In the heart of an old city stands a clock tower whose hands have never been adjusted since it was built. By how far it runs slow, you can calculate how much time has stretched since then. In space, the role of such towers is played by relic galaxies — dense stellar systems that formed in the first few billion years and have since neither merged with neighbors nor given birth to new stars. Their light is an imprint of age, frozen in their spectra.

Astronomers selected 189 such objects and measured for each the spectral index Dn4000—a quantity that changes as stars age: it is lower for young stars and higher for old ones. By tracking how this index varies from galaxy to galaxy at different distances, one can infer how fast the Universe was expanding when the light left those systems. This is the cosmic chronometer method: instead of assuming the composition of the Universe, we listen to the 'ticking' of aging stars.

The resulting estimate of the expansion rate is 85±53 kilometers per second per megaparsec. The statistical uncertainty is still large — due to the small number of suitable galaxies. But the systematic error is only 13%—one of the lowest among all cosmic chronometer measurements. Previously, it was unclear whether these 'fossils' could be trusted; now it is clear that their simplicity is an advantage.

With the advent of next-generation surveys such as Euclid and Rubin, the number of confirmed relic galaxies will increase by orders of magnitude. Then it will become possible to independently test the main puzzle of cosmology — the discrepancy between different ways of measuring the expansion rate. The clocks that have not been adjusted for 10 billion years will finally show the precise time of the Universe.