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The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature . [1] Various cosmological models of the Big Bang explain the evolution of the observable universe from the earliest known periods through its subsequent large-scale form. [2] [3] [4] These models offer a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements , the cosmic microwave background (CMB) radiation , and large-s
Crucially, these models are compatible with the Hubble–Lemaître law —the observation that the farther away a galaxy is, the faster it is moving away from Earth. Extrapolating this cosmic expansion backwards in time using the known laws of physics , the models describe an increasingly concentrated cosmos preceded by a singularity in which space and time lose meaning (typically named " the Big Bang singularity "). [5] In 1964 the CMB was discovered, which convinced many cosmologists that the competing steady-
There remain aspects of the observed universe that are not yet adequately explained by the Big Bang models. After its initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles , and later atoms . The unequal abundances of matter and antimatter that allowed this to occur is an unexplained effect known as baryon asymmetry . These primordial elements—mostly hydrogen , with some helium and lithium —later coalesced through gravity , forming early stars and galaxies. Astron
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