Big Bang
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The Big Bang is a cosmological model of the universe that has become well supported by several independent observations. After Edwin Hubble discovered that galactic distances were generally proportional to their redshifts in 1929, this observation was taken to indicate that the universe is expanding. If the universe is seen to be expanding today, then it must have been smaller, denser, and hotter in the past. This idea has been considered in detail all the way back to extreme densities and temperatures, and the resulting conclusions have been found to conform very closely to what is observed.
Ironically, the term Big Bang was first coined by Fred Hoyle in a derisory statement seeking to belittle the credibility of the theory that he did not believe to be true. However, the discovery of the cosmic microwave background in 1964 was taken as almost undeniable support for the Big Bang.
Analysis of the spectrum of light from distant galaxies reveals a shift towards longer wavelengths proportional to each galaxys distance in a relationship described by Hubbles law, which is taken to indicate that the universe is undergoing a continuous expansion. Furthermore, the cosmic microwave background radiation discovered in 1964 provides strong evidence that due to the expansion, the universe has naturally cooled from an extremely hot, dense initial state. The discovery of the cosmic microwave background led to almost universal acceptance among physicists, astronomers, and astrophysicists that the Big Bang describes the evolution of the universe quite well, at least in its broad outline.
Further evidence supporting the Big Bang model comes from the relative proportion of light elements in the universe. The observed abundances of hydrogen and helium throughout the cosmos closely match the calculated predictions for the formation of these elements from nuclear processes in the rapidly expanding and cooling first minutes of the universe, as logically and quantitatively detailed according to Big Bang nucleosynthesis.
However, there are mysteries of the universe that are not explained by the Big Bang model alone. For example, a region of the universe 12 billion lightyears distant in one direction appears little different than a region 12 billion lightyears distant in the opposite direction. But since the universe is only around 13.7 billion years old, it would appear these regions could never have been causally connected. How, then, can they be so similar? Alan Guths 1981 theory of cosmic inflation, a short, sudden burst of extreme exponential expansion in the very early universe, provided an explanation for this horizon problem and several of the features unaccounted for by the original Big Bang model. The successor to Guths original theory has found some circumstantial support, but it is not yet nearly as well supported as the Big Bang model.
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.Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from Albert Einsteins equations of general relativity, showing that the universe might be expanding in contrast to the static universe model advocated by Einstein.[4] In 1924, Edwin Hubbles measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Independently deriving Friedmanns equations in 1927, Georges LemaД®tre, a Belgian physicist and Roman Catholic priest, predicted that the recession of the nebulae was due to the expansion of the universe.
In 1931 LemaД®tre went further and suggested that the evident expansion in forward time required that the universe contracted backwards in time, and would continue to do so until it