http://en.wikipedia.org/wiki/Big_bang The Big Bang theory developed from observations of the structure of the universe and from theoretical considerations. In 1912 Vesto Slipher measured the first Doppler shift of a "spiral nebula" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way.[2] Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from Albert Einstein's equations of general relativity,
showing that the universe might be expanding in contrast to the static universe model advocated by Einstein.[3] In 1924, Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Independently deriving Friedmann's equations in 1927, Georges Lemaître, a Belgian Roman Catholic priest, predicted that the recession of the nebulae was due to the expansion of the universe.[4] In 1931 Lemaître went further and suggested that the universe began as a simple "primeval atom", echoing previous speculations about the cosmic egg origin of the universe.[5]
Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100 inch Hooker telescope at Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recession velocity—now known as Hubble's law.[6][7] Lemaître had already shown that this was expected, given the cosmological principle.[8]
During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model,[9] the oscillatory universe (originally suggested by Friedmann, but advocated by Einstein and Richard Tolman)[10] and Fritz Zwicky's tired light hypothesis.[11]
After World War II, two distinct possibilities emerged. One was Fred Hoyle's steady state model, whereby new matter would be created as the universe seemed to expand. In this model, the universe is roughly the same at any point in time.[12] The other was Lemaître's Big Bang theory, advocated and developed by George Gamow, who introduced big bang nucleosynthesis[13] and whose associates, Ralph Alpher and Robert Herman, predicted the cosmic microwave background (CMB).[14] It is an irony that it was Hoyle who coined the name that would come to be applied to Lemaître's theory, referring to it sarcastically as "this big bang idea" during a radio broadcast.[15] For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts, began to favor the latter. The discovery of the cosmic microwave background radiation in 1964[16] secured the Big Bang as the best theory of the origin and evolution of the cosmos. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the universe at earlier and earlier times, and reconciling observations with the basic theory.
Huge strides in Big Bang cosmology have been made since the late 1990s as a result of major advances in telescope technology as well as the analysis of copious data from satellites such as COBE,[17] the Hubble Space Telescope and WMAP.[18] Cosmologists now have fairly precise measurement of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating