Simple search of Wiki:
The Big Bang theory, built upon the equations of classical general relativity, indicates a
singularity at the
origin of cosmic time.
The
Big Bang theory is the prevailing
cosmological model for the
universe from the
earliest known periods through its subsequent large-scale evolution.
[1][2][3] The model accounts for the fact that the universe
expanded from a very high density and high temperature state,
[4][5] and offers a comprehensive explanation for a broad range of phenomena, including the abundance of
light elements, the
cosmic microwave background,
large scale structure and
Hubble's Law.
[6] If the known laws of physics are extrapolated beyond where they have been verified, there is a
singularity.
Some estimates place this moment at approximately 13.8 billion years ago, which is thus considered the age of the universe.
[7] After the initial expansion, the universe cooled sufficiently to allow the formation of
subatomic particles, and later simple
atoms. Giant clouds of these primordial elements later coalesced through
gravity to form
stars and
galaxies.
Since
Georges Lemaître first noted, in 1927, that an expanding universe might be traced back in time to an originating single point, scientists have built on his idea of cosmic expansion. While the scientific community was once divided between supporters of two different expanding universe theories, the Big Bang and the
Steady State theory, accumulated
empirical evidence provides strong support for the former.
[8] In 1929, from analysis of galactic
redshifts,
Edwin Hubble concluded that galaxies are drifting apart; this is important observational evidence consistent with the hypothesis of an expanding universe. In 1965, the
cosmic microwave background radiation was discovered, which was crucial evidence in favor of the Big Bang model, since that theory predicted the existence of background radiation throughout the universe before it was discovered. More recently, measurements of the redshifts of supernovae indicate that the
expansion of the universe is accelerating, an observation attributed to
dark energy's existence.
[9] The known
physical laws of nature can be used to calculate the characteristics of the universe in detail back in time to an initial state of extreme
density and
temperature.
[10][11][12]