[title: Excerpt from "The Big Bang Theory," from SchoolsObservatory.org.uk]

Willem de Sitter was the first to show that the universe must expand. His prediction was improved by Alexander Alexandrovich Friedmann in 1922, and further refined by Arthur Stanley Eddington in 1930. Georges Lemaitre, in 1927, thought about the consequences in a slightly different way. If the universe was alread expanding gravity could slow the expansion, but not necessarily stop it and cause collapse. He realised that an expanding universe would have been smaller yesterday than today and so on, all the way back to a 'day that would not have had a yesterday'. He argued that that instant would have been the moment of creation, and as he was also an abbot of the Roman Catholic church argued that God had created 'a primeval atom' which had grown to become the Universe. Lemaitre's main interest in the primeval atom was as a source of all other atoms, which he imagined taking place by a process of continual fission. Initially Einstein thought that Lemaitre did not understand the physics properly and dismissed the idea, but later after Hubble's discovery of the expansion, Lemaitre gave a lecture at which both Hubble and Einstein were present in which Einstein proclaimed, " [what he had just heard] was the most beautiful and satisfying interpretation I have ever listened to" and admitted that the cosmological constant hadbeen an error.

Not everyone accepted the idea of a moment of creation. Fred Hoyle was unhappy about accepting a God given creation, and as an atheist attempted to develop a theory without such an intervention. This was to become the steady state theory. His principal challenge to Lemaitre's theory was that there was no 'fossil' record. In an attack on the theory he had dismissively referred to "this hot Big Bang" and the name stuck. In the same year as steady state was published, 1948, George Gamow and his student Ralph Alpher, proposed that if the universe was created in a gigantic explosion that the various elements observed today would be produced within the first few minutes after the big bang, when the extremely high temperature and density of the universe would fuse subatomic particles into the chemical elements.

-- from http://www.schoolsobservatory.org.uk/study/sci/cosmo/internal/bigbang.htm