Big Bang

The Big Bang is a physical theory that explains how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept define a wide range of phenomena, comprising of the predominance of light elements, the cosmic microwave background (CMB) radiation, and tremendous-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 broad range of empirical evidence strongly favors the Big Bang event, which is now immensely accepted. Extrapolating this inordinate expansion backward in time using the known laws of physics, the models describe an extraordinarily hot and dense primordial universe. Physics lacks a broadly accepted theory that can model the earliest conditions of the Big Bang. As the universe expanded, it cooled sufficiently to allow the formation of subatomic particles, and later atoms. These primordial elements—predominantly 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. There maintain aspects of the observed universe that are not yet sufficiently described by the Big Bang models. These contain 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.