In physical cosmology, the term Big Bang has three related meanings. It refers to the observable facts of the evolution of the universe. It is also a cosmological model in which the universe has been expanding for around 13.7 billion years (13.7 Ga), starting from a tremendously dense and hot state. The term is also used in a narrower sense to describe the fundamental 'fireball' that erupted at or close to time t=0 in the history of the universe.
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. Combined with the assumption that observers located anywhere in the universe would make similar observations (the Copernican principle), this suggests that space itself is expanding. Extrapolation of this expansion back in time yields a state in the distant past in which the universe was in a state of immense density and temperature. This hot, dense state is the key premise of the Big Bang.
Theoretical support for the Big Bang comes from mathematical models, called Friedmann models, which show that a Big Bang is consistent with general relativity and with the cosmological principle, which states that the properties of the universe should be independent of position or orientation.
The theory of Big Bang nucleosynthesis predicts the rates at which various light elements are created in models of the early universe and gives results that are generally consistent with observations. 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 general acceptance among physicists that the Big Bang is the best model for the origin and evolution of the universe.