Glossary

Big Bang

The Big Bang is the prevailing model for the origin of the universe, describing how it expanded and cooled from an extremely hot, dense state about 13.8 billion years ago. It is not an explosion in space but an expansion of space itself. The model's successes include the cosmic microwave background, the observed abundances of hydrogen and helium forged in the first minutes, and the large-scale distribution of galaxies.

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