About how old is the universe?

Answer

This figure comes from measurements of the cosmic microwave background and the universe's expansion.

Context

The age comes from combining several lines of evidence rather than a single measurement. The most precise estimate uses the cosmic microwave background: ESA's Planck mission fitted the standard cosmological model to the background's temperature pattern and obtained roughly 13.8 billion years, with an uncertainty of only a few tens of millions of years. Fitting the same model requires assumptions about the contents of the universe, so the result is model-dependent.

Independent checks point to a similar range. The oldest known stars, found in globular clusters, are dated by stellar-evolution models to roughly 12 to 13 billion years, which must be younger than the universe. The expansion rate gives a rough age too: running Hubble's law backwards, the reciprocal of the Hubble constant is about 13 to 14 billion years, with corrections for how expansion has sped up and slowed over time.

There is an unresolved tension. Measurements of the present expansion rate using nearby supernovae calibrated by Cepheid stars give a higher Hubble constant than the value inferred from the microwave background, and a higher rate would imply a somewhat younger universe. Researchers are still debating whether this reflects measurement error or new physics.

The age is also not the 'age of the Big Bang' in the sense of an explosion at a point. It is the time since the hot, dense early state, measured on a clock for observers moving with the expansion.