Like de Sitter, Lemaître, who worked with Hubble in 1924, developed out a simple solution to Einstein’s equations that described a universe in expansion. Hubble’s stunning observation provided the evidence Lemaître was seeking for his theory. In 1933, Lemaître clearly described the expansion of the universe. Projecting back in time, he suggested that the universe had originated as a great “cosmic egg,” expanding outward from a central point. He did not, however, consider whether an explosion actually took place to initiate this expansion. George Gamow further investigated the origin of the universe in 1948. Because the universe is expanding outward, he reasoned, it should be possible to calculate backward in time to its beginning. If all the mass of the universe was compressed into a small volume 10–15 billion years ago, its density and temperature must have been phenomenal. A tremendous explosion would have caused the start of the expansion, left a “halo” of background radiation, and formed the atomic elements that are heavier than the abundant hydrogen and helium. Physicists Ralph A. Alpher and Robert C. Herman established a model to show how such heavier particles could form under these conditions.
Gamow’s theory implied there was a specific beginning and end to the universe. However, a number of other scientists, including Fred Hoyle, Thomas Gold, and Hermann Bondi felt that the theory of expansion required no beginning or end. Their model, called the steady state theory, suggested that matter was being continuously created throughout the universe. As galaxies drifted apart, matter would “condense” to form new ones in the void left behind. For nearly two decades, supporters of the competing theories seemed to be on equal footing.