The Big Bang and the Beginning of Time

In standard cosmology, space expands from a hot, dense early state, and cosmic time tracks the universe’s history from that early epoch. In the classical relativistic description, this time coordinate has a finite past boundary, which is sometimes summarized as time “beginning” with the Big Bang. But the Big Bang is not matter exploding into pre-existing space, and current theory does not reliably describe the very first instant. Whether “before” the Big Bang is meaningful remains open; speculative prehistory and bounce models exist.