Annu. Rev. Astron. Astrophys. 1984. 22: 157-184
Copyright © 1984 by . All rights reserved

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1. INTRODUCTION

At issue is one of the most significant questions in astronomy: What is the nature of the beginning of the Universe? While there is at present a widespread opinion that only one view (the "standard hot big bang," or SHBB) gives a viable picture of the physical origin of the Universe, there are alternatives that need consideration in any dispassionate review of models compatible with current evidence.

The concept of an expanding universe evolving from an initial singular state arose from the work (1922-30) of A.A. Friedmann, G. E. Lemaitre, and A.S. Eddington, together with E.P. Hubble's (1929) determination of a linear velocity-distance relation for distant galaxies. With the work of H.P. Robertson and A.G. Walker (1933-35), the spatially homogeneous, isotropic Friedmann-Lemaitre-Robertson-Walker or (FLRW) universes were established as the standard universe models. The steady-state universe (1952) of H. Bondi, T. Gold and F. Hoyle posited continuous creation in an expanding universe without a beginning. However, the discovery (1965) of the cosmic microwave background radiation (CMBR) firmly established the SHBB as the accepted theory, accounting not only for that radiation and the velocity-distance relation, but also for the observed abundances of elements.

In recent times, (a) alternative theories have been proposed for the redshifts observed for galaxies and QSOs; (b) new physical theories (such as the grand unified theories, or GUTs) have resulted in a reappraisal of the effective equation of state of matter in the early Universe; (c) new theories of gravity have been proposed as alternatives to Einstein's general theory of relativity; and (d) consideration has been given to more complex geometries than the FLRW models that underlie the standard picture.

In each case, new possibilities arise in which the origin of the Universe is different from that in the standard picture. It is important to consider such alternatives, particularly as there are a number of problems that are not resolved by the SHBB; without such an examination, the argument for the standard view is clearly incomplete. (One cannot make a rational choice between alternative explanations on the basis of an examination of only one of them.)

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