolled up to a tiny size. What does this mean? Imagine viewing a hosepipe from a distance. It looks like a wiggly line, but on closer inspection 100

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each point on the line is seen to be a little circle going around the circumference of the pipe (see Figure 15). By analogy, suppose each point of three-dimensional space is in fact a tiny circle (tiny, as in twenty powers of ten smaller than a proton) going around a fourth space dimension. We would never notice. You can hide any number of additional dimensions by ‘compactifying’ them in this way, but with more than one dimension there are many choices of how to do it. For example, two dimensions can be rolled up as the surface of a sphere or as a doughnut with a hole in the middle (called a

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Figure 15. How to hide a dimension of space. From a distance, the hosepipe looks like a wiggly line. Close up, a ‘point’ P on the line is revealed as a little circle going around the tube.

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torus). Mathematicians describe these two arrangements as different ‘topologies’. When it comes to six additional dimensions, the number of compact topologies is stupendously huge. Whole new branches of mathematics are needed to classify them and determine their properties. The idea is that the strings thread through these higher-dimensional labyrinths, which serve as byzantine echo chambers. History has, it seems, come full circle. The ancient Greeks built music and harmony into the macrocosm and celebrated the music of the spheres. Today, string theorists are tuned reverentially to the music and harmony of the microcosm. Initially physicists held out the hope that string theory might describe the actual physical world, incorporating the known list of particles and forces as we observe them, possibly even accounting for the force strengths and particle masses. Nobody believes that now. In fact, it seems likely that the theory is consistent with an almost limitless number of possible worlds. Does that make it useless? Not all theorists despair over this ambiguity. What is regarded as a sin by some has been seized on as a virtue by others. As we shall see, a rich smorgasbord of alternative universes is precisely what some physicists seek in order to explain the universe that is ours. The dry list of particles that I have introduced in this chapter raises the question of why. Is there a deeper reason for the list to be populated in just that way, by just those particles, with just those masses, electric charges and spins? And is there something magical about four (really three) basic forces rather than five, or fifty, or five hundred? Also, what is the point of having not merely an electron, but its two heavier cousins  –  the muon and the tau  –  extras that 102

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accomplish little more than vanishing in a fraction of a second? Is there an abstract mathematical subtext that threads these subatomic splinters into a coherent unity  –  string theory perhaps, or some other branch of mathematics awaiting a flash of inspiration by a future generation of theoretical physicists? Or might the universe simply be an arbitrary ragbag of components thrown together, which, merely fortuitously, proves sufficient to coax thinking, bewildered beings into existence?

19. Fossils from the Cosmic Dawn Cosmology’s golden age began with a picture, and a rather unprepossessing one at that. The blotchy panorama of the sky created by COBE (Figure 1, p. 1) can’t compete with the images from the Hubble Space Telescope, or the highresolution close-ups of the planets taken by space probes. But what COBE’s output lacked in visual appeal, it more than made up for in significance, for when studying that picture, we are in a very real sense looking directly at the primordial furnace itself. The light from the nascent universe imaged there has travelled to us for over 13.7 billion years, during which time it has remained largely undisturbed. That means the CMB is a type of fossil: fossilized light. To be sure, that light has been stretched in wavelength a thousand times, but it’s still distinctive, the graceful profile and purity of its spectrum immediately recognizable. Contrast the pristine CMB with the corrupted fossil remains collected by palaeontologists, from which they try to reconstruct a plausible narrative. There’s a lot of guesswork involved, because palaeontologists can’t physically see the past in the way astronomers can directly see the birth of the universe. Admittedly, the CMB isn’t a relic of the very instant of creation; it’s what’s left of the glow present at about 380,000 years after the big bang, an epoch about 0.003 per cent of the present age of the universe. Expressed differently, we can look back 99.997 per cent of the way to the beginning. That’s 104

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undeniably impressive, but can we do better? Conventional radio and optical telescopes can’t help much, because the universe was an opaque fiery plasma before that point. We can’t see into it for the same reason that we can’t see inside the sun. But telescopes aren’t the only way to uncover a record of our deep past: fossilized chemical elements are also important remnants of bygone times. George Gamow, the nuclear physicist who put the big bang theory on the map, reasoned that if there was a primordial phase when the temperature of the universe was millions of degrees, then a range of nuclear reactions would have occurred, opening up the prospect that their products might be identified in the chemical elements around us. In the 1960s, astronomers attempted to reconstruct the nuclear story in detail. At one second after the big bang, they calculated, the universe was too hot for nuclei to exist. Space was filled with a dense soup of untethered neutrons, protons, electrons and neutrinos. Soon thereafter the universe had cooled enough for neutrons and protons to interact, react, combine and rearrange. That phase didn’t last long –  just a few minutes. Once the temperature fell below about a hundred million degrees, nuclear reactions ground to a halt, leaving about a quarter of the primordial material as helium, almost all the rest as hydrogen, plus a smattering of deuterium. These numbers turn out to closely match astronomical observations. The element helium, then, can be regarded as a cosmic fossil dating from about three minutes after the origin of the universe.* Buoyed by this success, cosmologists sought out yet * Balloon lovers take note: most of the industrial helium used today comes from radioactive decay in the Earth, not from the big bang.

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earlier fossils. Neutrinos offered an intriguing possibility. They would have been made in prodigious quantities in the hot early universe, and, because they interact so weakly with ordinary matter, they will have travelled to us untrammelled from a second or so after the big bang. Like the CMB, there should be a cosmic background of neutrinos too: four neutrinos for every eleven CMB photons, to be precise. Which implies that the material universe is bathed in a sea of neutrinos that outnumber atoms by many billions to one. Although the primordial neutrinos collectively exert a measurable gravitational brake on the expansion of the universe, directly detecting them individually looks to be a lost cause, so tiny is their effect on matter. Still, it’s creepy to think that every second, our bodies are being penetrated (albeit harmlessly) by trillions of these cosmic fossils. Another great leap backwards came in the 1980s with the development of the inflationary universe scenario. Here I had a modest hand to play myself. As I explained in Chapter 9, the essential idea of inflation is that the universe jumped in size by an enormous factor a split second after its birth, when quantum effects would have been important. By a happy coincidence, a student of mine, Timothy Bunch, had already worked out the properties of the quantum vacuum –  the void seething with evanescent energy I mentioned in Chapter 11 – in precisely such a scenario. When the inflation theory became popular soon after, theorists were ready with the so-called Bunch-Davies vacuum to calculate what pattern of quantum density fluctuations should have been imprinted on the universe when inflation ended. A few years later they were able to match up those predicted variations with the splodges mapped by COBE, and later recorded in 106

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much higher resolution by a European satellite called Planck. The concordance was, and remains, very good. If the inflationary theory is right, it means that quantum fluctuations in the very early universe are writ large and frozen in the sky for all to see. The variegated blotches that graced the front pages of the world’s newspapers on 14 January 1990 are fossil remnants laid down just a temporal sliver away from the very origin of the cosmos. There is a postscript to this remarkable story. The quantum calculation that Tim and I did considered a simple type of field (for example, electromagnetic) in a background of expanding space described by general relativity. We ignored the quantum properties of gravity itself, because there was no good theory of it. But physicists have long been obsessed with detecting quantum effects of the actual gravitational field (see ‘The holy grail of quantum gravity’, below). Although the theory of quantum gravity remains a work in progress (and string theory is one contender), enough is known to work out the expected signature of quantum gravity effects emanating from the inflation phase. It would show up as distinctive twisting patterns in the CMB splodges  –  specifically in the pattern of polarization. Great excitement thus attended the announcement in 2014 that a special telescope in Antarctica had detected a matching pattern from a patch of sky above the South Pole. Alas, the result turned out to be caused by foreground dust in the Milky Way; it didn’t come from the dawn of creation after all. Still, the obscure field of palaeo-cosmology is in its infancy, and quantum gravitational effects may yet be found. That’s not necessarily the end of cosmic fossil hunting. It’s possible that the laws of physics themselves are fossils from 107

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Figure 16. Theory suggests that the electromagnetic radiation around the CMB splodges should be polarized in a distinctive pattern that could embed clues from the earliest phase of the universe, just after the big bang. The lines in the figure depict the direction of polarization.

the primordial era. As I explained in the previous chapter, many of the known features of the forces of nature and the list of subatomic particles have no rhyme or reason. Some physicists insist there really is no reason – it was just the way the cosmic cookie crumbled. And it’s true that the universe is replete with ‘frozen accidents’ –  the size of the Earth, the pattern of craters on the moon, the number of planets in the solar system. The same might be true of the masses of subatomic particles and the strengths of the forces between them. Even the number of forces and the number of spatial dimensions –  all might have fallen out arbitrarily as the universe cooled, with no deeper explanation for why they are as they are. They are just random accidents, forged in the heat of creation and frozen for ever after. If that is so, then the 108

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The holy grail of quantum gravity The twentieth century’s greatest triumphs in physics were quantum mechanics and Einstein’s general theory of relativity. Unfortunately, the two theories are incompatible. Quantum mechanics successfully accounts for the workings of three of the four fundamental forces. It describes electromagnetism especially brilliantly, by treating the force between charged particles as due to the exchange of photons –  quantized packets of light. Not so for gravity. You can attempt to apply quantum mechanics to gravity in the same way, by considering the exchange of ‘gravitons’ – hypothetical quanta of the gravitational field. But calculations based on general relativity soon run into trouble, yielding an unending series of infinite or meaningless answers. There have been many ingenious attempts to circumvent this problem by modifying the theory of gravity, string theory being one such approach. A further difficulty is that, even with a theory that yields finite answers, the effects of quantum gravity are likely to be incredibly small and hard to detect. On general grounds, physicists expect significant quantum effects of gravity to manifest themselves on a tiny length scale of 10−33 centimetres, often called the Planck length after Max Planck, who first defined it. The Planck length is twenty powers of ten smaller than a proton and is the scale at which strings operate in string theory. The corresponding timescale is 10−43 seconds – the Planck time. Assuming this expectation is correct, it implies that at these ultra-microscopic dimensions, space itself would be profoundly different, perhaps

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forming a maelstrom of shifting complex foam. When a black hole evaporates to a Planck size, its behaviour is likely to change in important, but as yet unknown, ways. Likewise, in the absence of an agreed workable theory of quantum gravity, we can say very little about the behaviour of the universe before the Planck time.

laws of physics themselves could be time capsules from the earliest conceivable moments of history. The success in using physics to explain the cosmos as far back as the first split second must count as one of humanity’s greatest achievements. Nevertheless, people still ask questions about the ultimate origin of the universe. It’s one thing to give a good account of what the universe was doing at some moment just after it sprang into existence. But what about the originating event itself ? What has science got to say about that?

20. Can the Universe Come from Nothing? Children have a disconcerting habit of asking their parents where babies come from. Mostly they are fobbed off with vague replies. It’s something of a surprise that before the invention of the microscope, even adults didn’t really know the answer. In fact, the very topic was somewhat delicate and contentious in Christian Europe, because the Church taught that God alone could bring forth new life. Then, in 1677, Antonie van Leeuwenhoek discovered sperm cells, and the fascinating story of fertilization, zygotes and embryo development followed. On the whole, science gives a good account of origins –  babies, hurricanes, mountains, planets, stars  . . . But what about the origin of the entire universe? Where did it come from? What caused it to come into being? And can scientific reasoning even be applied to grand existential questions of that nature? Explanations for cosmic existence fall into two broad categories. The first is that the universe popped into being from nothing. The second is that something always existed –  maybe not the universe we see now, but not nothing, anyway. Logically, it seems it has to be one or the other. During my career, sentiment has swung back and forth. When I was a teenager, the steady state theory, which posited a universe with no origin, was very popular. By the time I began doing my own research in cosmology, it had been thoroughly 111

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eclipsed by the big bang theory, which assumed a beginning at a finite moment in the past. Though the big bang idea was very successful, few cosmologists wanted to be drawn on awkward questions like how, or why, a big bang happened in the first place. Many of them simply declared it to be beyond the scope of science and not a job for cosmologists. Leave it to the philosophers, was a common reply. Well, what did philosophers have to say on the subject? Not very much, as it happened. One of the twentieth century’s leading philosophers of science, Bertrand Russell, engaged in a BBC radio debate with an English Jesuit priest, Fr Frederick Copleston, in 1948, and was asked about God and the origin of the universe. This is the downbeat exchange that followed: Copleston : . . . But your general point then, Lord Russell, is that it’s illegitimate even to ask the question of the cause of the world? Russell : Yes, that’s my position. Copleston : If it’s a question that for you has no meaning, it’s of course very difficult to discuss it, isn’t it? Russell : Yes, it is very difficult. What do you say – shall we pass on to some other issue?

Russell wasn’t being evasive. There is indeed a fundamental problem with the conventional idea of a cause being applied to the whole universe: it carries the implication that there was something there before the big bang, something to make it happen. But that would not have been the case if the big bang marked the origin of time itself. The idea that time began with the universe was suggested by St Augustine 112

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of Hippo, the fifth-century Christian theologian, who proclaimed that ‘the world was made with time and not in time.’ This ruse was to avoid the tricky question of what God was doing before creating the universe, but Augustine was right to regard time as part of the physical world and therefore to have come into existence with the physical world. The possibility that space and time can have edges or boundaries, technically known as singularities, with literally nothing beyond, is an integral feature of modern gravitational theory. There’s a singularity right there in Friedman’s original 1921 analysis of an expanding universe based on general relativity: it describes a universe that springs into being from a state of infinite density, expanding infinitely fast, implying that there was no time (or space) before the big bang. (There is also a singularity at the centre of a black hole, according to Schwarzschild’s original solution of general relativity.) However, the singular origin wasn’t taken too seriously for many decades because Friedman’s model was very simplified, describing a perfectly smooth universe in which all matter is squeezed into a single point at the beginning. Perhaps a slightly irregular arrangement would avoid this singular crush? In the late 1960s, ingenious mathematical analyses by Roger Penrose and Stephen Hawking proved that a singularity of some sort at the start of the universe can’t be dodged by minor changes to the geometry of space or the distribution of the cosmological material. These theorems were so powerful they spawned an entire field of research into singularity formation and structure, an accomplishment deservedly recognized by the award of the 2020 Nobel Prize in Physics to Penrose. Who would have guessed 113

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that the edge of nothing could be such a fertile area of mathematical complexity? If time did not exist before the big bang, the concept of a prior physical cause is meaningless. Asking what was there before the big bang is, as Hawking once put it, like asking what lies north of the North Pole. The answer is ‘nothing’, not because there is a mysterious Land of Nothing there, but because there is no such place as ‘north of the North Pole’. In the same vein, there is no such place (or time) as ‘before the big bang’. It would be ‘nothing’ (no-thing) in the same sense as the nothing lying north of the North Pole. The ‘nothing-before’ version of the big bang theory reached its zenith in the 1980s with the work of Hawking in collaboration with James Hartle. They bravely tried to explain how nothing can become something without violating the laws of physics. To do this, they appealed to that magic box of wonders, quantum mechanics. It had long been realized that time can’t be simply subdivided into ever finer intervals: quantum processes will make something weird occur on a short-enough timescale  –  the so-called Planck time (see p. 109). Particle physicists often detect effects that take a mere trillion trillionth of a second and, so far, nothing untoward has been seen concerning the nature of time. But, according to theory, at a billion billionth of this already slender temporal slice, the very concept of time breaks down. If time itself can’t be traced smoothly back to the origin, then maybe it’s possible to avoid time abruptly ‘switching on’ at an initial singular moment; time may have ‘congealed’ from some fuzzy quantum ‘pre-time’. Hartle and Hawking bolstered this general idea with detailed calculations based on quantum gravity. Although many scientists dismissed their model as a 114

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bit of mathematical chicanery, Hartle and Hawking at least demonstrated the possibility of giving an account, entirely within the framework of physical law, of a universe that has a finite age, yet for which no miraculous or singular first event was involved. If that essential idea can be made to work, we could follow the historical record back to the epoch when time simply evaporated away. By any account, that would truly be the end of history. Explanations for the origin of the universe based on physical laws, like that of Hartle and Hawking, tacitly assume that the laws are in some sense ‘already there’ to permit the universe to come out of nothing. Some sceptics say, ‘Oh, so the universe didn’t come from nothing, after all. The laws of physics were there before the big bang!’ But that’s not really true since there was no ‘before’. It’s more accurate to say that the laws of physics transcend space and time; they exist in a mathematical realm that is not part of the physical universe. Some distinguished scientists do indeed take that position. They argue that in the matter of existence, the set of physical laws are the primary entity and have within them universe-creating capabilities. Others disagree and deny that one can think of laws as ‘things’ that exist independently of the physical universe. They reason that the laws of physics are relationships between physical things that necessarily exist only after the origin of the universe. But you can’t have it both ways. Either unexplained laws are already ‘out there’ and account for how the physical cosmos (lawfully) appeared spontaneously from nothing, or laws + universe together burst into being ready-made  –  a joint package of marvels with no explanation whatsoever. It’s Catch 22. If this discussion makes your mind reel, don’t worry. 115

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Scarcely was the ink dry on Hartle and Hawking’s paper when sentiment swung back, and cosmologists started suggesting that maybe the big bang wasn’t the beginning of time after all.

21. How Many Universes Are There? On the list of Really Big cosmic Big Questions, they don’t come much bigger than this: how many universes actually exist? As the answer is unlikely to be, say, 153, the choice would seem to be 1, 2 or infinity. We can be sure there is at least one. In Chapter 12, I floated the notion of a matching Anti-World to balance everything, so that would make two. How about infinity? If the big bang was a natural event, surely it could happen more than once? Might there be many bangs scattered throughout space and time? Why would there be any limit on this process? The argument is persuasive, and these days most cosmologists I know think that there is indeed an infinity of universes. Such a scenario happens naturally in the inflationary theory. In the original version, there is a bang, then a stupendous antigravity force seizes the new-born universe and enormously inflates it, ironing out any preceding messiness. After that, inflation stops and the standard big bang story unfolds. But if inflation erases all record of the cosmic past, how do we know there was any beginning at all? What we have been calling the big bang could simply be what came after inflation. If so, what might have come before inflation? Well, maybe more inflation, back to eternity. Eternity being a long time, if there is a probability, however small, of an exit from eternal inflation into a regular universe, then our universe cannot be the only example. Taking a god’s-eye-view of this ‘eternal 117

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inflation’ scheme, the system as a whole just inflates and inflates, endlessly and frenetically. But within the overall superstructure, universes pop up randomly, separating out from the inflating region like bubbles in a fizzy drink. Since eternal inflation was mooted in the 1980s, it has grown steadily in popularity, not least because it avoids the vexed issue of a universe coming from nothing. The essential idea is reminiscent of the steady state theory, but instead of particles of matter being created, entire bubble universes come into being. Each bubble starts with a big bang, and embarks on a life cycle of beginning, middle and (maybe) an end. The overall system, however, is eternal. What we have all along been calling ‘the universe’ is actually just ‘our universe’  –  a tiny fragment of a much vaster, more complex ‘multiverse’. Where are all those other bubbles? Most cosmologists think they are much too far away for us to ever detect them. In the simplest multiverse variant, the bubbles fly apart from each other much faster than they themselves expand, so there isn’t any risk of our universe bumping into another one. However, some versions of the multiverse do feature cosmic encounters, an alarming prospect that I shall come back to later. While the multiverse idea undoubtedly has its attractions –  such as sidestepping the problem of a singular, magical origin of one universe – it comes with its own uncomfortable philosophical issues. If there really are an infinite number of universes, then there will be others out there identical to our own, in every respect, including duplicate versions of you and me. In fact, there will be an infinity of such replicas. The same basic problem arises even in a single universe. Roughly speaking, in an infinite universe with uniform laws and conditions, 118

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if something can exist it will exist, and exist infinitely many times. That something includes you and your entire life history, meaning that in an infinite universe or an infinite multiverse there will be a limitless number of other yous, identical in every conceivable respect. Furthermore, for every identical you there will be vastly more almost-yous. To help get to grips with this dizzying idea, think of tossing a coin. What are the chances of obtaining heads a million times in a row? Extremely small, but not strictly zero. Infinity beats not-quite-zero every time. In an infinite number of trials, a million heads in a row is assured – not just once, but an infinite number of times. Same is true of a billion heads, or a trillion. If you think that life is a lottery and you are the jackpot, well, the odds are certain that the same jackpot will come up somewhere else, sometime else. Of course, none of this reasoning applies if the universe isn’t actually infinite but just very, very big. It’s not all that hard to work out how big it would need to be to contain at least one other you. To be safe that we are not discussing just a momentary replication but an entire identical life history, we should consider an exact replica of the whole observable universe, because any bit of the universe can cause a small disturbance on a human being. The cosmologist Max Tegmark did the sums and determined that the volume of space you would need to explore before having a good chance of encountering a precise match – Universe 2.0 – is a volume of space out to a distance of 10 to the power 10120 metres; that’s 1 followed by a trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion zeros. For comparison, our observable ‘Universe 1.0’ is only 10 trillion trillion metres across. With numbers like these, it’s clear you are unlikely to 119

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actually run into any of your duplicates. Nevertheless, most people find the very notion of duplicate beings disturbing. Which one is the ‘real you’? There is a much-discussed variant of this large numbers game that can be applied to infinite time rather than infinite space. If the universe is set to endure for ever, isn’t that time enough for another you to be reconstituted from scratch? I mentioned the quantum vacuum as otherwise empty space seething with temporary particles, flitting in and out of existence randomly. But randomly means there is always a probability, albeit exceedingly small, that the quantum vacuum will generate something more organized than a sea of half-real electrons, quarks, neutrinos and photons. In principle, the quantum vacuum could create a human being – any human being – or maybe just their brain – though this luckless individual probably wouldn’t last long enough to know much joy before dissolving back into the quantum quagmire. The study of this offbeat subject has even been dignified with a name: Boltzmann brains, after Ludwig himself, who first toyed with the idea of an infinite universe randomly arranging itself into one containing a population of humans. To avoid such disconcerting scenarios, one could throw out the very notion of infinity. Scientists have always had an uneasy relationship with the concept, given that it can have such troublesome consequences. Perhaps infinity is good for mathematics but has no place in the real universe? Some cosmologists have argued as much. It’s hard to see how the matter can ever be resolved by observation, because we can only ever measure finite quantities. An actual infinity can be grasped by the human mind but never encountered in reality. 120

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It is an idealization, useful for computational purposes, but of dubious applicability to the real world. So much for philosophy. What about actual science? Is there any conceivable evidence for a multiverse? Possibly. In Chapter 19, I suggested that the laws of physics we know today might, in fact, be fossils from the big bang, forged in the hottest phase of existence when the most basic aspects of our universe were being established. In the case of more familiar fossils, such as dinosaurs, they come in many different species: you might find a brontosaurus here and a triceratops there. If the laws of physics are a type of fossil, then we can imagine different laws of physics in other bubble universes. If the ‘universe experiment’ is endlessly repeated, we can imagine that each bubble universe would come complete with its own set of laws, and that every variant, every possibility, would be instantiated somewhere (an infinity of times). Given that we are stuck in this universe, with the laws it has, how can we possibly obtain evidence for other universes with different laws? Well, there’s a rough and ready line of reasoning based on statistics. Let me give an analogy. In the United States bears are brown or black. In the far north of Canada they are white. Polar bears have an advantage in being camouflaged against the snow. There are two explanations for why polar bears are white. One is that they just got lucky. They were white anyway, and happily found themselves living in a snowy environment. The other explanation is that there was a large ensemble of bears of a range of colours, and evolution selected the fittest colour for the frozen north. In science, luck is sometimes the right explanation, but if something looks suspiciously, awfully, lucky, then the 121

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Is the universe a fake? In 1641, the philosopher René Descartes floated the notion that the universe might be a fake, conjured up by a cunning and malicious demon to deceive us. The basic idea has been revived recently in light of artificial intelligence and ever more lifelike virtual reality. If you believe consciousness is a product of physical processes then, given sufficient resources, it must be possible to simulate those processes in a computer and to create a fake virtual world for the simulated beings to inhabit, as in The Matrix series of movies. Many scientists and philosophers find this plausible. So how can we be sure that our universe is real, and not a simulation on somebody’s super-duper computer? The idea gives an intriguing twist to the multiverse theory: given that fake universes are far easier to make than real ones, the fakes might well outnumber the genuine articles, so that most conscious beings will in fact be living in a simulation.

chances are that we have overlooked a selection process. In that vein, evidence for the multiverse theory might be found if there seems to be something peculiar or special or really lucky about the laws in our universe – the one we call home. Is there? Well, how about the fact that it is home?

22. The Goldilocks Enigma There’s a story of a man who has fallen on hard times. His business has gone bust, his wife has left him and, to cap it all, he’s just dropped his freshly buttered toast on the carpet. Gloomily reaching down, he’s astonished to see the toast has fallen buttered side up. Could this be a sign? Has his luck begun to turn? He carefully picks up the toast, puts it in a ziplock bag, and hurries off to see his local priest. ‘Father,’ asks the man, showing him the toast, ‘what is the meaning of this? Are there better times ahead?’ The priest peers at the toast, then examines it with a magnifying glass. Reaching for his Bible, he suggests the man comes back in an hour. ‘Well, father,’ asks the man on his return, ‘has my luck changed?’ ‘No,’ replies the priest, shaking his head sorrowfully. ‘You buttered the wrong side, that’s all.’ We find this story amusing because we all have a natural desire to seek meaning even in the minutiae of our lives, to find a deeper significance beyond our humdrum existence. Astrology, mysticism, shamanism and religion all address this basic human need. But are we fooling ourselves? It is an ever present danger. One of the sharpest schisms in science erupted after a group of scientists claimed to have found hidden meaning in the fine details of the laws of physics. This stormy dispute goes right to the heart of the place of human beings in the great cosmic drama. The argument began in the 1950s, when Hoyle was trying to 123

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work out how carbon  –  the life-giving element  –  was made inside stars by nuclear collisions. At first it didn’t look possible –  there was no way a carbon nucleus could form from the fusion of two lighter nuclei. But as carbon is an abundant element there had to be a reaction pathway. Hoyle guessed there must exist a happy coincidence in the arrangement of nuclear states that enabled carbon to form from rare simultaneous encounters of three helium nuclei. Experiment proved him right. ‘It seemed as if a super-intellect had been monkeying with the laws of physics,’ he later wrote, noting that if the strength of the nuclear force were just a bit weaker or stronger, the coincidence would go away and the universe would be devoid of carbon, and therefore life. To Hoyle it was as if the universe were, to use his pithy description, ‘a put-up job’. Following this lead, distinguished physicists and astronomers began trawling through the details of other physical laws, looking for more examples of where the cosmic toast had fallen buttered side up. What if gravity was a bit weaker, or electrons a bit heavier? Suppose the universe were expanding faster or slower, or the big bang was hotter? Some of these changes turn out not to matter much as far as the existence of life is concerned, but others would be decidedly lethal. As the list of propitious coincidences grew, scientists became uneasy. Like in the story of Goldilocks, our universe seems to be ‘just right’ for life. It looks too much like divine providence, another Big Fix. Then a convenient explanation came to hand: the multiverse.* Among the countless * This is the version of the multiverse in which the laws of physics emerge randomly in the intense heat of the big bang and get fossilized in place, i.e. they are frozen accidents.

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universes churned out by the multiverse’s creation mechanism, surely some fraction will have life-encouraging laws replete with happy coincidences, purely by chance? Obviously, we couldn’t find ourselves living in an uninhabitable universe, so the freakish bio-friendliness of this one is no surprise really. We have selected it by our very existence. An explanation of this type is unusual in science, and it sparked heated debate. Some physicists, dreaming of a final theory of everything, hate the multiverse. They hope their theory will one day describe a unique universe with unique laws  –  thus demonstrating that it couldn’t have been otherwise –  and if that universe just happens to permit life, well, that’s simply the way it is. A third group, including some of a religious persuasion, see the multiverse explanation as an untestable cop-out. They also dismiss the unique final theory as a case of promissory triumphalism, given that there is no such theory in existence. This entire three-way spat revolves around something termed the anthropic principle, which is a misleading designation because it’s not intended to refer to human beings specifically, but to any sentient life form. At one level, the anthropic principle is innocuous enough; it merely says that the universe we observe has to be consistent with the existence of life. Thus, for example, it’s no surprise that we live on a planet near a stable star. We wouldn’t expect to be observing a region of the universe from intergalactic space. But sometimes it takes on a more mystical flavour, hinting that the laws of physics have been ‘monkeyed’ with, to use Hoyle’s evocative description. In the context of the multiverse, however, it makes no difference how rigged our universe looks because there are plenty of universes to go 125

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round, and there will always be one somewhere that will be inhabited and celebrated, no matter how quirky it is. I haven’t conducted a straw poll, but there is a list of very distinguished physicists and cosmologists who are totally persuaded that we do indeed live in a universe delicately poised in the Goldilocks zone, and that – theological explanations being anathema – there must be a multiverse. My own feeling is that, even if a multiverse exists, it still doesn’t explain everything. In the eternal inflation version of a multiverse, for example, there needs to be a universe-creating mechanism –  a bubble generator –  based on some physical laws. And the inflating superstructure itself uses the laws of quantum physics and general relativity. The origin of those laws remains unexplained. You could cook up any number of different multiverse models with different overarching laws and different bubble universe generators. So, the problem is just shifted up a level: instead of ‘Why this universe?’ one can ask, ‘Why this multiverse?’ There may be no end to this ontological paper trail.

23. What’s Eating the Universe? When COBE uncovered a sky full of splodges, theoretical cosmologists began poring over the data. Were the splodges random? Were the bigger splodges more pronounced than the smaller ones, and, if so, did the variation follow a mathematical law? How consistent were the patterns with the inflation theory? More satellites were launched, with better equipment, producing a deluge of additional data. Today, cosmologists think they have a good overall grasp of splodgology. But, frustratingly, there are persistent anomalies. Among the many splodges in the heat map of the sky, one in particular stands out as enigmatic. Located in the constellation of Eridanus in the Southern hemisphere, it is about five degrees across  –  the equivalent of ten full moons  –  representing an enormous volume of space. What is odd about this particular splodge is that it is far colder than it should be –  nearly eight times colder than the average temperature variation in the CMB. Its origin remains a mystery. At first sight, it looks as if a cosmic giant has taken a huge bite out of the universe, leaving a super-void. Astronomers have long accepted that the universe we can see –  the stars, gas and dust – is being slowly but inexorably eaten by supermassive black holes lurking in the centres of galaxies. But the cold spot in the Southern sky suggests something on a much, much larger scale, a blemish in the very structure of the cosmos itself. 127

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No easy explanation works, so there have been some wild speculations to explain it. One of these, championed by Laura Mersini-Houghton, posits an awesome collision involving another universe crashing into ours, leaving a fossilized scar in sky. Such a titanic encounter would generate copious quantities of gravitational waves and produce a distinctive pattern of polarization in the CMB. So far, searches for this pattern have come up empty. But in the multiverse of many bubbles, universes crashing into neighbours is just one of several available cosmic catastrophes, limited only by the ingenuity of theorists. Not only might our universe be bumped by another; it could be gobbled up wholesale, swallowed in toto by a bigger, more powerful universe blundering into ours. Or vice versa. Even ignoring the menace from without, there are also threats from within. One idea that has been around since the 1970s is that our universe might eat itself from the inside out. This could happen at any time, on account of a general feature of quantum systems.  The story of the self-devouring universe unfolds something like this. When an atom is excited, i.e. in an elevated energy level, then it will make a transition (it will ‘decay’, to use the correct technical term) to a lower level, emitting a photon. But what’s good for atoms is good for the quantum vacuum too. If, as most scientists believe, dark energy has a quantum origin (that is, it is the energy of the quantum vacuum –  see pp. 54–6), then, like atoms, there might be many possible values or levels of vacuum energy. Our universe happens to be in one of these vacuum energy levels, but it may not be the lowest.  The worry is, an excited, or elevated, state of the quantum vacuum wouldn’t be completely stable. There would always be a 128

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risk it would transition to a lower energy state – the vacuum could ‘decay’  –  thereby releasing a stupendous amount of energy. If this happened anywhere in the universe, the consequences would be apocalyptic. A tiny bubble of the new, lower energy vacuum would spread out at nearly the speed of light, with the released energy concentrated into the bubble wall. As this boundary expanded it would destroy everything in its path. There may be no warning: we might know our universe was being wrecked only when the wall arrived and annihilated us – and everything else – faster than the speed of thought. There is an eerie variant to this scenario. The expanding bubble might contain, not a lower energy quantum vacuum, but nothing at all. It would be a hole in space, not a black hole, but a bubble of no-space, expanding uncontrollably, sweeping all before it and eventually gobbling up the entire universe, leaving only nothing: space totally engulfed by spacelessness. The basic idea of space vanishing without warning in this alarming manner was first suggested in 1982 by theoretical physicist Edward Witten from the Institute of Advanced Study in Princeton (where Einstein used to work), based on his analysis of string theory. He described it thus: ‘A hole spontaneously forms in space and rapidly expands to infinity, pushing to infinity anything it may meet.’ Envisage space as the substance in a Swiss cheese. Now imagine the holes in the cheesy matrix growing ever bigger until there is no cheese left. The mysterious Cold Spot of Eridanus is just one feature in a collection of anomalies in the CMB that has left cosmologists scratching their heads. There is evidence that the overall power of the afterglow is lopsided between the 129

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hemispheres. Even stranger is that this asymmetry seems to align with the plane of the solar system, a finding so bizarre and heretical it has been dubbed the Axis of Evil. It’s almost as if the entire universe is being squeezed by a giant vice. What is going on? Is something outside the visible universe assaulting it? Could the cold spot be a sort of giant porthole into ‘pre-creation’, a cosmic epoch that preceded the big bang, very different from the cosmos we know? Might there be other portholes providing glimpses of this veiled anteworld? Windows into the wider multiverse? Or will all the puzzling anomalies turn out to be just statistical flukes? Watch this space . . . Although the cosmic cataclysms outlined here are the stuff of nightmares, content yourself with the fact that the universe has endured for many billions of years without mishap so far. Of all the things to worry about, the possibility that our universe might be eaten by another should be low down on your list. But from the scientific point of view, hints that the cosmos might be blemished and wonky –  that it is not quite perfect –  raises an interesting and profound question: might it be flawed in other ways too?

24. Is the Universe Actually a Botched Job? The idea that our universe is in some way special goes back to Isaac Newton’s continental rival, the seventeenth-century philosopher Gottfried Leibniz, who claimed that ours is actually the best of all possible worlds. He based this on theological grounds, reasoning that a perfect Divine Being wouldn’t make a flawed or ugly universe. Leibniz attracted much derision for his rosy view of the world, famously being lampooned as ‘Dr Pangloss’ in Voltaire’s 1759 novel Candide. Is there any sense in which our universe would in fact come out top in a cosmic beauty contest? Imagine playing God and consider giving the universe a makeover. Could it be improved? It’s easy to think of simple things – one more bottle of champagne, prettier sunsets, fewer wars. However, these aren’t the sort of things that scientists normally have in mind when considering optimization problems. If the universe were to get an upgrade to something better, we need to have a clear idea of what ‘better’ means. An obvious definition has to do with the existence of life. Life would seem to be a good thing (where would we be without it?), so a universe with more life might be an improvement. As I have explained, our universe is good at life, but could it be better? Could the laws of physics be tweaked so as to generate more habitable real estate? The astronomer Fred Adams thinks the answer is yes. ‘The universe is not fully optimized for the emergence of life,’ he 131

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says. ‘One can readily envision more favourable universes.’ Adams bases his conclusion on an exhaustive study of the consequences for life if many of the unexplained constants of nature were different. By ‘constants of nature’ I am referring to seemingly fixed but unexplained quantities, such as the charge on the electron or the mass of the proton. To make this analysis vivid, imagine a Designer Machine with knobs to adjust the various constants one by one. If you take the Standard Model of particle physics, and standard cosmology, there are thirty-something knobs determining such things as the strengths of forces, the masses of particles and the density of dark energy. Turn this knob and make all neutrinos a bit heavier; turn that one and make the weak force weaker. And so on. What happens to the universe? Adams twiddled the knobs (metaphorically speaking) and found numerous cases where the upshot would be a more bio-friendly universe than ours. For example, if the strong force were stronger, the nuclear pathway to synthesizing carbon would be much more efficient. Also, stars could live much longer and sustain habitable planets for trillions of years. He looked at many features like this. His overall conclusion: the Great Cosmic Creator botched the numbers and he, Fred Adams, could do a better job if only he could get his hands on the Creation Machine. Given that neither Adams nor anyone else has access to a Creation Machine, we can at least try to determine by observation just how life-friendly our universe is. The anthropic principle only requires someone to inhabit the universe and observe it; it says nothing about the total population. Did life just squeak home, a marginal happy accident on a handful of planets in a universe that permits life without encouraging it? Or is the universe teeming with life? 132

25. Are We Alone? I often give public lectures on astrobiology with the title: ‘Are we alone in the universe?’ From time to time somebody will raise a hand during the Q&A and announce that they know the answer, because they have met, or have even been abducted by, aliens. I don’t believe them, but I try not to be too dismissive. Occasionally my sense of mischief gets the better of me, though, and if I am asked whether I know anything about secret US government facilities with alien bodies, or crashed flying saucers, I reply by saying that I do know something, but I’m afraid I’m not at liberty to disclose it. That either causes uproar or brings the lecture to a convenient close. Much as I would love to think we are not alone, I accept that there is still no sound scientific evidence for any life whatsoever beyond Earth, let alone intelligent life. The search for alien beings is another of those pendulum stories. Early scientists had little problem with the possibility of extra-terrestrial life, once it became established that the planets were other worlds. The seventeenth-century astronomer Johannes Kepler, for example, believed that Jupiter  –  and even the moon  –  were inhabited. But arguments for or against the existence of extra-terrestrials rested more on the arcana of Christian dogma than scientific inquiry. Interestingly, the person who gave us the word ‘scientist’, William Whewell, master of Trinity College, Cambridge, chose to 133

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reject the notion of alien beings on religious grounds, as being inconsistent with Christian salvation and the uniqueness of the Incarnation. The scientific arguments against life elsewhere in the solar system began to strengthen with better telescopes, although the possibility of life on Mars persisted into the twentieth century (I’ve already mentioned Percival Lowell’s belief in Martians). It took the advent of the space age and a succession of Mars fly-bys, followed by orbiters and landers, to finally put paid to speculation about Martians. While it’s possible that Mars may have hosted primitive life in the far past – and there remains a faint hope it may still do so today –  recent attention has focused on the icy moons of the outer solar system, such as Europa and Enceladus. Beneath their thick ice crusts they have liquid water, which just might provide suitable conditions for biology. However, nobody supposes that these bodies would harbour more than microbial life. In the last decade, astronomers have discovered many extra-solar planets, a fraction of which will surely resemble Earth. Some estimates suggest a billion earth-like planets in the Milky Way alone. A major challenge is to find ‘biosignatures’, detectable from Earth, that might indicate the presence of some form of life on these distant worlds. For example, oxygen in the atmosphere would be a positive (though not definitive) sign, because on Earth oxygen is a product of photosynthesis. While it’s promising to know there are plenty of habitable planets out there, there is a huge difference between habitable and inhabited. For a habitable planet to spawn life, somehow a mish-mash of chemicals has to turn into something living. 134

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During my career, opinion has shifted from deep scepticism that life would readily arise in a chemical soup, to the currently fashionable view that the universe is seething with life. Contrast Francis Crick’s 1973 statement that ‘life seems almost a miracle’, so many are the conditions necessary for it to get going, with Christian de Duve’s 1995 opinion that the emergence of life is ‘a cosmic imperative’. In the absence of hard facts there is plenty of scope for disagreement. At the heart of the problem is an almost complete ignorance of the process that turned non-life into life. Because scientists have only scrappy ideas of what that process was, they are unable to reliably estimate the odds of it happening. The answer could lie anywhere on a spectrum from complete fluke to almost inevitable. If you believe the universe is rigged in favour of life, you would opt for the latter. Just a single data point –  just one instance of a second sample of life – would clinch the matter.

26. Is ET in Our Backyard? When most people refer to extra-terrestrial life, they usually have in mind not microbes but intelligent aliens. The search for extra-terrestrial intelligence –  SETI –  is a small but growing branch of science. It began in earnest in 1960 when a few optimistic astronomers, led by Frank Drake, began sweeping the skies with radio telescopes in the hope of coming across a message from an alien civilization. In my student days, I was a strong fan of SETI – I still am – but in the 1960s the idea of searching for aliens was regarded as decidedly crackpot. One might as well have professed an interest in looking for fairies. The prevailing scientific opinion was well captured in a 1964 article by the distinguished biologist George Simpson, entitled ‘The nonprevalence of humanoids’. The quest for them was utterly futile, wrote Simpson, so heavily are the odds stacked against it. In this climate of scepticism, SETI limped along on a shoestring budget. NASA first supported it, but pulled out in 1993, and the whole project was obliged to rely on private donations, most notably by Paul Allen, co-founder of Microsoft. Then came a dramatic reversal of fortunes. In July 2015 the business magnate Yuri Milner, founder of the Breakthrough Prizes, announced a commitment of a hundred million US dollars to boost SETI research, with more to follow.* * In recent years, NASA has embraced SETI once again as part of its broader astrobiology programme.

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In spite of the extra cash, traditional radio SETI remains hampered by a fundamental scientific obstacle. Our biggest satellite dishes lack the power to pick up an alien broadcast unless it is beamed directly at Earth. But why would an alien civilization deliberately send us radio messages unless they could be sure that our planet hosted a civilization with radio technology? They would of course know this by detecting our own radio transmissions. But these have scarcely penetrated a hundred light years into space, so unless there are aliens extremely close, by astronomical standards, we can’t expect anyone to be purposefully signalling us. Still, we don’t need to pick up a customized radio message to discover that we are not alone in the universe. All it needs is convincing evidence for non-human technology – a ‘technosignature’ to use the buzzword. Earth abounds with technosignatures: ecosystems modified by agriculture, the Great Wall of China, CFCs in the atmosphere, night-time illumination . . . Given the millions of earth-like planets in the galaxy, there are plenty of potential targets to search for such telltale signs. Unfortunately, detection of any technology restricted to a planetary scale is way beyond our current capabilities, so researchers have focused their deliberations on two other avenues: megastructures and probes. An advanced alien civilization could have extended its technological footprint beyond the home planet and modified its astronomical environment in some conspicuous way. An early suggestion, made by the physicist and futurist Freeman Dyson in 1960, is that an energy-hungry supercivilization might build an enclosure round its host star to trap all its light – a solar energy project on a grand scale. Such a ‘Dyson sphere’ would show up as a strange infrared stellar-mass object. 137

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Nothing quite like that has been found, but in 2015 there was a flurry of excitement when Tabetha Boyajian of Georgia State University identified a star – popularly called Tabby’s star – that keeps changing its brightness in erratic ways. Is it being partially eclipsed by some sort of artificial orbiting megastructure? Although graphic artists had a field day with this concept, most astronomers think there is a natural explanation. Another weird object is named after the PolishAustralian astronomer Antoni Przybylski. The spectrum of Przybylski’s star suggests the presence of a host of radioactive elements, including thorium, uranium, plutonium and einsteinium. How they got there remains a mystery. Is this some sort of extra-terrestrial nuclear waste dump? A civilization able to build a megastructure would presumably also have the know-how to send probes to other star systems. So, could there be an alien technosignature in our cosmic backyard? If there were, where should we look? A few years ago, with the help of a resourceful undergraduate, I tested the feasibility of finding an alien artefact on the moon. The lunar surface is relatively unchanging, and a moderate-sized object might remain exposed for many millions of years. The Lunar Reconnaissance Orbiter (LRO) spacecraft operated by NASA, with help from Arizona State University, is photographing the moon to half-metre resolution. The LRO pictures clearly show technosignatures. To date, however, they are all of human origin. But the moon isn’t our only close astronomical companion. There are also a few small asteroids stalking our planet. Physicist James Benford has suggested that these co-orbital rocks would be handy places for a probe to lurk for a few thousand years if it was designed to monitor Earth. (Jim is, 138

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by the way, the twin brother of Greg Benford, of Timescape fame – see p. 78.) For longer-term parking, a good location would be one of the so-called Lagrange points – regions of space where the gravitational forces of the Earth and sun are in balance, thus obviating the need for orbital corrections. If a dormant alien probe were to ‘wake up’ and start communicating, the job of SETI scientists would be easy. However, finding a defunct or dormant probe somewhere in the solar system would be like looking for a needle in a haystack  –  even more so when you consider not only the vastness of space, but the immensity of time. At 4.6 billion years old, the solar system is only about one-third of the age of the universe: there may have been inhabited planets around before Earth even existed. If interstellar missions are feasible, an alien probe could have arrived at any time in our planetary history. When it comes to probes, my guess is that we are more likely to encounter robotic surrogates than biological organisms. On Earth, designed intelligence – usually called artificial intelligence or AI  –  is already penetrating many areas of human activity; it can do mathematics better than we can, and, increasingly, make decisions about complex systems. It is easy to imagine that in a few decades most of the intellectual heavy lifting on Earth will be done by AI that will likely far outstrip the human brain in capability. Besides, machines can be built to go almost anywhere, tolerating conditions well outside the comfort zone of living organisms.  If we ever do detect extra-terrestrial intelligence, it will almost certainly be of this super-advanced post-biological variety. This conclusion carries the depressing implication that whatever intellectual prizes might be stored in an artificial 139

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mega-brain, they would lie well beyond our realm of understanding anyway. Nevertheless, merely to know of the existence of this god-like entity – thereby confirming that we are not alone –  would have enormous implications for our view of ourselves and our place in the universe.

27. Why Am I Living Now? My parents were married in London on the first day of the Blitz, with dog-fights over the church, shrapnel falling and the taxi driver blaring his horn in panic. If one of the Luftwaffe pilots had sneezed, he might have dropped a bomb on the church, instead of the next street over, and I would not be writing these words now. I remember being shocked as a child to learn that if my parents had never met, I wouldn’t exist. Even weirder, if a different sperm had made it ahead of the actual winner, I would be someone else! I began to fret: why am I me ? This existential angst kept me awake at night, until a related question began to bother me even more: why am I living now –  say, 1956 – rather than in 1066, or 2176? Many years later I found that there might actually be a rough-and-ready answer to this conundrum, on a cosmic timescale at least. Let me explain. First, when precisely is ‘now’? In cosmic terms, it is something like year 13,775,248,929 since the beginning of the universe (I just made up the last eight digits for purposes of illustration). So, there is a lot of time in the past when I might have existed, but didn’t. (And quite likely even more time in the future.) Which makes me living now seem even more baffling and special. But hold on, could a sentient being have lived a billion years ago? Ten billion years ago? Maybe, maybe not. In 1936, the physicist Paul Dirac noted something odd 141

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about ‘now’, that is, about the cosmic date corresponding to 1936. The terrestrial year is of course just an arbitrary unit. But physics, noted Dirac, supplies a fundamental unit of time, good for anywhere in the universe. It is the duration it takes for light to cross the nucleus of an atom, which is about a ten-trillionth of a second. Expressed in that atomic unit, the age of the universe is roughly 1040, which is one followed by forty zeros. It occurred to Dirac that he’d come across that enormous number before. It is about the same as the ratio of the electric to the gravitational attraction in a hydrogen atom, the combined force between the proton and its orbiting electron. Was this just a coincidence? Generally speaking, scientists hate coincidences –  or ‘Big Fixes’ –  as explanations, as I have stressed on several occasions. That two huge numbers, arising in totally different contexts, should be even approximately the same, hinted at a deep link of some sort. Now, the thing about the age of the universe is that it’s not a fixed number. In the past, when the universe was younger, that number was smaller. At one hundred years after the big bang, it was only about 1033, which is ten million times smaller than the ratio of forces in the hydrogen atom as measured today, throwing the coincidence out of whack. But what if, conjectured Dirac, gravity was ten million times stronger then (i.e. a hundred years after the big bang) than it is today? The two numbers would then have matched at both epochs. So Dirac proposed that gravity slowly weakens over time in order to keep the two big numbers always in lockstep. I heard Dirac lecture on this topic in Trieste in 1971, at a conference held in his honour. He was clearly still very much enamoured of it. Sadly, he died before the matter was 142

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resolved. In 1976 two spacecraft landed on Mars. Called Viking, they went there to find life. In that, they were unsuccessful, but whenever they sent signals back to mission control, astronomers were able to compute Mars’s orbit to very great precision by exact timing of the radio traffic. If gravity was slowly weakening over time, the orbit should be getting slightly bigger, and the Viking radio signals would have revealed that after a few years. They didn’t. If the strength of gravity is fixed, it seems we are living at an atypical cosmic epoch after all, when the two big numbers just happen to coincide. Isn’t that peculiar? The Princeton astronomer Robert Dicke thought there had to be a scientific explanation, and in 1961 he came up with a clever argument. Why are we living now should be interpreted, he said, as why we are living now. Life is now. There could be no life a hundred years after the Big Bang, for example. Quite apart from anything else, it was far too hot for organic chemistry. More seriously, there was almost no carbon around then. Recall that carbon is the key life-giving element – it puts the organic into organic chemistry. It was made by stars and spewed out into space by stellar explosions (see Chapter 10). Thus, reasoned Dicke, life cannot arise in the universe until at least one generation of stars has lived and died, and disseminated its manufactured carbon around the galaxy. Dicke did a rough calculation to estimate the expected lifetime of a typical star, when expressed in the same atomic units of time that Dirac used. He knew that the answer would depend on both the strength of gravity, which squeezes the star and heats up its core, and the strength of the electromagnetic force, which determines the rate at which heat can flow out of the star. What Dicke found was that the answer 143

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hinges only on the ratio of those two forces, i.e. 1040, the same large number that Dirac puzzled over when contemplating the age of the universe. In other words, the lifetime of a star in atomic units will, simply by reason of basic physics, necessarily be the ratio of the two forces. Dicke also pointed out that life would struggle to survive in the very far future, when all the stars had burned out, because complex life needs sunlight as an energy source. In other words, the epoch of the universe in which carbon-based life can exist is bracketed by the lifetime of the first generation of stars (which lived and died rather quickly about 12 billion years ago), and the dwindling population of stars in the far future – maybe in another hundred billion years. Therefore, the habitable epoch of cosmic history will lie between one and a few stellar lifetimes, a duration that in turn is determined by the ratio of the electric to gravitational forces, 1040. Ergo, no coincidence after all! This line of reasoning is a successful example of the use of the anthropic principle (see p. 125). Dicke’s argument convinces me: I’m not surprised to be living at this cosmic epoch. But what about on the timescale of human history? I might still have found myself living in Shakespeare’s day, or a million years from now in some wonderful utopia-to-be. Why am I around in the twentieth and twenty-first centuries? There’s a chilling answer to that question. Maybe I am living now because most people live about now. It’s a sobering fact that a fair fraction of all humans who have ever lived are currently alive. That’s because the population has grown so explosively; for tens of thousands of years there were only a few million people on the entire planet. Now we have 8 billion and rising. A person chosen at random from the set of 144

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all humans who have ever existed has a reasonable chance of living in the twentieth or early twenty-first century, simply on statistical grounds. However, that doesn’t explain why I am not living in the future. Given exponential population growth, surely the vast majority of humans will find themselves alive in future centuries? These will be the countless billions of our descendants who will populate the planet (maybe even the galaxy) in millennia after millennia to come. Why am I not one among these teeming trillions? Perhaps because they will never exist. Imagine that we could take an overview of all human history. For millennia, Homo sapiens jogged along, gradually growing in numbers. Then about 500 years ago the numbers began to surge, rising sharply to, say, 10 billion by the mid-twenty-first century. After that there is an equally precipitous plunge towards zero. With such a narrow population spike, most people who ever exist will live about now, give or take a few decades. So, my existence (and yours) at this time is explained by this gloomy analysis, which goes by the name of the Doomsday argument. Its somewhat back-to-front rationale is that, since there is no reason to suppose you or I are special, we are probably living when most people are living, which of course implies that few people will live in our future. It is Doom Soon. I hate to end on a depressing note, so I should counter that the same reasoning could be applied to any sentient being. True, humans may be in their heyday, but the universe could be seething with life. There may have been all manner of extra-terrestrials around for billions of years, and many more to come in the future – assuming they aren’t all snuffed out by a cosmic-scale catastrophe. And we can add to that all the super-smart post-biological beings mentioned in the 145

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previous chapter. If you count ‘me’ as a member of this broader class – a typical sentient observer – then even if the present cosmic epoch represents ‘peak sentience’ there could still be thinking, joyful beings of some sort inhabiting the universe for billions of years to come. But probably not for ever, as we shall now see.

28. The Fate of Our Universe On 21 December 2012, I addressed an audience in New Delhi on the subject of the end of the universe. By coincidence, that was the very day that the world was scheduled to end according to the ancient Mayan calendar. In fact, it was just about the right time of day too. I remarked that I wouldn’t waste time discussing the Mayan prediction, on the basis that, if it were correct, we’d all know about it before the end of the lecture. Humans have a peculiar fascination with eschatology – the study of the final state of things  –  particularly apocalyptic endings. The Bible prophesies Armageddon and the four horsemen of the Apocalypse. Norse mythology foretells a period of cold, darkness, devastation and despair followed by Ragnarök, the end-of-world battle to decide the fate of the gods. In the Sermon of the Seven Suns, the Buddha describes a period of accumulating chaos culminating in the Earth being engulfed in fire. Cosmologists are not immune from gloomy theorizing either, as I recounted in Chapter 23, but our timescales tend to be longer – mostly billions of years. Whereas astrologers and mystics might study occult numerology in the Mayan calendar, cosmologists base their predictions on mathematical modelling using gravitational theory. The same equations that describe how the universe evolved into its present state from the big bang can be extrapolated into the far future to determine its likely fate. 147

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I’m talking now just about our very own universe –  the one we know and love – and not a bigger multiverse, which, if it exists, might contain any number of horrid universes, some grotesquely flawed or, worse still, full of unremitting evil, the demise of which would only be a good thing. Before the discovery of dark energy, now thought to dominate the mass of our universe, the choice of fate was binary: either the universe would go on expanding, or it would turn around and collapse. In the latter case, nothing could prevent it from imploding to a zero volume and an infinite density, like the big bang in reverse. This ‘big crunch’ wouldn’t happen any time soon, as the turnaround would take many billions of years. Actually, some cosmologists think the big crunch could really be a big bounce, following which a new phase of expansion and contraction would ensue. Nor does there have to be a single bounce. It’s possible that the universe has been bouncing along for ever in a cyclic manner, each phase of expansion eventually slowing to a halt and then turning into contraction, presaging the next bounce. There are other bouncy cosmological models around, involving higher dimensions of space and various obscure refinements. All these theories come with their own problems of physics, not to mention plausibility. Additional destinies are on offer when dark energy is in play, which has the effect of hastening the long-drawn-out death of the universe. If dark energy remains constant, as Einstein originally posited, the end state is just an everexpanding, featureless vacuum. On the other hand, dark energy might not be simply the energy of empty space but a weird new substance, sometimes called quintessence, with 148

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unusual properties we don’t need to get into here.* Suffice it to say that from the point of view of eschatology, the critical feature is only whether the density of dark energy increases or decreases over time, which in turn affects its power to accelerate the expansion of the universe. It’s possible that dark energy will decline and slowly fade to nothing, or even become negative. In the latter case, the universe could eventually collapse to a big crunch. On the other hand, if dark energy grows with time then a very different cosmic fate lies in store. The rate of expansion will get faster and faster, accelerating without limit, culminating in a so-called ‘big rip’, an alarming state of affairs where space, in effect, comes apart at the seams and ceases to exist. All the above predictions describe a lingering demise, but as we saw in Chapter 23, there is always the threat of sudden, unexpected death from the decay of the quantum vacuum or an outbreak of ravaging spacelessness. Even if the solar system avoids vanishing into a new quantum vacuum or being swallowed up by a marauding bubble of nothing, doom still lurks close to home. When I was a teenager, I told a girl in our road that the sun could explode at any time. It was clear she was somewhat rattled by this dark warning because she often alluded to it. My motivation for raising the subject with her was a pathetic attempt to win her affections with my loveable nerd persona. It failed utterly. She either considered me an incipient sociopath or an insufferable know-all. Even my Buddy Holly glasses failed to * For the curious: it would be some sort of slowly evolving, selfinteracting field with a novel equation of state, including negative pressure.

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impress; a boring owl was the description that got back to me. Anyway, my dire prediction was completely wrong. The sun won’t suddenly blow up. But it won’t stay the same indefinitely either. Over the next few hundred million years it will steadily swell in size and heat output, until the oceans boil. In the fullness of time, Earth may even be engulfed and dragged into a vaporizing death spiral. Stars can burn for billions of years but sooner or later they either explode, leaving a remnant core as a black hole or a neutron star, or swell up and then contract to form a slowly fading white dwarf (see Chapter 13). But the action doesn’t stop there. We’ve already seen how black holes gorge themselves on anything going, shredding passing stars and guzzling the gas, or swallowing other black holes. The supermassive black holes at the centres of galaxies do it on a grand scale. As a result of the undiscriminating ingestion, black holes just go on getting bigger, while the rest of the universe becomes progressively denuded of matter. In spite of their voracious appetite, black holes are not, in fact, the final state. Recall Stephen Hawking’s discovery that black holes generate a faint glow of heat. And it really is faint. A stellar mass black hole has a temperature of a hundredmillionth of a degree above absolute zero, while supermassive black holes (like the one at the centre of the galaxy M87) have an even lower temperature. That’s far cooler than the CMB right now, so the net heat flow is from the cosmos into the black holes. But if the universe continues to expand, the cosmic background temperature will eventually fall below any given black hole temperature. At that point, the black hole will start to lose more energy than it gains, as a result of which its mass will go down, and it will shrink in size. 150

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One of the oddities of this process can be seen on the Westminster Abbey headstone (Figure 13) that displays Hawking’s formula for the temperature of a black hole, denoted by the T on the left-hand side of the equation. On the other side, M stands for the black hole’s mass. You can see that if the black hole loses mass, so M gets smaller, T will get bigger. Unlike, say, a mug of coffee that cools as it emits heat, black holes grow hotter as they radiate, making it a runaway process that speeds up over time. The black hole will shrink faster and faster, finally disappearing completely in a flash of gamma rays plus an assortment of subatomic particles. But black hole radiance is so feeble that the timescale for this denouement is mind-stretching. A solar-mass black hole takes 1067 (that’s 10 million trillion trillion trillion trillion trillion) years to evaporate, while the one in M87 would endure for 1097 (10 trillion trillion trillion trillion trillion trillion trillion trillion) years! Most of the mass released from black hole radiation will end up as photons, some as neutrinos and gravitons, and a tiny fraction in the form of more massive particles like electrons and protons. This wispy soup of particles will become ever more dilute with the continued expansion, as the universe empties, leaving little but a thinning background of photons, neutrinos and gravitons with energies that sink towards zero. Dark energy, assuming it stays positive, accelerates the emptying-out process. The ultimate end state for such a universe is a total void, at 10−28 degrees above absolute zero, that being  –  according to quantum mechanics  –  the irreducible temperature of pure dark energy. Although the idea is currently out of fashion, it remains possible that the universe will eventually collapse to a big crunch. If it does, the consequences for the objects within it 151

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are hardly less rosy. When contraction begins, the CMB temperature starts to rise, eventually exceeding that of any stars that may be left, causing them to explode. Near the crunch point it will become too hot for atoms, then nuclei, to exist. Matter will be squeezed to higher and higher densities as the cosmos enters its death throes. The final state is as uncertain as the initial state of the universe. It could spell the end of spacetime in a singularity, or become a gateway to a new phase of cosmic existence we can only guess at. The various scenarios are summarized in Figure 17. It says something about the heroic nature of the human spirit that, faced with these equally unappealing cosmic destinies, some theoretical physicists have nevertheless tried to

D

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Figure 17. How will the universe end? These schematic graphs show the alternative fates of crunch, rip or eternal expansion (not to scale).

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put a brave face on it by asking whether life, or intelligence, or sentience –  or something –  might cling on for ever if the universe is eternal, or at least go out in a blaze of glory when caught in a crunch or rip. Their analyses are quite technical so I won’t dwell on them here, but the issue, at rock bottom, is whether information processing of some sort can continue unabated in the face of a dying universe. Or if it can’t, whether pseudo-resurrection in the guise of Boltzmann brains (p. 120) is a cause for cheer. It all boils down to resources and thermodynamics, and there is no definitive answer. I doubt that this grim prognosis offers much eschatological solace, but we can at least content ourselves that, in the countless aeons left, there is plenty of time for our descendants to come to terms with it. To finish once again on a more cheerful note, let me point out that there is a possible escape clause from these dismal predictions. Part of the appeal of the steady state theory was its cosmological and biological immortality. The universe could just go on replenishing itself for ever, and life along with it. As ageing galaxies ran out of habitable locales, advanced civilizations could simply decamp to a younger galaxy, given the many billions of years at their disposal to implement the move. Life could pervade the universe in both space and time. Although the steady state theory is now passé, eternal inflation does much the same job, on a grander scale. Now there is the possibility of entire new universes erupting into existence. If ours is heading for Armageddon, could we not take up residence in another? Indeed, could we engineer our own ‘baby universe’ and fix it up with just the right laws to permit life? Or even, as we explored in Chapter 24, make it more congenial? 153

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The idea of baby universes has been taken seriously by many eminent physicists – Stephen Hawking even published a book on the subject. This takes us back to the rarefied field of quantum gravity (see pp. 109–10), where adventurous theorists are wont to play fast and loose with the laws of physics. One idea that has cropped up is that the formation of a baby universe might be triggered by the gravitational collapse of a star, with the infant cosmos located through a sort of umbilical tube or wormhole that opens up inside the resulting black hole. In that case, if you fell into the black hole, it might not spell doom. Although you couldn’t come back out again in our universe you might end up in a baby one, new-born, raring to go and expanding in big bang-like fashion. Leaping into black hole portals to universes new would surely be the ultimate in emigration –  getting out for good, abandoning our entire cosmos to its miserable fate in favour of another with better prospects.

29. Is There a Meaning to It All? ‘The most incomprehensible thing about the universe is that it’s comprehensible.’ Albert Einstein

I once took part in an impassioned television debate about science and religion. At one point, the conversation turned to the philosophy of reductionism, also known as nothingbuttery, which holds that true reality lies with the fundamental physical building blocks of the world, and the great edifice of human achievements and values and culture is, at rock bottom, no more than an illusory embellishment; to maintain otherwise is sentimental twaddle. One of the panellists used a striking illustration to denounce this harsh viewpoint. Am I to suppose, he said, that when I tell my wife I love her, it’s nothing but one meaningless mound of molecules transmitting sound waves to another meaningless mound of molecules? The philosopher A. J. Ayer, an enthusiastic reductionist and prominent atheist, objected strongly to this comment, claiming that he too loved his wife very much, but that the meaning attached to that endearment was entirely a human construct. It is people who create meaning in their lives, he pointed out: it doesn’t descend from on high. ‘But,’ countered another panellist, Hugh Montefiore, the bishop of Birmingham, ‘you’re claiming there is no ultimate meaning.’ 155

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At that point Ayer became exasperated. ‘I don’t know what ultimate meaning means!’ he fumed. And there we have it. Meaning is a concept that enriches human lives. A person can lead a meaningful and rewarding life. But does it make any sense to attribute meaning to nature, or the universe? I’m with Ayer in thinking that the meaning of meaning is fraught with difficulties, so let me come at this question from a different tack. When I was sixteen, I became friendly with a young lady in the same year at school. Because I was studying science and she arts, we never shared a class. The only time we saw one another was in the school library. I remember her sitting opposite me one day as I ploughed through a physics calculation. ‘What are you doing?’ she asked, frowning at my scribbles. ‘Working out the range of a ball thrown up an inclined plane,’ I replied. She thought for a moment, then said, ‘But how can you do that by writing things on bits of paper?’ At the time, I dismissed her question as silly. After all, this was my homework, so it had to make sense! But now I realize that her comment touched on something profound. Scientists and engineers can use abstract mathematics to work out in advance what will happen in the physical world because mathematics, which is a rational construct of the human mind, is also found to align with the deep order of nature. Successful prediction is only one facet of the role of mathematics in describing nature. Another is understanding. Merely describing the world, however accurately, is not at all the same as making sense of it. Physicists often refer to ‘curve fitting’ as simply matching up data with the mathematical function that best fits, without that function having any broader linkage with a law or a deeper set of concepts. 156

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However, science is full of ‘Ah!’ moments – discoveries when things connect and fall into place. Let me illustrate this point. In the 1950s, particle accelerators were producing a host of new subatomic splinters. So many, in fact, that physicists ran out of names for them. There were pions and kaons and sigmas and lambdas, and then just a whole bunch of letters and numbers. The rapidly proliferating list of particles looked bewildering and arbitrary. Then, in 1961, Murray Gell-Mann came up with a mathematical scheme based on a branch of mathematics called group theory to bring some order to the data. All those diverse subatomic entities were, he said, made of smaller particles he dubbed quarks – three inside a proton, two inside a kaon, and so on. He produced neat-looking schematic patterns to show how it all hung together. Gell-Mann was able to predict a hitherto undiscovered particle, which rejoiced in the name of the omega minus, based on the fact that there was a gap in one of his pretty group theory patterns. In 1964, Gell-Mann’s missing particle was found. Suddenly the particle zoo ‘made sense’. And quarks are real: you can detect them jiggling about inside protons. The list of successful predictions in theoretical physics is extensive –  the Higgs boson, antimatter, black holes, gravitational waves  –  they all provide clear examples of things ‘falling into place’, sometimes after decades of experimental searching. It seems to me that if we can extract sense from nature, then there must be something like ‘sense’ in nature. By this I mean that nature is ‘about’ something, an interconnecting rational scheme that for some reason can be grasped by the human mind. It is what I called ‘the key to the universe’ in Chapter 2. Finding that key was by no means 157

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inevitable. For a start, there is no absolute reason for nature to have a straightforward mathematical subtext in the first place. And even if it does, there is no reason why humans should be capable of comprehending it. You couldn’t tell from daily experience that the disparate physical systems making up the natural world are linked, deep down, by a network of coded mathematical relationships. How has this come about? How have human beings become privy to nature’s subtle and elegant scheme? Somehow the universe has engineered, not just its own awareness, but its own comprehension. Mindless, blundering atoms have conspired to spawn beings who are able not merely to watch the show, but to unravel the plot, to engage with the totality of the cosmos and the silent mathematical tune to which it dances. In this book, I have focused on the great questions of existence viewed through the lens of science, which is my own perspective. However, it’s fair to say that the majority of scientists aren’t comfortable trespassing into philosophical questions, some of which seem to stray into theology. Challenged on whether the universe might have some sort of meaning or purpose, most would either say no or, like Ayer, dismiss the question itself as meaningless. One distinguished scientist bold enough to address the topic is Steven Weinberg, who wrote, ‘The more the universe seems comprehensible the more it also seems pointless.’ Weinberg was roundly condemned for deigning to dignify the concept of a universe with a ‘point’, even if only to deny it. A universe that ‘just exists’ for no reason, with specific properties that ‘just are’, is correctly described, in formal logic, as ‘absurd’. But if there is no rational coherent scheme beneath 158

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the surface phenomena of nature, if things ‘just are’, if the universe is absurd, then the success of the scientific enterprise is totally enigmatic. It cannot be pursued with any expectation that the methods adopted hitherto will continue to work, that we will go on uncovering new mechanisms and processes that make sense, for how can sense be rooted in absurdity? Some years ago, I committed these deliberations to an article in the New York Times. The editor chose the by-line ‘Having faith in science’. It provoked a furious backlash from some of my peers, who counsel against anything that blurs the boundary of science and religion, even on topics where their agendas overlap, and even though the word ‘faith’ has many shades of meaning. One of the more polite responses came from the renowned cosmologist and writer Sean Carroll, who expressed the consensus on the dependability of the laws of nature in characteristically eloquent fashion: ‘There is a chain of explanations concerning things that happen in the universe, which ultimately reaches the fundamental laws of nature and stops . . . at the end of the day the laws are what they are . . . that’s okay. I’m happy to take the universe just as we find it.’ Every scientist who opts to work on profound cosmic questions is confronted by this stark choice: either, like Carroll, take the universe for what it is –  an inexplicable brute fact –  and get on with the practical job of doing science, or accept that the entire scientific enterprise rests on a deeper layer of rational order. If the latter is the case, a pressing question is whether science will ever advance to the point where we can fully grasp that deeper layer. That is the biggest of all the big questions discussed in this book.

30. What’s New on the Cosmic Horizon? Historians will look back at the last few centuries as an exceptional time in human history, when explanations of the world transitioned from magic and miracles to mechanisms.  The success of the scientific method in accurately describing so much of nature is astonishing. Yet the ambitious project to explain the universe is a work in progress. There are many loose ends – plenty left for future generations to investigate, contemplate and resolve. Some of this unfinished business has been touched upon in the preceding chapters, like the cold patch in Eridanus. Nobody can say what, if anything, preceded the big bang, whether there is one universe or many, or even how many dimensions of space there may be. Was there really an inflationary phase in the very early universe and, if so, what is the nature of the inflaton field that drove it? The new discrepancy in the value for the Hubble constant (see ‘Hubble wars’, p. 18) rings alarm bells, as do the results of a mammoth survey of dark matter showing huge cosmic voids. The observations suggest that dark matter is not as clumpy as Einstein’s general theory of relativity might predict. In addition, we are in the dark about a vast slice of time, from about 400,000 years until half a billion years after the big bang – appropriately called the cosmic Dark Age. And the speed-up of the expansion rate that took everyone by surprise when it was discovered in the 1990s raises the question of whether the acceleration is constant, or is itself increasing 160

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or decreasing with time. Nothing less than the fate of the universe hangs on it. Some of these outstanding questions will benefit from a range of new experiments. A huge system of coupled radio antennae, called the Square Kilometre Array (SKA), is being built in South Africa and Western Australia and will enable astronomers to penetrate the cosmic Dark Age. The James Webb optical and infrared telescope  –  a successor to the Hubble Space Telescope – should launch soon and will probe the very distant universe in far greater detail. To investigate whether dark energy is changing with time, the European Space Agency is due to launch a special satellite called Euclid in 2022, while NASA is planning a Wide Field Infrared Survey Telescope that should also cast light on the problem. Both will garner precision data over billions of years of cosmic history to determine more accurately how the expansion rate has changed. Gravitational wave detectors will be refined and improved in the coming years, providing an abundance of data on the most violent cosmic events, especially those involving black holes. Because microcosm and macrocosm –  the very large and very small  –  are so intimately linked, unresolved issues in particle and quantum physics also have a bearing on cosmology, such as the crucial asymmetry between matter and antimatter that isn’t completely explained. The nature of dark matter and dark energy remains enigmatic, but it’s possible that any day a dark matter particle detector will get a positive signal. Something unexpected may be found in cosmic rays, or in the large underground neutrino experiments. New particles may yet be created in an upgraded LHC or in one of its mooted successors. 161

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Theoreticians also have a long list of open questions. There is no fully unified theory of the forces of nature, and supersymmetry, which has been part of the theoretical physicists’ armoury for decades, remains unconfirmed. A quantum theory of gravity, crucial for explaining the earliest moments after the big bang, the end state of black hole evaporation and the paradox of missing information, is still not definitive, although string theory remains a popular contender. These difficulties suggest that we still do not possess the complete set of fundamental physical laws. The Standard Model of particle physics is clearly an approximation to something that might turn out to be very different in form. There could even be totally new kinds of laws to be uncovered. On top of these specific technical problems, there are foundational open questions, such as whether quantum physics can be applied to the universe as a whole and what determined the cosmological initial conditions. Another profound question concerns the status of the laws of physics: are they immutable, or can they vary from one cosmic region to another, or one bubble universe to another, or even change with time within our universe? Can the laws of physics themselves actually be explained, or will they for ever lie outside the scope of the scientific method? In the previous chapter I argued that science goes beyond just explaining the physical world; it also provides a basis for understanding it. A fully satisfactory scientific explanation is one that ‘makes sense’ and can be incorporated into a broader rational account of existence. The catalogue of loose ends I listed above indicates that there is much we do not understand about the universe. Sometimes when scientists encounter a deep problem it is solved by a new discovery or by working harder at the theory. But when there is a real dead end, 162

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the reason why we are stuck is often because scientists are thinking about the problem the wrong way. The theory of relativity resolved many troubling issues of physics by abandoning the natural assumption that space and time provide a fixed scaffold in which matter moves about; instead, space and time need to be treated as flexible and dynamic. Quantum mechanics rescued microphysics by relinquishing the intuitive notion that the atomic domain is just a scaled-down version of the everyday world, with electrons and protons behaving like little billiard balls obeying the usual rules of cause and effect. Both relativity and quantum theory succeeded by fundamentally altering the entire conceptual framework of the subject matter. I suspect that some of the problems faced by physicists and cosmologists today will likewise require a radical overhaul of existing concepts. I have explained how the key to the universe came with the realization that the physical world is mirrored by a mathematical world. Mathematics is, of course, a limitless ocean of forms and relationships. The mathematical structures employed in theoretical physics, however, are actually of a very special nature, often selected on grounds of simplicity and even beauty. There is no guarantee that restricting ourselves to this specific limited class will always yield results in physics. To make the latter point explicit, the theories I have discussed in this book –  general relativity, quantum mechanics, electromagnetism, the weak and strong forces, Newtonian mechanics and gravitation –  are all what we might call bottom-up theories. They are cast in terms of what happens at each point of space and time, focusing on the simplest level. Physicists refer to these as ‘local’ laws. Adopting this standard approach, complexity in physical systems is regarded as a secondary or derivative 163

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quality that emerges in some large systems, such as living organisms, and not something to be built into the basic description – an example of reductionism (see p. 94). There is also an assumption that the laws are fixed, immutable and universal. All of these assumptions can be challenged. Let me illustrate a possible alternative way forward. Sometimes our wonderful and much-loved theories lead to bizarre predictions. In Chapter 16 I mentioned how the laws of electromagnetism can describe radio messages going back in time. Is that reason to throw out the whole theory as absurd? No, because it turns out that reversed-time radio waves are not impossible, just very improbable, like shuffling cards back into numerical order. More seriously, I also discussed how general relativity permits time travel into the past, arguably undermining the rational basis of cause and effect. Does that suggest general relativity should be discarded in favour of a theory of space, time and gravitation that is paradoxfree? Similar causality issues arise if singularities can form by gravitational collapse without being trapped inside black holes. If these so-called naked singularities form, or could be engineered, they could emit influences that lie completely outside the ambit of rational inquiry because cause and effect break down at singularities. One way to avoid such unpalatable scenarios is to start with certain overarching principles, as Einstein did when formulating the theory of relativity, or Mach did when trying to attribute the origin of centrifugal force to the structure of the universe. Adopting this ‘top-down’ approach, one might use the opportunity to rule out certain philosophically repugnant or outlandish possibilities, of the sort I touched on in the earlier chapters. Many scientists regard duplicate worlds, 164

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duplicate beings, or Boltzmann brains, as unacceptable. Should we invent a new principle of nature to ban them, and place it alongside the laws of physics, requiring us to rule out any cosmological models that predicted them? Similar ‘no-go theorems’ have been invoked for other unpalatable scenarios, such as perpetual motion, naked spacetime singularities and time travel – recall Hawking’s Chronology Protection Hypothesis. Science has long been informed by the Principle of Sufficient Reason, first articulated by Leibniz, which says there should be reasons for why things are the way they are. Perhaps we need a Principle of Sufficient Reasonableness too? One could use that as a starting point to identify appropriate mathematical structures, rather than setting out with the standard bottom-up theories and hoping they never predict absurdity. The requisite mathematics might involve novel features, such as non-local laws, or laws that vary across space and time or within systems above a certain level of complexity. Perhaps they will have a form that nobody has yet thought of. What it needs is the proverbial ‘next Einstein’ to pull some of these ideas together into a fully worked-out theory. Such is my hope and vision. It could be that the golden age of cosmology has been a lucky streak, a dream run of discoveries and theoretical advances that may not last. After all, there is no guarantee that the human intellect has the ability to complete the glorious programme of explanation and unification I have described in this book. The precious cosmic key we have in our possession may serve to unlock the outermost box containing the secrets of nature, but the inner sanctum may for ever elude us. I fervently believe, however, that, while the scientific method continues to deliver results, humanity must persevere in its quest for the ultimate prize. 165

Index acceleration, 32–5, 90 Adams, Fred, 131–2 alien beings, 133–5 Allen, Paul, 136 alpha decay, 20 Anderson, Carl, 60, 62 Andromeda galaxy, 13, 17, 24n anthropic principle, 125, 132, 144 antigravity, 48, 54, 56–7 antimatter, 61–7 anti-particles, 60–62, 95 Anti-World, 67, 117 artificial intelligence (AI) see designed intelligence astrobiology, 133 astronomy, 5, 13, 15–17, 39, 50–51, 71 asymmetries, 65–7, 83, 86–7 atomic unit of time, 142–4 Atomism, 94 atoms, 94–5 Augustine of Hippo, 112–13 Ayer, A. J., 155–6

Benford, Gregory, Timescape, 78, 81 Benford, James, 138–9 beta decay, 63–4 big bang afterglow, 1, 46, 104, 127–30 big bang theory, 19, 22–3, 25, 29–30, 112 black holes evaporation, 90, 162 formation, 71 gravitational fields of, 41, 75–7, 82, 86 temperature, 89, 90, 150, 151 the ‘Hawking effect,’ 88–92, 150 and spacewarps, 39 supermassive, 86, 127, 150 Boltzmann, Ludwig, 84, 120 Boltzmann brains, 120, 153 Boyajian, Tabetha, 138 Bunch, Timothy, 106–7

‘baby universes’, 153–4 Bell, Jocelyn, 73–4 Bell Laboratory, New Jersey, 22

carbon, 124, 143–4 Carroll, Sean, 159 causal loop paradox, 80–82

167

index Chandrasekhar, Subrahmanyan, 68–71, 76, 86 chemical elements, cosmic abundance, 105 formation of, 49, 50, 105 Chronology Protection Hypothesis, 82, 165 CMB (cosmic microwave background), 18, 21, 22–3, 25–6, 45, 46, 48, 63, 104, 107, 127, 150 COBE (Cosmic Background Explorer) satellite, 1–2, 23, 85, 104, 106, 127 Cold Spot of Eridanus, 127, 129 Comte, Auguste, 49 constants of nature, 132 Contact (film, 1997), 82 Copernicus, 2–3 Copleston, Fr Frederick, 112 cosmic Dark Age, 160–61 cosmic fossils, 104–10 cosmic rays, 61–2, 79 cosmology modern, 2, 16, 22 Ptolemaic, 5 Crab nebula, 70 creation myths, 2 Crick, Francis, 135 curved space, 40–45 Cygnus constellation, 71

dark energy, 54, 56–8, 128–9, 148–9, 151, 161 dark matter, 50–52, 160 Democritus, 94 Descartes, René, 122 designed intelligence, 139 deuterium, 105 Dicke, Robert, 143–4 differential calculus, 7 dimensions, 100–102 Dirac, Paul, 59–61, 141–2 Doomsday argument, 145 Drake, Frank, 136 Duve, Christian de, 135 Dyson, Freeman, 137 E = mc², 55 Eddington, Arthur, 37–9, 69 Einstein, Albert antigravity, 53–4 and black holes, 71 on the comprehensibility of the universe, 155 ‘cosmic religious feeling’, 92 and dark energy, 148 and dark matter, 160 scepticism about big bang, 19 general theory of relativity, 32, 160, 164 gravitational theory, 38–42, 79 and Mach’s principle, 35

168

index model of a static universe, 40–43 and time warps, 73, 81 unifying theory, 97 electromagnetism, 96, 109, 164 electrons, 59–60, 66, 98, 151 Enceladus (moon), 134 entropy, 84–5, 86 Eridanus constellation, 127 eschatology, 147–9, 153 Escher, M. C., 41 Euclid (satellite), 161 Europa (moon), 134 European Space Agency, 161 event horizon, 75, 91–2 Everest, Mount, 73 expanding universe, 17–19 extra-terrestrial life, 133–40 fluxions, theory of, 7 Fraunhoffer, Joseph, 49 Friedman, Alexander, 19, 113 Fulling, Stephen, 91 galaxies, 17, 24n Galileo, 6–7, 12, 31 Galle, Johann Gottfried, 51 gamma rays, 61, 63 Gamow, George, 20, 22, 105 Geiger counter, 62 Gell-Mann, Murray, 157 general relativity, theory of and black holes, 71

and gravity, 38–43, 54 significance of, 163 and time travel, 78–9, 164 and time warps, 81–2 Global Positioning System, 73 gluons, 96–7 Gödel, Kurt, 81–2 gravitational clumping, 86 gravitational collapse, 68–71 gravitational lensing, 39 gravitational theory, 113, 147 gravitational waves, 41, 161 gravitons, 97–8, 109, 151 gravity and black holes, 75 and the expanding universe, 17–19 fundamental force, 96, 97–8 and general relativity, 34, 35 Newton’s discovery of, 7–9 and spacewarps, 37–8 and time, 86–7 and time warps, 73–7 group theory, 157 Guth, Alan, 48 Hartle, James, 114–15 Hawking, Stephen and baby universes, 154 and big bang theory, 22 Chronology Protection Hypothesis, 82

169

index Hawking, Stephen – cont’d. on the COBE satellite big bang photo, 1 the ‘Hawking effect’, 88–92, 150–51 origins of the universe, 113–15 ‘theory of everything’, 92, 93, 98 Westminster Abbey headstone, 89 helium, 49, 105, 124, produced in the big bang, 105 Heraclitus of Ephesus, 94 Herschel, William, 50–51 Hewish, Antony, 73–4 Higgs, Peter, 96 Higgs boson, 96, 98, 157 horizons (in space), 26–7 event horizon, 75, 91–2 Hoyle, Fred, 21–2, 77, 83, 123–4 Hubble, Edwin, 16–19, 44 Hubble Space Telescope, 13, 18, 161 ‘Hubble wars’, 18 hydrogen, 105 hypersphere, 42, 43 infinity, of the universe, 8–9, 117–22 inflation theory, 47–8, 87, 106–7, 117–18, 153

intrinsic spin, 66, 99 Io (Jupiter’s moon), 12–13 James Webb optical and infrared telescope, 161 Jeans, James, 7 Kelvin, William Thomson, 1st Baron (Lord Kelvin), 84–5 Kepler, Johannes, 133 Lagrange points, 139 Large Hadron Collider (LHC), 55, 62, 79, 96, 100 laws of nature, 6, 28–9 laws of physics, 107–10, 115, 121, 162 Leeuwenhoek, Antoine van, 111 Leibniz, Gottfried, 7, 131, 165 Lemaître, Georges, 19, 20 Leucippus, 94 levitation, 53–4 life carbon basis, 124, 132, 144, extraterrestrial, 15, 133–4, 143 origin, 134–5 intelligent (see also SETI), 136–40 life-sustaining universes (see also anthropic principle), 125–6, 131–2 light, speed of, 12–13, 32

170

index light years, 13 Lowell, Percival, 15, 134 Lowell Observatory, 15–16 Loys de Cheseaux, Jean-Philippe, 11 Lunar Reconnaissance Orbiter (LRO), 138 M theory see string theory M87 galaxy, 76, 150 Mach, Ernst, 33–5, 164 Mars, life on, 15, 134, 143 mathematics, 6–8, 156–8, 163, 165 Matrix, The (films), 122 Maxwell, James Clerk, 97 megastructures, 137–8 Mersini-Houghton, Laura, 128 Michell, John, 75 Michelson, Albert, 93 Milky Way, 51, 76, 134 Milner, Yuri, 136 mirror symmetry, 63–4 Montefiore, Hugh, Bishop of Birmingham, 155 motion, and relativity, 31–2 multiverse, 118–22, 124–6 muons, 95, 98n, 102–3 NASA, 23, 136, 138, 161 nebulae, 15–17 negative energy, 91 Neptune, 51

neutrinos, 51–2, 65, 95, 98, 106, 151 neutron stars, 70, 73–4 neutrons, 95 New Horizons spacecraft, 14 Newton, Isaac, 35, 75 bucket experiment, 33 infinite universe model, 8–9, 11–13, 40 law of gravity, 7–8, 34, 50, 56 laws of motion, 7, 97 nuclear forces, 96 Olbers, Heinrich, 11 ‘Olbers’ paradox’, 11–14 Ophiucus constellation, 76 palaeo-cosmology, 107 particle accelerators, 157 particle physics, 95–103, 114, 157 Penrose, Roger, 22, 113 photons, 51–2, 63, 96, 98, 151 Pierre Auger Observatory, 79 Planck (satellite), 107 Planck, Max, 109 Planck length/ time, 109–10, 114 population, world, 144–5 positrons, 60–61, 62 Principle of Sufficient Reason, 165 protons, 95, 151

171

index Przybylski, Antoni, 138 Ptolemy (Claudius Ptolemaeus), 5 quantum fluctuations, 107 quantum gravity, 107, 109–10, 114, 154 quantum mechanics, 54, 56–8, 59, 82, 88–92, 98, 109–10, 114, 163 quantum physics, 162 quantum gravity, 162 quantum vacuum, 90, 91, 106–7, 120, 128–9 quarks, 95, 98, 157 quasars, 22 quintessence, 148–9 radiation, 56–7 red shift, 25–6 reductionism, 94, 155, 164 relativity see general relativity, theory of Rømer, Ole, 12–13 Royal Astronomical Society, 69 Russell, Bertrand, 112 Sakharov, Andrei, 65, 67 Schiaparelli, Giovanni, 15 Schwarzschild, Karl, 71, 88–9, 113 Sciama, Dennis, The Unity of the Universe, 28

SETI (search for extraterrestrial intelligence), 136–40 Simpson, George, 136 singularities (in spacetime), 69, 75, 113, 164 Sirius, 10, 13 Slipher, Vesto, 16–17, 24n Smoot, George, 1 solar eclipse, 37, 38 space curvature, 38, 41, 44–5 flatness of, 44–5 warped, 37–9 spin, of electrons, 66 Square Kilometre Array (SKA), 161 Standard Model of particle physics, 97, 98n, 99, 132, 162 starlight, 10–14 stars composition and life cycle of, 49–50 death of, 89–90, 150 gravitational collapse, 68–71 steady state theory, 21–2, 111, 153 Stonehenge, 53–4 stretching of space, 25–6 string theory, 100–102, 107, 109, 129, 162 supernova, 70

172

index supersymmetry, 100, 162 symmetry (in physical laws), 61–7 Tabby’s star, 138 tau particles, 95, 102–3 technosignatures, 137–8 Tegmark, Max, 119 telescope, Galileo’s use of, 7 ‘theory of everything’, 93–103 thermodynamics, second law of, 84 Thompson, Francis, 35–6 time speed of light, 12–13 and the universe, 112–13 time reversal, 63–5 time travel, 78–82 time warps, 72–7, 79–80, 82 time’s arrow, 83–7 topology, 101–2 torus, 102 universe age of, 18 end of, 147–54 expanding, 17–19, 24–5, 43, 54 flawed, 131–2 and infinity, 8–9, 117–22 and laws of nature, 28–9 and mathematics, 6–7 mechanistic view of, 4–6

origins of, 111–16 self-devouring, 128–30 shape of, 39–45 simulated, 122 unified theory of, 93–103 uniformity (homogeneity) of, 46–7, 87 Unruh, William, 90, 91 Uranus, 50–51 vacuum, quantum see quantum vacuum Viking spacecraft, 143 Voltaire, Candide, 131 W particle, 97, 98 Weinberg, Steven, 158 Wells, H. G. The Time Machine, 80 The War of the Worlds, 15 Westminster Abbey, 59, 60, 89, 90, 151 Whewell, William, 133 white dwarfs, 68, 150 Wide Field Infrared Survey Telescope, 161 WIMPS (Weakly Interacting Massive Particles), 52 Witten, Edward, 129 wormholes, 67, 82 Z particle, 97, 98 Zeno of Elea, 94