How We Know the Age of the Universe

For the first few hundred thousand years, the Universe was hot and opaque. When it cooled enough for light to travel freely, that burst spread in every direction and has been stretching along with space ever since. Today, it appears as faint radiation across the entire sky.

It contains tiny ripples: patches that are slightly warmer or cooler. The size of these patches depends on how much matter the Universe contains, what it is made of, and how quickly everything was expanding. Plug the measured pattern into the equations, and you get a set of parameters, including the age.

The second method is simpler in principle: measure how quickly galaxies are moving away from us, then rewind the clock. The difficulty is that the speed has changed, so you also need a model of the Universe's composition. The measurements are made directly, using supernova explosions in distant galaxies.

The third method does not depend on cosmology at all. For the oldest star clusters, you can estimate their age by looking at which stars have already left the main sequence. This gives a lower limit: the Universe cannot be younger than its oldest stars.

It is the agreement between these three independent estimates that makes the result reliable. An error in one method would not be reproduced by the other two, which work in completely different ways.

The expansion rate measured from ancient radiation and the same quantity measured from nearby galaxies differ by roughly a few percent. The discrepancy is persistent and cannot be explained by measurement errors, so it is called a tension.

It has little effect on the age itself, but it means something is missing from the model. The possibilities under discussion range from unaccounted-for systematic errors to new physics. For now, the question remains open, and that is a normal working state for science.