Big Bang

The Big Bang is the cosmological model of the universe whose primary assertion is that the universe has expanded into its current state from a primordial condition of enormous density and temperature. The term is also used in a narrower sense to describe the fundamental "fireball" that erupted at or close to an initial time-point in the history of our observed spacetime.
Theoretical support for the Big Bang comes from mathematical models, called Friedmann models. These models 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.
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 evidence that observers located anywhere in the universe make similar observations (the Copernican principle), this suggests that space itself is expanding. The next most important observational evidence was the discovery of cosmic microwave background radiation in 1964. This had been predicted as a relic from when hot ionized plasma of the early universe first cooled sufficiently to form neutral hydrogen and allow space to become transparent to light, and its discovery led to general acceptance among physicists that the Big Bang is the best model for the origin and evolution of the universe. A third important line of evidence is the relative proportion of light elements in the universe, which is a close match to predictions for the formation of light elements in the first minutes of the universe, according to Big Bang nucleosynthesis.