The universe is about 13.8 billion years old. That figure comes mainly from measurements of the cosmic microwave background, and it is supported by independent checks on the oldest objects we can see.
Method 1: the cosmic microwave background
The cosmic microwave background is light released about 380,000 years after the Big Bang. The size and pattern of its tiny temperature variations depend on the universe’s ingredients (ordinary matter, dark matter, dark energy) and its expansion history. Fitting these measurements with the standard cosmological model, the European Space Agency’s Planck satellite gave an age of about 13.8 billion years with an uncertainty of only a few tens of millions of years. See the evidence for the Big Bang.
Method 2: the expansion rate
If you know how fast the universe is expanding today, the Hubble constant, and how that rate changed over time, you can work out how long ago everything was together. Roughly, a faster expansion means a younger universe.
This is where the Hubble tension comes in. Measurements based on the early universe give a Hubble constant around 67 to 68 kilometers per second per megaparsec. Measurements using nearby Cepheid stars and supernovae give around 73. The difference is small but persistent and could hint at new physics, or at errors not yet understood. See how astronomers measure distances.
Method 3: the oldest stars
The universe must be older than anything in it:
- Globular clusters: by comparing their stars with models of stellar evolution, astronomers estimate ages of about 12 to 13 billion years, see star clusters.
- White dwarf cooling: white dwarfs cool at a predictable rate, so the coolest ones in a cluster give its age, see white dwarfs.
- Radioactive dating of uranium and thorium in very old stars gives similar ages.
These are all consistent with 13.8 billion years.
Is the number settled?
The age is very well constrained within the standard model. If the Hubble tension turns out to reflect new physics, the figure could shift somewhat, but not dramatically. And the age of the observable universe is not its size: because space has been expanding, the most distant regions we see are now about 46 billion light-years away.
