The Big Bang
Space, time and everything in them begin expanding from an unimaginably hot, dense state.
- When
- Known to
- ± 20 million years
- Era
- Primordial
The universe begins in a state so hot and so dense that no known physics describes its first instant. What the Big Bang model actually describes is not an explosion into space but the expansion of space itself, carrying everything apart from everything else — which is why it has no centre and no edge. Every other date on this timeline is measured from this moment, fixed by Planck satellite observations of the cosmic microwave background at 13.787 billion years ago, give or take about twenty million.