By 1950, there were two rival interpretations of Bubble’s observations : the BIG BANG and the STEADY STATE theories. Although we shall have more to say about theories of the Universe later on, it is appropriate to mention this famous controversy at this point since it was instrumental in motivating many important observations bearing on cosmology. Both theories accepted the simple view that the Universe, in the large, is homogeneous and isotropic, and expanding against the pull of gravity. Where they differed was that whereas the Big Bang theory took the conservative view that matter is neither created nor destroyed spontaneously, the Steady State theory did not. A direct consequence of the Big Bang hypo¬thesis is that, at a finite time in our past (roughly 20 billion years 1 ago as calculated on the basis of current data), all the matter hi the Universe was piled up in a state of infinite density – the COSMIC SINGULARITY. On largely philosophical grounds, this singularity was regarded by some as a serious defect of the theory, and an ingenious way out was suggested by Herman Bondi, Tommy Gold and Fred Hoyle. It was proposed that matter might be spontaneously created at just the rate required to make up for the decrease of density resulting from the cosmic expansion. The density of the Universe thus remains constant and there is no singular event of the kind encountered in the Big Bang theory; such a Universe has an infinite past and infinite future.

One of the strongest protagonists of the Big Bang theory was George Gamow. Gamow saw the early Universe not only as a dense place, but also a hot one where nuclear reactions might take place. At that time, Gamow’s hope had been to synthesize all the elements in their cosmically-observed amounts. Although this hope was not to be fulfilled, Gamow’s theory nevertheless resulted in two important predictions. It was possible to predict firstly that there should be a universal cosmic abundance of helium of about 25 per cent by mass. Secondly, that we should now observe the left-over radiation from this early hot phase as an isotropic radiation field having a black-body spectrum corresponding to a temperature of a few degrees absolute. The first of these has since been well verified; nowhere has an abundance of helium which is low compared with the cosmic value been observed. Moreover, astrophysicists find it difficult to understand under what circumstances this helium might have been produced if not in a hot Big Bang. The second prediction has led to the most important discovery in cosmology since Hubble’s discovery of the expansion of the Universe. In 1965, Arno Penzias and Robert Wilson discovered an isotropic radiation field whose intensity corresponded closely to Gamow’s prediction. Ten years of subsequent observations have strongly confirmed the isotropy of the radiation and have shown, in addition, that the spectrum of the radiation is thermal with a temperature of 2.7K. The remarkable isotropy of the radiation convinces astrophysicists of its cosmic origin. The shape of the spectrum tells us that the radiation was once in thermodynamic equilibrium with matter, from which we conclude that the Universe must have been considerably denser and hotter in the past than now.

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