Cosmological Implications of the 3 K BackgroundAuthor(s): D. W. SciamaSource: Proceedings of the Royal Society of London. Series A, Mathematical and PhysicalSciences, Vol. 368, No. 1732 (Sep. 17, 1979), pp. 17-18Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/79920 .

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Proc. R. Soc. Lond. A 368, 17-18 (1979)

Printed in Great Britain

Cosmological implications of the 3 K background

BY D. W. SCIAMA

Department of Astrophysics, University of Oxford, Oxford, U.K.

Three recent developments involving the 3 K background were described. (i) The measurement by Smoot et al. (I977) of an angular distortion of the back-

ground which could be due to a Doppler effect arising from the Earth's motion relative to the Universe. After correcting for the rotation of the Galaxy they obtain a net motion of 600kms-1 towards galactic longitude 2610, latitude 33?. This motion presumably involves the whole of the Local Group of galaxies and is surprisingly large. It could arise from the action on the Local Group of a nearby supercluster of galaxies. If this explanation is correct it would be interesting to identify the supercluster responsible.

(ii) It has been known for some time that the observed isotropy of the 3 K back- ground (AT/T < 10-3) implies that the Universe itself is also nearly isotropic at the present time (Collins & Hawking I973 a). In particular, this would mean that the vorticity (absolute rotation) and the shear (anisotropy in the expansion rate) are very small. Still more stringent limits on the shear can be derived from the requirement that the abundance of helium formed 100 s after the hot big bang be in agreement with observation (Barrow I976; Olson I978). Moreover, if any non-trivial amount of shear were dissipated in the early stages of the Universe, it would have produced more entropy per baryon than is observed in the 3 K back- ground (Barrow & Matzner I977).

It seems likely, then, that the Universe has always been nearly Robertson- Walker. The reason for this is unknown. Possible explanations involve (a) Mach's principle (Raine I975), (b) the anthropic principle (Collins & Hawking I973b), (c) a boundary condition on the initial singularity related to the second law of thermo- dynamics (Penrose I978, I979) and (d) an ultra-stiff equation of state in the very early Universe (Barrow I978).

(iii) The origin of the 3 K background is not understood, but could be due to the Hawking evaporation of small primordial black holes. This evaporation may be slightly asymmetric between baryons and anti-baryons, and this could account both for the present (presumed) baryon asymmetry of the Universe and the entropy per baryon currently associated with the 3 K background. A recent attempt to construct such a scheme has been made by Toussaint et al. (I979).

It is also possible that the background arises from pair creation associated with the rapidly changing gravitational field in the first 10-43 s after the big bang (Parker I977). It is difficult to define quantum particles in this situation, but one can discuss the quantum dissipation involved by using the fluctuation-dissipation theorem (Candelas & Sciama I977; Sciama I979).

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18 D. W. Sciama (Discussion Meeting)

One can also discuss what happened before 10-43 s, at least in the approximation of classical general relativity. One knows then from the Penrose-Hawking singu- larity theorems, in conjunction with the 3 K background, that there was a singu- larity in our past (Hawking & Ellis i968). Now we have learned from studies of the Hawking radiation process that the expectation value of the energy momentum tensor of a quantum field can be negative in some gravitational situations. If this occurs near the big bang to a significant extent it is possible that the repulsive gravitational field associated with the negative energy may prevent the occurrence of a singularity. It may then be possible to discuss a collapse phase preceding the present expansion phase. The 3 K background itself may have been produced in this collapse phase.

REFERENCES (Sciama)

Barrow, J. D. 1976 Mon. Not. R. astr. Soc. 175, 359. Barrow, J. D. 1978 Nature, Lond. 272, 211. Barrow, J. :D. & Matzner, R. A. 1977 Mon. Not. R. astr. Soc. 181, 719. Candelas, P. & Sciama, D. W. 1977 Phys. Rev. Lett. 38, 1372. Collins, C. B. & Hawking, S. W. 1973a Mon. Not. R. astr. Soc. 162, 307. Collins, C. B. & Hawking, S. W. i973b Astrophys. J. 180, 317. Hawking, S. W. & Ellis, G. F. R. i968 Astrophys. J. 152, 25. Olson, D. W. 1978 Astrophys. J. 219, 777. Parker, L. 1977 In Asymptotic structure of space-time. Penrose, R. 1978 a In Theoretical principles in astrophysics and relativity (ed. N. R. Lebovitz,

W. H. Read & P. 0. Vandervoort), Chicago. Penrose, R. 1979 In General Relativity: an Einstein Centenary survey (ed. S. W. Hawking

& W. Israel). Cambridge University Press. Raine, D. J. 1975 Mon. Not. R. astr. Soc. 171, 507. Sciama, D. W. 1979 In Einstein centenary volume (ed. de Finis). Giunti Barbera. Smoot, G. F., Gorenstein, M. V. & Muller, R. A. 1977 Phys. Rev. Lett. 39, 898. Toussaint, D., Treiman, S., Wilezek, F. & Zee, A. 1978 Phys. Rev. D 19, 1036.

Nrote Added in proof, 20 July 1979. In an important development since the Meeting, grand unified theories of the strong, electromagnetic and weak inter- actions have been invoked to explain the baryon asymmetry of the Universe and the observed 108 photons per baryon in the 3K background. This explanation depends on the violation of baryon number conservation predicted by these theories. In particular the proton would be expected to decay into leptons with a half life of order 1032 years. An experiment to test this prediction is now being set up.

Dimopoulos, S. & Susskind, L. I979 Phys. Lett. 81 B, 416. Ellis, J., Gaillard, M. K. & Nanopoulos, D. V. 1979 Phys. Lett. 80 B, 360. Weinberg, S. 1979 Phys. Rev. Lett. 42, 850. Yoshimura, M. 1979 Phys. Rev. Lett. 42, 746.

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