Big Bang

Summary

The Big Bang is the hot, dense initial state from which the universe has been expanding and cooling. In the Friedmann–Lemaître–Robertson–Walker models it appears as a past time at which the scale factor tends to zero. There the density and the curvature blow up, so it is an initial gravitational singularity. The Big Bang model explains the expansion of the universe, the cosmic microwave background and the abundances of the light elements. The singularity itself marks the limit of classical general relativity rather than an observed event.