Cosmology

The analysis of Einstein, de Sitter, Friedmann, Lemaitre, and others showed that the dynamic future of the expanding universe depended on the local density. Simply put, if the density of the universe were sufficiently high, then the gravitational pull of the matter in any given volume on itself would be sufficient to eventually stop the expansion. Within the description given by the General Theory of Relativity, the matter would be said to warp space to such an extent that the space would be called closed. The structure of such a universe would allow the expansion to continue until it filled the interior of a black hole appropriate for the mass of the entire universe at which point it would begin to collapse. A universe with less density would exhibit less space warping and be said to be open and would be able to expand forever.

There exists a value for the density between these extremes where the matter of the universe can just stop the expansion after an infinite time. Such a universe is said to be flat. One of the central questions for observational cosmology continues to be which of these three cases applies to our universe.

George Gamow concerned himself with the early phases of an expanding universe and showed that Lemaitre's primeval atom would have been so hot that it would explode. After World War II, a competing cosmology developed by Hermann Bondi, Thomas Gold, and Fred Hoyle was put forth in order to avoid the ultimate problem with the expanding universe, namely, it must have had an origin. The Steady State Cosmology of Bondi, Gold, and Hoyle suggested that the universe has existed indefinitely and that matter is continuously created so as to replace that carried away by the observed expansion.

This rather sophisticated cosmology replaced the origin problem of the expanding universe by spreading the creation problem out over the entire history of the universe and making it a part of its continuing existence. It is somewhat ironic that the current name for the expanding universe cosmology as expressed by Gamow is derived from the somewhat disparaging name, big bang, given to it by Fred Hoyle during a BBC interview.

Gamow and colleagues noted that a very hot primeval atom should radiate like a blackbody (i.e., a perfect thermal radiator), but that radiation should be extremely red-shifted by the expansion of the universe so that it would appear today like a very cold blackbody. That prediction, made in 1948, would have to wait until 1965 for its confirmation. In that year Arno Penzias and Robert Wilson announced the discovery of microwave radiation which uniformly filled the sky and had a blackbody temperature of about 2.7K (-454.5°F [-270.3°C]).

While Gamow's original prediction had been forgotten, the idea had been re-discovered by Robert Dicke and his colleagues at Princeton University. Subsequent observation of this background radiation showed it to fit all the characteristics required by radiation from the early stages of the big bang. Its discovery spelled the end to the elegant Steady State Cosmology which could not easily accommodate the existence of such radiation.