The Big Bang Theory

In physical cosmology, the Big Bang is the scientific "theory" that the universe emerged from a tremendously dense and hot state about 13.7 billion years ago.

Observational evidence for the Big Bang includes the analysis of the spectrum of light from galaxies which reveal a shift towards longer wavelengths proportional to each galaxy's distance in a relationship described by Hubble's law. If this observation that galaxies are moving away from us is not at any special position in the universe (the Copernican principle), then this suggests that space itself is expanding. Extrapolation of this expansion back in time yields an initial state in the distant past in which all matter and energy was at an immense temperature and density. This initial state is the key premise of Big Bang theory.

Theoretical support for the Big Bang comes from mathematical models, called Robertson-Walker models, which show that a Big Bang is consistent with general relativity and with the principle that properties of the universe should be independent of position or orientation (the cosmological argument).

The Big Bang model predicts the rates at which various elements are synthesized in the early universe, as originally suggested by Alpher, Bethe, and Gamow. Modern (and more refined) calculations of this process, known as Big Bang nucleosynthesis (BBN), produce results that are consistent with observations of the present universe.

The Big Bang model also predicts the cosmic microwave background radiation (CMB), a background of weak microwave radiation filling the whole universe. The discovery of the CMB in 1964 led to the general acceptance amongst physicists of the Big Bang as the best theory of the origin and evolution of the cosmos.

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