Big Bang

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Physical cosmology
Big Bang · Universe Age of the universe Chronology of the universe
Early universe Inflation · Nucleosynthesis Backgrounds Gravitational wave (GWB) Microwave (CMB) · Neutrino (CNB)
Expansion · Future Hubble's law · Redshift Expansion of the universe FLRW metric · Friedmann equations Lambda-CDM model Future of an expanding universe Ultimate fate of the universe
Components · Structure Components Dark energy · Dark matter Photons · Baryons Structure Shape of the universe Galaxy filament · Galaxy formation Large quasar group Large-scale structure Reionization · Structure formation
Experiments Black Hole Initiative (BHI) BOOMERanG Cosmic Background Explorer (COBE) Dark Energy Survey Planck space observatory Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey ("2dF") Wilkinson Microwave Anisotropy Probe (WMAP)
Scientists Aaronson Alfvén Alpher Copernicus de Sitter Dicke Ehlers Einstein Ellis Friedmann Galileo Gamow Guth Hawking Hubble Huygens Kepler Lemaître Mather Newton Penrose Penzias Rubin Schmidt Smoot Suntzeff Sunyaev Tolman Wilson Zeldovich List of cosmologists
Subject history Discovery of cosmic microwave background radiation History of the Big Bang theory Timeline of cosmological theories
Category Astronomy portal
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The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature.^[1] Various cosmological models based on the Big Bang concept explain a broad range of phenomena,^[2][3][4] including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure. The uniformity of the universe, known as the horizon and flatness problems, is explained through cosmic inflation: a phase of accelerated expansion during the earliest stages. A wide range of empirical evidence strongly favors the Big Bang event, which is now essentially universally accepted.^[5] Detailed measurements of the expansion rate of the universe place the Big Bang singularity at an estimated 13.787±0.02 billion years ago, which is considered the age of the universe.^[6]

Extrapolating this cosmic expansion backward in time using the known laws of physics, the models describe an extraordinarily hot and dense primordial universe. Physics lacks a widely accepted theory that can model the earliest conditions of the Big Bang.^[7] As the universe expanded, it cooled sufficiently to allow the formation of subatomic particles, and later atoms. These primordial elements—mostly hydrogen, with some helium and lithium—then coalesced under the force of gravity aided by dark matter, forming early stars and galaxies. Measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to a concept called dark energy.

The concept of an expanding universe was scientifically originated by the physicist Alexander Friedmann in 1922 with the mathematical derivation of the Friedmann equations.^{[8][9][10][11]} The earliest empirical observation of an expanding universe is known as Hubble's law, published in work by physicist Edwin Hubble in 1929, which discerned that galaxies are moving away from Earth at a rate that accelerates proportionally with distance. Independent of Friedmann's work, and independent of Hubble's observations, physicist Georges Lemaître proposed that the universe emerged from a "primeval atom" in 1931, introducing the modern notion of the Big Bang. In 1964, the CMB was discovered, which convinced many cosmologists that the competing steady-state model of cosmic evolution was falsified, since the Big Bang models predict a uniform background radiation caused by high temperatures and densities in the distant past.^[12]

There remain aspects of the observed universe that are not yet adequately explained by the Big Bang models. These include the unequal abundances of matter and antimatter known as baryon asymmetry, the detailed nature of dark matter surrounding galaxies, and the origin of dark energy.^[13]