How do we know the universe is about 13.8 billion years old?

The universe is currently estimated to be about 13.8 billion years old. No one measures that number directly. It comes from running models of the universe backwards, using the expansion rate and the cosmic microwave background, and from checking the result against the ages of the oldest stars. [1] [2]

Galaxies are moving away from us, and the farther away they are, the faster they recede. The Hubble constant measures the present expansion rate. Cosmologists extrapolate it back to the Big Bang. The result depends on the density and composition of the universe. For a flat universe made mostly of matter the age is 2/(3 H0). For a very low-density universe it is 1/H0. Adding a cosmological constant to general relativity can make the inferred age larger still. [3] [2]

Distances to remote galaxies are built up in steps. Supernovae whose brightness is thought to be well understood can be seen at great distances, so they extend the distance scale beyond Hubble's original nearby galaxies. [1]

The cosmic microwave background is light released after about 380,000 years, when the universe had cooled enough for neutral hydrogen atoms to form and light could travel without scattering. NASA's WMAP spacecraft gave an age of 13.77 billion years, plus or minus 0.059 billion. ESA's Planck spacecraft refined it to 13.8 billion years. Both used detailed observations of fluctuations in the background, together with general relativity, to run the clock back to time zero. [2] [1]

Globular clusters are dense collections of close to a million stars that formed at roughly the same time, so they work as cosmic clocks. Massive stars burn out quickly. The oldest clusters contain only stars below 0.7 solar masses, which suggests ages between 11 and 18 billion years. NASA notes that this range is wide because cluster distances are hard to pin down and some details of stellar evolution are not fully understood. The stellar ages are therefore a rougher check than the cosmological methods, and they are consistent with them. [3] [2]

A Harvard-Smithsonian Center for Astrophysics report describes a disagreement between two methods. One uses the cosmic microwave background. The other uses galaxies and supernovae to find the Hubble parameter. Each has narrowed its uncertainty to a few percent, yet they do not agree, and both rest on the same underlying model. The report says this suggests that our understanding may be wrong, perhaps fundamentally. Some researchers propose that gravitational waves from merging black holes or neutron stars could settle it. That method is independent of the other two. A simulation study projected that it could reach 2% precision within five years of the report. The one early gravitational-wave estimate then available spanned 11.9 to 15.7 billion years, which covered both other values. That report is an earlier snapshot, so more recent results may differ. [1]