How old is the universe, and how do we know?

The standard figure is 13.8 billion years, derived principally from precise measurements of the cosmic microwave background, the faint radiation released about 380,000 years after the Big Bang when the universe first became transparent. The pattern of tiny temperature fluctuations in that radiation, mapped in detail by the WMAP and Planck satellites, encodes the composition and expansion history of the universe, from which the age follows. Independent checks including the ages of the oldest known star clusters and the abundances of light elements produced in the first minutes agree with that number.

The evidence for expansion itself dates to Edwin Hubble's observation in the 1920s that distant galaxies are receding, with more distant ones receding faster, which implies everything was closer together in the past. Running that expansion backward gives a beginning. Crucially, the Big Bang describes the expansion of space itself rather than an explosion into pre-existing space, and it has no center, since every observer everywhere sees galaxies receding from them.

There is an unresolved problem worth knowing about. Measuring the current expansion rate using the cosmic microwave background gives a value meaningfully different from measuring it using nearby distance indicators such as Cepheid variable stars and supernovae. The gap has persisted and tightened as both methods improved, which makes measurement error a less comfortable explanation than it once was, and it may indicate that something is missing from the standard cosmological model. This is the Hubble tension, and it is among the most actively worked problems in cosmology, with the James Webb Space Telescope contributing new measurements.