The “Hubble tension” remains one of the most stubborn mysteries in modern cosmology, and it centers on a deceptively simple question: how fast is the universe expanding right now?
Astronomers use the Hubble constant to describe today’s expansion rate, but two leading methods keep producing answers that don’t agree. One approach relies on the local distance ladder, measuring distances to relatively nearby galaxies and using those observations to infer a faster expansion rate—an outcome that tends to imply a younger universe. The other approach looks much farther back in time by studying the cosmic microwave background, the faint afterglow of the early universe. That early-universe method points to a slower expansion rate and an older universe. The mismatch between these results is what researchers call the Hubble tension.
To sidestep the expansion-rate debate, a new analysis focused on a different but closely related clue: the age of stars. Instead of trying to calculate how quickly space is stretching today, astronomers examined an existing catalogue of stellar ages to estimate the universe’s age from the bottom up—using some of the Milky Way’s oldest stars as cosmic timekeepers.
The analysis drew on data from the European Space Agency’s Gaia mission, which has transformed astronomy with its ultra-precise measurements of stellar parallax. Parallax is the tiny apparent shift in a star’s position caused by Earth’s movement around the Sun, and it allows scientists to determine distances with exceptional accuracy. With better distances, astronomers can more reliably pin down key stellar properties—and from there, estimate stellar ages.
Researchers worked with a catalogue containing age estimates for more than 200,000 stars in the Milky Way. To make the results as dependable as possible, they narrowed the sample to the oldest stars and prioritized the most reliable data. By focusing on this population, the study aimed to identify a robust “cosmic minimum age” anchored in ancient stars that formed early in the galaxy’s history.
From this deep dive into stellar ages, the most probable age of the universe was placed at about 13.6 billion years. Notably, that figure lines up closely with the age inferred from cosmic microwave background observations—supporting the idea of an older universe consistent with the early-universe picture.
Even so, the finding doesn’t fully close the case on the Hubble tension. Stellar age dating is powerful, but it isn’t free from uncertainty. The results still depend on how well scientists understand stellar evolution models, the techniques used to translate observations into ages, and the precision of chemical composition measurements that influence how stars change over time. Small errors in any of these areas can shift age estimates.
The good news is that this is exactly where future Gaia data releases could make a major impact. As Gaia continues refining its measurements and expanding its dataset, uncertainties in stellar distances and properties should shrink—helping astronomers determine stellar ages, and ultimately the age of the universe, with even higher precision. If that happens, stellar archaeology may become one of the most decisive tools for testing whether the Hubble tension points to new physics—or simply to hidden measurement gaps in how we read the cosmos.
