The Big Bang
The Big Bang is the prevailing model of how the universe began: about 13.8 billion years ago the cosmos expanded from an extremely hot and dense state and has been cooling and expanding ever since. Three independent lines of evidence converge on it — the expansion of the universe described by Hubble's law, the cosmic microwave background at 2.725 K released about 380,000 years after the beginning, and the predicted abundances of hydrogen, helium and lithium forged in the first minutes. This article follows that evidence chain from Hubble's 1929 observations to the Planck and DESI results, and explains what the theory does — and does not — claim.