How old is the universe?
Scientists have long tried to answer this question, but different measurement methods have produced conflicting results.
Now, researchers from the University of Bologna and the Leibniz Institute for Astrophysics Potsdam (AIP) have taken a new approach.
Instead of focusing on how fast the universe is expanding, they looked at the ages of some of the oldest stars in our galaxy. Their findings suggest the universe is about 13.6 billion years old.
The research, published in Astronomy & Astrophysics, offers a fresh way to think about a long-standing puzzle in cosmology known as the Hubble tension.
The Hubble tension refers to a disagreement between two major methods used to measure the universe’s expansion rate. One method studies nearby objects such as Cepheid variable stars and exploding stars called supernovae. These measurements suggest the universe is expanding faster, which implies it is younger—around 13 billion years old.
Another method examines the cosmic microwave background, the faint radiation left over from the Big Bang. This approach suggests the universe is expanding more slowly and is therefore older—about 14 billion years old.
Because the expansion rate and the age of the universe are closely linked, these different results have puzzled scientists for years.
The new study takes a different perspective. Instead of directly measuring expansion, the researchers asked a simpler question: How old are the oldest stars in our galaxy? The universe must be at least as old as the oldest stars it contains.
To investigate this, the team analyzed a large catalog of stellar ages based on data from the European Space Agency’s Gaia space mission. Gaia has mapped the positions, distances, and brightness of more than a billion stars in the Milky Way with remarkable precision.
Using this data, scientists examined over 200,000 stars and carefully selected about 100 of the oldest stars whose ages could be measured with the greatest reliability. They used a method called the StarHorse code, which combines information about a star’s brightness, distance, and chemical composition to estimate its age.
The result was striking. The most probable age of these ancient stars was about 13.6 billion years.
This age is too old to fit comfortably with the younger universe suggested by Cepheid and supernova measurements. However, it matches well with the older age derived from the cosmic microwave background.
The findings do not fully resolve the Hubble tension yet. Measuring the ages of stars is still difficult, and uncertainties remain. But the study provides an independent way to estimate the age of the universe, which could help scientists better understand the disagreement between different cosmological measurements.
The research also highlights the growing importance of what scientists call “near-field cosmology.” By studying stars in our own galaxy in extraordinary detail, astronomers can learn about the history and evolution of the entire universe.
Future data releases from the Gaia mission are expected to improve the accuracy of stellar age measurements even further. With better data, the oldest stars in the Milky Way could become powerful tools for answering one of the biggest questions in science: just how old our universe really is.
