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

We know the universe's age by rewinding its expansion. Galaxies are moving away from us, and the farther away they are, the faster they recede: this is Hubble's law. If everything is flying apart, you can calculate when it must all have started from the same point. A rough answer is simply the inverse of the expansion rate, called the Hubble time. With today's best models and data from the Planck space probe, the answer is 13.8 billion years.

Astronomers check that number in two independent ways. One reads the oldest light in the universe, the cosmic microwave background, and matches it to a model of the early universe. The other measures distances and speeds of stars and galaxies nearby, rung by rung, like a ladder. The two give slightly different expansion rates, a puzzle still being worked on. And the oldest stars we know come out with ages that fit.

The first attempt went badly wrong. Edwin Hubble's 1929 data gave an expansion rate of 500 kilometres per second per megaparsec, far higher than today's value, because he thought galaxies were much closer than they are. His age for the universe was far too young. And the expansion itself had been published first, in 1927, in French in a little-read journal, by a Belgian priest and astronomer, Georges Lemaître.