n. (Astron.) The explosive event marking the beginning of the known universe, according to big bang theory; the beginning of time. The big bang is likened to a massive explosion from a point containing all of the matter and energy of the beginning universe, which started the expansion and evolution of the universe which continues today. The evidence for such an event includes the apparently expanding size of the present universe, and a background microwave radiation of about 3° Kelvin, equal to that predicted from mathematical models of such an explosive event. [ PJC ]

n. (Astronomy, Cosmology) The theory that the known universe originated in an explosive event (the big bang) in which all of the matter and energy of the universe was contained in a single point and began to rapidly expand and evolve, starting as high-energy particles and radiation, and, as it cooled over time, evolving into ordinary subatomic particles, atoms, and then stars and galaxies. According to this theory, the four-dimensional space-time continuum which we perceive as our universe continues to expand to the present time, but it is unknown whether the expansion will continue indefinitely or eventually stop or even reverse, possibly leading to a contraction to a single point sometimes referred to as the “big crunch”. The competing “Steady-state Theory” gradually lost favor in the 1980's and 1990's. See also big bang. [ PJC ]