Fortschritte der Physik / Progress of Physics: Band 16, Heft 10 1968 [Reprint 2021 ed.] 9783112500583, 9783112500576

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FORTSCHRITTE DER PHYSIK HERAUSGEGEBEN IM AUFTRAGE D E R PHYSIKALISCHEN GESELLSCHAFT IN D E R DEUTSCHEN DEMOKRATISCHEN R E P U B L I K VON F. KASCHLUHN, A. LÖSCHE, R. RITSCHL UND R. ROMPE

B A N D 16 • H E F T 10 • 1968

A K A D E M I E - V E R L A G

•

B E R L I N

Prof. A. A. S O K O L O W

Elementarteilch en Übersetzung aus d e m Russischen Bearbeiter der deutschen Ausgabe Dr. U L R I C H K U N D T (Wissenschaftliche T a s c h e n b ü c h e r , R e i h e M a t h e m a t i k / P h y s i k ) 2. Auflage 1968. 95 Seiten — 14 Abbildungen — 5 Tabellen — 8° — 4,— M Auf leichtverständliche Weise e r l ä u t e r t der Verfasser die gegenwärtig wichtigen P r o b l e m e der Elementarteilchenphysik: Das Antiteilchenproblem, die innere S t r u k t u r des Nukleons, die N i c h t e r h a l t u n g der P a r i t ä t , die Symmetriegesetze der Wechselwirkungen u n d die Bildung von R e s o n a n z z u s t ä n d e n . E r geht in seiner Darstellung auf die gesicherten Forschungsergebnisse der letzten J a h r e ein u n d v e r m i t t e l t die neuesten wesentlichen E r k e n n t n i s s e der E l e m e n t a r t e i l c h e n p h y s i k . Auf diesem Gebiet w a r bisher keine deutschsprachige L i t e r a t u r erhältlich. Bestellungen

durch eine Buchhandlung

AKADEMIE-VERLAG PERGAMON

•

erbeten

BERLIN

PRESS • OXFORD

VIEWEG & SOHN •

BRAUNSCHWEIG

BEZUGSMÖGLICHKEITEN Sämtliche Veröffentlichungen unseres Verlages sind d u r c h j e d e B u c h h a n d l u n g im I n - u n d Ausland zu beziehen. Falls keine Bezugsmöglichkeit v o r h a n d e n ist, wende m a n sich in der D e u t s c h e n D e m o k r a t i s c h e n R e p u b l i k a n d e n A K A D E M I E - V E R L A G , G m b H , D D R - 1 0 8 Berlin, Leipziger S t r a ß e 3 - 4 in der D e u t s c h e n Bundesrepublik a n K U N S T U N D W I S S E N , E r i c h Bieber, 7 S t u t t g a r t 1, W i l h e l m s t r a ß e 4 - 6 in Österreich a n den G L O B U S - B u c h v e r t r i e b , W i e n I , Salzgries 16 in Nord- u n d S ü d a m e r i k a a n G o r d o n a n d B r e a c h Science Publishers, Inc., 150 F i f t h A v e n u e , N e w Y o r k , N . Y . 11 11 U.S.A. im sozialistischen Ausland a n die B u c h h a n d l u n g e n f ü r f r e m d s p r a c h i g e L i t e r a t u r bzw. d e n zuständigen P o s t z e i t u n g s v e r t r i e b bei W o h n s i t z im übrigen nichtsozialistischen Ausland a n d e n D e u t s c h e n B u c h - E x p o r t u n d - I m p o r t G m b H , D D R - 7 0 1 Leipzig, L e n i n s t r a ß e 16.

Fortschritte der Physik 16, 545—593 (1968)

The Cosmic Blackbody Radiation G . DAUTCOTOT a n d G . W A L L I S

Babelsberg Observatory and Physical-Technical German Academy of Sciences Berlin,

Institute GDR

Contents Introduction

545

A. Observations of the Cosmic Blackbody Radiation 546 1. Radiometer measurements (direct observations) 547 a) The spectrum and the effective temperature 547 550 b) The isotropy of the Cosmic Blackbody Radiation 2. Measurements of the rotational temperature of interstellar molecules (indirect observations) 554 B. Local (galactic and metagalactic) consequences of the Cosmic Blackbody Radiation 1. The galactic and metagalactic interaction scheme 2. Inverse Compton and synchrotron processes 3. The influence of processes with the Cosmic Blackbody Radiation on galactic and metagalactic models 4. The interaction of cosmic ray protons and high-energy photons with the Cosmic Blackbody Radiation

556 556 557

563

C. The cosmological interpretation of the Cosmic Blackbody Radiation 1. General-relativistic thermodynamics 2. Outline of the thermal evolution of the metagalaxy 3. Equilibrium distributions for the primordial gas 4. Non-equilibrium processes 5. Deviations from homogeneity and isotropy 6. Rival explanations of the Cosmic Blackbody Radiation 7. Concluding remarks

564 564 570 573 579 582 586 588

Literature

589

561

Introduction During the last years several important discoveries with great influence on fundamental aspects of astrophysics have been made. Besides t h a t of quasistellar objects it is the detection of a cosmic blackbody radiation field (over the range of frequencies from ~ 1 GHz to ~ 4 0 GHz) corresponding to a temperature between 2 . 5 ° K and 3 ° K with the resulting enormous photon number density of 10 8 to 10 9 times the mean intergalactic particle density of 1 0 - 5 t o 5 • 10~ 7 c m - 3 . 40

Zeitschrift „Fortschritte der Physik", Heft 10

546

G . DAUTCOTJRT a n d G . W A L L I S

The existence of a radiation background assumed to fill the whole observable Universe as a relict of a high temperature state of cosmic matter had been postulated by theoretical cosmologists many years earlier. Other rival explanations in terms of thermalized starlight, free-free-emission from a hot primordial gas or radiation of a large number of weak and unresolved radio sources with uncommon spectra, can probably be ruled out (s. chapter C 6). In the present review we first give a survey of the observations of the cosmic blackbody or background radiation (CBR). We then proceed to a discussion of the many consequences this radiation has in the galaxy and in near intergalactic space in spite of its low mean photon energy (7 • 10~4 eV) and low energy density (0.4 eV/cm3). The energy density of the CBR is of the same order as that of the cosmic rays, of the magnetic fields in the galactic plane or of the turbulent motion of the interstellar gas. The possibility, that this coincidence is not by chance, gave the impetus for several attempts of explanation of the CBR. On the other hand the mere existence of the CBR resolves several astrophysival puzzles which hitherto could not be removed. In a third part the cosmological interpretation of the CBR is reviewed. A. Observations of the Cosmic Blackbody Radiation We find the CBR at the high frequency end of the radio channel of our Earth's atmosphere, placed very well between the nonthermal microwave radiation of our galaxy and the mean stellar radiation. The CBR flux level is quite well detectable by high precision radiometers with absolute calibration. Besides this possibility of

The Cosmic Blackbody Radiation

547

direct measurement, astrophysicists were fortunate in finding a good interstellar molecular thermometer for the CBR, the "CN thermometer". Thus an advance in the state of our knowledge of the spectrum and thereby of the effective temperature was attained as illustrated by fig. 1, which will be discussed in more detail. Immediately after the discovery of this radiation there rised the question of its angular distribution. Small scale variations in this highly isotropic radiation could give some interesting indications on possible anisotropy in the early stage of our universe, or by regarding the component of the angular distribution with a 24 h period, on the motion of our solar system with respect to the distant matter which last scattered this radiation. Besides the observational problem of the exact value of the effective temperature and the detailed spectrum the second mean topic therefore consists in a highly accurate determination of the possible anisotropics of this radiation. 1. R a d i o m e t e r measurements (direct observations) a) The spectrum and the effective temperature The first attempt to find the C B R was initiated by R O L L and W I L K I N S O N (cf. [ J ] ) at a wavelength of 3.2 cm. This choice of wavelength was dictated by considerations concerning atmospheric absorption on the one side and extragalactic emission on the other. While this first attempt was not successful, P E N Z I A S and W I L S O N [2] observed at 7.35 cm wavelength (4080 MHz) an effective zenith noise temperature of about 3.5 °K higher than expected. This excess temperature was found to be, within the first rough limits of errors, isotropic, unpolarized, and free from seasonal variations over a period of 9 months. The antenna used in [2] was a 20-foot horn-reflector. Its gain was obtained by comparison with that of a standard horn using a helicopter-borne source [3]. The radiometer employed a traveling-wave maser and a liquid helium-cooled reference termination. The estimated error in the measured value of the total antenna temperature was 0.3 °K. The contribution to the antenna temperature due to atmospheric absorption was eliminated by variation of the elevation angle. The result, (2.3 ± 0.3) °K, is in good agreement with other published values. The contribution from ohmic losses, (0.8 ± 0.4) °K, has been calculated. The backlobe response to ground radiation was checked to be less than 0.1 °K. The total antenna temperature measured at the zenith was 6.7 °K. A combination of these values yielded a remaining unaccounted for antenna temperature of (3.5 ± 1) °K. This value was later on corrected to (3.1 ± 1) °K (s. [4]). A short time later, R O L L and W I L K I N S O N [ 4 ] succeeded in measuring the flux of the new background radiation at 3.2 cm wavelength, using a Dicke type radiometer. In analogy to the observation at 7.35 cm, the following apparent temperature contributions had to be taken into account: °K radiometer output (antenna pointing at the zenith) 3.03 - 4 . 4 8 2RO radiation of the cold load 6.71 - 6 . 9 8 Tc L asymmetry of the switch 3.51 - 4 . 2 0 Tgw horn losses 1.07 - 1 . 1 1 TBI radiation of the atmosphere Taiu 2.9 - 3 . 2 TCBR. cosmic balckbody radiation (background) 2.76 - 3 . 3 2

40*

548

G. DATJTCOTJBT and G. WALLIS

The last column gives the lowest and highest values obtained during 10 runs made at Princeton during October/November 1965. Averaging the values for TcBB one gets a mean of (3.0 ± 0.5) °K, the value of 0.5 ° K representing estimated limits of systematic error (random errors resulting from the 10 measurements are ± 0 . 0 6 °K only). A third measurement at 20.7 cm wavelength (1407 MHz) by H O W E L L and SHAKESHAFT [5], using a Dicke type radiometer with an electron beam parametric amplifier, yielded a value of (3.3 ± 0.5) °K for the minimum background brightness temperature. In this wavelength range, however, the galactic contribution to the brightness temperature (TGAL) becomes appreciable. The contribution to be expected at X = 20.7 cm had been calculated in a manner which will be described in the following. The results was T G A L = ( 0 . 5 ± 0 . 2 ) °K, so that finally TCBn = ( 2 . 8 ± 0 . 6 ) ° K at X = 20.7 cm. An unpublished measurement of A. A. PENZIAS and R . W. WILSON (Amer. Astron. Soc. Meeting, Los Angeles 1966) at 21 cm wavelength yielded a value of (3.2 ± 1.0) ° K for T C B R . I n view of the importance of further information on the long wavelength spectrum of the C B R H O W E L L and SHAKESHAFT [6] attempted to eliminate its contribution at 49.2 cm and 73.5 cm wavelength, (the frequencies are 610 and 408 MHz respectively), where the radiation from the Galaxy is the dominant component. As shall be discussed later they yielded a value of (3.7 ± 1.2) °K in this range. The experimental method was the same as that in [5] (difference of noise output from an antenna and from a reference termination immersed in liquid helium). B y using optimal horns with scaled dimensions (E-plane slant heights of 6 X) and therefore with identical reception patterns they reached a simplified comparison of the antenna temperatures at both frequencies. On the other hand important observational work has been done to determine the spectrum also in the short wavelength range [7—11]. To identify the C B R as truly blackbody radiation one has to check the spectrum near its maximum, that is in the range of 1 to 2 mm wavelengths, or at even shorter wavelengths to cover also the descending Wien'spart of the spectrum. Since this is not yet possible one can look for the curvature in the spectrum at somewhat longer wavelength. At mm wavelength there is very little emission from the Galaxy or the known extragalactic radio sources. The main limitation in pursuing short wavelength is atmospheric radiation. To reduce this confusing background the measurements [7—70] were made from the high altitude Barcroft-Laboratory of the University of California (Bishop, White Mountains). The main contribution to the atmospheric emission is due to the proximity of the 1.35 mm-Line of water vapor and the 0.5 cm band of oxygen. New techniques were developed to overcome these difficulties. There is at first an observation at 1 . 5 cm wavelength by W E L C H , K E A C H I E , THORNTON and W R I X O N [7], The measurements of STOCKES, PATRIDGE and WILKINSON [ 5 ] at 1 . 5 8 cm and 3 . 2 cm wavelength were done with the same method used by WILKINSON [9] to determine the temperature of the C B R at 8 . 5 6 mm wavelength. Another method was developed by EWING, B U R K E and STAELIN [10] for a measurement at 9 . 2 4 mm wavelength. Finally there are an observation at 8 . 2 mm wavelength done by SALOMONOVICH, STANKEVICH and PUZHANO [ 1 1 ] and a measurement at 3 . 3 mm reported by PARTRIDGE on the Tbilissi Conference 1968. The measurement at 1.5 cm wavelength [7] run in the following manner: The antenna temperature was measured a different zenith angles. A plot of this tempera-

The Cosmic Blackbody Radiation

549

ture against the secant function convolved with the antenna pattern gave a straight line (a separate calibration was made at each angle), showing again t h a t the CBR temperature is isotropic. The intercept at sec z — 0 then should just be the CBR temperature. 33 separate runs yielded an average CBR temperature of (2.0 ± 0.8) °K. W I L K I N S O N and his coworkers [ 5 , 9] developed a new technique for absolute radiometry in the mm- and cm- wavelength range. The optimum horn antennas were designed to give the same beamwidth at all three wavelengths (3.2 cm, 1.58 cm and 8.56 mm). The reference source used was a good absorber immersed in liquid helium. A further improvement in accouracy over older techniques consisted in the possibility to replace the sky radiation by the reference source without moving the radiometer. A large metal reflector which could be tilted was used for this. So the antenna beam could be tipped to various zenith angles, and the atmospheric radiation temperature was obtained also without moving the radiometer. The values for TATM and TCBn found in several runs covered the following ranges : X

^ATM

3.2 cm

5.71-7.48

2.07-3.13

2.69

f+0.161 1-021/

1.58 cm

2.83-4.96

2.50-4.05

2.78

{^on}

8.56 mm

1.32-1.44

2.27-3.13

2.56

{+JJJ}

9.24 mm 8.2 mm

2.88-4.95

[°K]

3.16 ± 0.26 2.9 ± 0 . 7

—

The last two measurements are reported in [10] and [11]. All direct radiometer measurements are summerized in the first part of Table 1 (page 553). Comparison with the galactic radio emission The flux densities and the spectrum of the galactic radio emission have been investigated in a number of observations which were extended and after critical analysis included in two recent papers of A N D R E W [12] and P T J E T O N [ 1 3 ] . Combining all measurements, one gets approximately a straight line spectrum over the range from 10 to 404 MHz or 3 m to 74 cm wavelength. More exactly however, an increasing curvature to higher frequencies is indicated. Putting the power spectrum in the form S, ~ v~the values of the spectral index x increase with frequency: Range [MHz]

oc

13 - 1 0 0 10 - 1 7 8 10 - 4 0 4 18-- 4 0 0 400 408-- 6 1 0 400 - 4 0 8 0

0.38 ± 0.05 0.43 ± 0.03 0.7 0.65 ± 0.15 0.9 0.8 ± 0.1 0.9

[13] [12] [14]

[5] [13]

[«] [15]

550

G . DATJTCOTTRT a n d G . WALLIS

From this some uncertainity arises in the extrapolation of the galactic radio spectrum into the lower decimeter region. The value at 20.7 cm, mentioned above in discussing the measurement of H O W E L L and S H A K E SHAFT, was derived from the brightness at 74 cm with a spectral index