Age of the universe: how we know it’s 13.8 bn

The universe is about 13.8 billion years old. The most precise figure, from the European Space Agency’s Planck mission in 2018, is 13.787 billion years, give or take 20 million. That number does not come from one measurement but from fitting a model of the expanding universe to the cosmic microwave background, the oldest light there is. Two other clocks, the ages of the oldest stars and the decay of radioactive elements inside them, agree that the universe is at least 12 to 13 billion years old.

To see where the number comes from, we redid the calculation ourselves. It is simpler than it sounds, and it shows why the current argument over how fast the universe is expanding matters for its age.

How do scientists know the age of the universe?

Clock one: run the expansion backwards

Galaxies are moving apart, and the farther away they are the faster they recede. The rate is the Hubble constant, H₀: about 67.4 km per second for every megaparsec of distance, according to Planck. If the universe had always expanded at that rate, you could find its age by dividing distance by speed, which gives a figure called the Hubble time. For 67.4 it is 14.5 billion years.

But the rate has not been constant. For most of cosmic history, the gravity of matter slowed the expansion; for the past few billion years, dark energy has been speeding it up. To get the real age you add up the expansion step by step, using how much matter, radiation and dark energy the universe contains. With Planck’s values (67.4 for H₀, 31.5% matter, the rest almost all dark energy), our calculation gives 13.79 billion years, matching Planck’s published figure. The slowing and the speeding up nearly cancel, which is why the age comes out so close to the simple Hubble time.

Line chart of the age of a flat universe against the Hubble constant from 60 to 80. The age falls steadily from about 15.5 billion years at 60 to about 11.6 at 80. Planck’s value of 67.4 gives 13.8 billion years; the SH0ES value of 73.0 gives 12.7. A shaded band from 12 to 13.5 billion years marks typical ages of the oldest star clusters. — A faster expansion rate means a younger universe. If the true rate were 73 and nothing else changed, the universe would be barely older than its oldest stars. Calculated by CosmosLens.Image: CosmosLens (© CosmosLens)

This is where the Hubble tension bites. Measurements of the nearby universe using Cepheid stars and supernovae, led by the SH0ES team, find a faster rate of about 73. Keep everything else the same and our calculation gives an age of 12.7 billion years. That is uncomfortably close to the ages of the oldest stars, which cannot be older than the universe that made them. In a real model, changing H₀ would shift other quantities too, so this is an illustration rather than a prediction, but it shows why astronomers care which number is right.

Clock two: the oldest stars

Stars age in predictable ways. A star burns hydrogen at a rate set by its mass, so in a cluster of stars born together, the heaviest stars to have run out of fuel mark the cluster’s age. The oldest globular clusters in the Milky Way come out at around 12 to 13 billion years. Individual old stars can be dated too. In 2013 a team led by Howard Bond estimated the star HD 140283 at 14.46 billion years, give or take 0.8 billion, which is older than the universe on its face, but the uncertainty overlaps 13.8 comfortably. This clock does not depend on the expansion at all, and it agrees.

Clock three: radioactive dating

Some very old stars contain uranium and thorium, made in the explosions of even earlier stars. These elements decay at known rates, like carbon dating on a cosmic scale. In 2007 a team led by Anna Frebel measured uranium in the star HE 1523-0901 and found it to be about 13.2 billion years old. Again: younger than the universe, but not by much.

All three clocks also lean on the cosmic microwave background, light released about 380,000 years after the Big Bang, whose pattern of hot and cold spots fixes the amounts of matter and dark energy that the expansion calculation needs.

If the universe is 13.8 billion years old, how is it 93 billion light years across?

Because space has been stretching while the light was on its way. Light from the most distant objects set out more than 13 billion years ago, but the space it crossed has expanded since, so those objects are now much farther away than 13 billion light years. Using the same model, we calculate the radius of the observable universe today at about 46 billion light years, a diameter of about 92 billion.

Take MoM-z14, one of the most distant galaxies confirmed by the James Webb Space Telescope, reported in 2025 at a redshift of 14.44. Our calculation says we see it as it was only about 280 million years after the Big Bang. Its light has been travelling for 13.5 billion years. The galaxy itself, or whatever it has become, is now about 34 billion light years away.

How old will the universe get?

As far as anyone can tell, it has no set end date. If dark energy stays constant, the universe will keep expanding forever, growing colder and emptier as stars burn out. That “if” is now being tested. Results from the DESI survey hint that dark energy may be weakening over time, which would change the long-term forecast; we looked at that evidence in our story on whether dark energy is running out of steam.

Quick revision

Age of the universe: about 13.8 billion years (Planck 2018: 13.787 ± 0.020 billion).
Main method: fit a model of the expanding universe to the cosmic microwave background.
Independent checks: ages of the oldest star clusters (about 12–13 billion years) and uranium dating of old stars (about 13 billion years).
Light from the Big Bang’s afterglow was released about 380,000 years after it began.
The observable universe is about 93 billion light years across because space has expanded while light travelled.

How we worked this out

We integrated the Friedmann equation for a flat universe: the age is the integral of 1/(a·H(a)) over the scale factor a from 0 to 1, where H(a) = H₀√(Ω_ra⁻⁴ + Ω_ma⁻³ + Ω_Λ). We used Planck 2018 values, H₀ = 67.4 km/s/Mpc and Ω_m = 0.315, with radiation (photons and neutrinos) at Ω_rh² = 4.18 × 10⁻⁵ and dark energy making up the rest. Lookback times and distances use the same model. Our results match Planck’s published age to within a few million years. The chart holds Ω_m fixed while varying H₀, which is a simplification.

How old is the universe? 13.8 billion years, and three ways we know