In physical cosmology, the Big Bang is the scientific theory that the Universe emerged from an enormously dense and hot state about 13.7 billion years ago. The Big Bang is a consequence of the observed Hubble's law velocities of distant galaxies that when taken together with the cosmological principle imply that space is expanding as per the Friedmann-Lemaître model of general relativity. Extrapolated into the past, these observations show that the Universe has expanded from a primeval state, in which all the matter and energy in the Universe was at an immense temperature and density. Physicists do not widely agree on what happened before this, eventhough general relativity predicts a gravitational singularity.

The term Big Bang is used both in a narrow sense to refer to a point in time when the observed expansion of the Universe (Hubble's law) began-calculated to be 13.7 billion (1.37 × 1010) years ago-and in a more general sense to refer to the prevailing cosmological paradigm explaining the origin and expansion of the Universe, as well as the composition of primordial matter through nucleosynthesis as predicted by the Alpher-Bethe-Gamow theory.

One consequence of the Big Bang is that the conditions of today's Universe are different from the conditions in the past or in the future. From this model, George Gamow in 1948 was able to predict, at least qualitatively, the existence of cosmic microwave background radiation (CMB). The CMB was discovered in the 1960s and further validated the Big Bang theory over its chief rival, the steady state theory.

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