How do we know how old the universe is? Expansion, the cosmic background and stars
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The figure usually quoted for the age of the universe is about 13.8 billion years. It is a remarkable claim: nobody was there to start a clock. Yet the number comes from several independent lines of evidence that broadly agree. This guide explains how cosmologists arrived at it and where the remaining arguments lie.
Clue one: the universe is expanding
In the 1920s, astronomers including Edwin Hubble found that distant galaxies are moving away from us, and that the farther away a galaxy is, the faster it recedes. The theoretical groundwork, including the idea that the universe is expanding, also came from Georges Lemaitre and others.
- This does not mean we are at the centre. Space itself is stretching, so observers in any galaxy would see the same pattern.
- Run the film backwards and everything was once much closer together, hotter and denser. That early state is what is meant by the Big Bang.
- The rate of expansion today is described by a number called the Hubble constant. Roughly speaking, if you know how fast things are separating and how the rate has changed over time, you can work out how long they have been separating.
Clue two: the cosmic microwave background
For the first few hundred thousand years, the universe was a hot, glowing fog of particles that light could not travel freely through. Once it cooled enough, at around 380,000 years old, atoms formed and light could stream away.
- That light is still arriving, stretched by expansion into faint microwaves coming from every direction. It is called the cosmic microwave background.
- Its tiny ripples carry information. Satellites including NASA's WMAP and the European Space Agency's Planck mapped these slight temperature differences in detail.
- Fitting a model to the ripples tells cosmologists how much ordinary matter, dark matter and dark energy the universe contains and how it has expanded. The Planck results gave an age of about 13.8 billion years.
Clue three: the oldest stars
Stars offer an independent check. Astronomers understand quite well how stars burn through their fuel and change over time.
- Globular clusters are dense balls of very old stars. By comparing the brightness and colours of their stars with models of stellar ageing, astronomers can estimate how long ago they formed.
- The oldest stars come out somewhat younger than 13.8 billion years, which is what you would expect: the universe has to be older than anything in it. If stars had turned out older than the universe, something would be badly wrong.
- Radioactive dating of elements in some ancient stars, working like a cosmic version of carbon dating, offers another rough cross-check.
The Hubble tension
The picture is not entirely tidy, which is part of what makes it interesting.
- The early-universe method, using the microwave background and the standard model of cosmology, predicts a particular expansion rate today.
- Direct local measurements, using pulsating stars and exploding stars called supernovae as distance markers, tend to give a noticeably faster rate.
- A faster rate would on its own imply a somewhat younger universe, so the gap matters.
- Astronomers are still debating whether the difference comes from subtle measurement problems or points to something missing from current theory. Observatories including the James Webb Space Telescope are being used to test the distance measurements.
What the number does and does not mean
- It is the time since the hot, dense early state, not a claim about what, if anything, came before.
- It carries uncertainty. The 13.8 billion figure has a small error bar within the standard model, but the Hubble tension is a reminder that the model itself is being tested.
- It does not set the size of the observable universe at 13.8 billion light years. Because space has kept expanding while light travelled, the most distant regions we can see are now much farther away than that.
Want to argue about the Hubble tension or ask how distances are measured? Join in below in the comments.