Age of the universe

Measurements of the oldest light in the sky put the age of the universe at about 13.8 billion years. ESA's Planck mission gave 13.82 billion, and NASA's WMAP mission gave 13.77 billion.

Different missions agree to within about a percent, but small differences remain.

Context

The age figure comes from fitting a model of the universe to the cosmic microwave background, the oldest light we can detect. The fit uses the pattern of tiny temperature ripples in that light, together with the known ingredients of the cosmos: ordinary matter, dark matter and dark energy. Once those values are pinned down, the model can be run backward to the moment of the Big Bang.

Planck and WMAP used different instruments and slightly different datasets, which explains why their central values differ by a few tens of millions of years. Each result also comes with an uncertainty, so the two are consistent with each other. Later Planck analyses landed close to 13.8 billion years as well.

One caveat is the so-called Hubble tension. Measurements of the present-day expansion rate from nearby stars and galaxies give a somewhat faster rate than the one inferred from the early-universe light. If the faster rate were used in the same model, the universe would come out a little younger. Researchers are still working out whether this reflects measurement problems or new physics.

The number matters because it sets a timescale for everything else. Stars, galaxies and planets all have to fit inside it, and no object we observe is allowed to be older.