Big Bang Theory

When Einstein developed his general theory of relativity he added a term, the cosmological constant, to his equations in order to permit a static universe which was neither expanding nor contracting. (He later came to regret this, calling it one of the worst mistakes he ever made; however, the recent discovery that the expansion of the Universe is actually accelerating has brought Einstein's cosmological constant back into favor). Despite his use of this extra term, however, Russian mathematician Alexander Friedmann (1888–1925) and Belgian astronomer Georges LeMaître (1894–1966) found solutions (in 1922 and 1927, respectively) to Einstein's equations that permitted an expanding universe. After Hubble's 1929 discovery there was a great deal of interest in these models, which could be used to explain the observations.

A big bang seemed an obvious implication of the new data, but steady-state models that avoided the embarrassment of a universe with a definite beginning had adherents for decades after Hubble's measurements were made. One of the more promising models, "constant creation," postulated that new hydrogen atoms form constantly and spontaneously throughout space, out of nothing, providing the material for new galaxies as the older galaxies move apart. On this theory, the Universe has always been expanding, had no beginning, has always looked as it does now, and will always look as it does now. This theory predicted that nearby galaxies will look similar to those far away, but it was found that distant galaxies are in fact different from nearby ones, which agrees with the big bang's claim that the Universe is not in a steady state. It was one of the originators of this steady-state theory, British astronomer Fred Hoyle (1915–2001), who coined the term "big bang," now used to describe the expanding-universe model based on Hubble's observations. Hoyle chose the expression to ridicule the theory, but the name stuck.