Abstract
The observational basis for the world models of general relativity is now much more secure than it was ten years ago. The discovery of the microwave background radiation and the high degree of its isotropy indicate that the universe can be adequately described by world models of the Robertson-Walker type. The thermal nature of the microwave background radiation can be naturally explained within the framework of evolutionary world models in which the universe passed through a high density, high temperature phase.
The parameters which describe the dynamics of the expansion of the universe at the present day are not known with much greater precision than they were ten years ago. The most recent determinations of the Hubble constant, H0 similar 50-75 km s−1 Mpc−1, suggest longer timescales, T similar 1·3-2·0 × 1010 years, than have been considered in the past. These estimates are consistent with the timescale of the galaxy derived from studies of stellar evolution in globular clusters and from the isotopic abundances in meteorites (nucleocosmochronology). There is still considerable uncertainty about the value of the deceleration parameter, q0, despite much detailed study. The present evidence suggests that q0 is probably positive.
Several discoveries have led to a much more detailed understanding of the physical evolution of the universe. These include the discovery of objects with large redshifts - the quasistellar objects - and the isotropic x ray background radiation. Counts of radio sources have been made by many observatories and an excess of faint sources is consistently found in comparison with the predictions of all simple uniform world models.
Because of the increased confidence in the hot model and the larger number of real facts about the universe, topics which in the past were questions of pure speculation have become susceptible to detailed quantitative analyses which can be checked against the observations. Topics in which this interaction between theory and observation has been most fruitful include (a) the present state and past history of the intergalactic gas, (b) the helium problem, (c) nucleosynthesis in the primordial fireball, (d) galaxy formation and (e) the singularity problem. In all cases, a significantly improved understanding has been gained of the essential problems to be solved.
The counts of radio sources and the space distribution of quasistellar objects suggest that in the recent past physical conditions in the universe have evolved from a state of `violent activity' at z similar 3 (when the universe was about 20% of its present age) towards a rather more quiescent phase at the present epoch.
Whilst a strong case can now be made for the hot model of the universe, many other theories have been proposed which would drastically change this picture. The status of a number of these hypotheses, including alternative theories of gravitation, noncosmological redshifts, steady state theory and variations of the constants of nature with epoch, are discussed in the light of recent observations. This review was completed in December 1971.