How Old Is the Universe? The Oldest Stars Reveal a Clue
Scientists Used Ancient Stars to Measure the Age of the Universe
One of the most fundamental questions in modern science is surprisingly difficult to answer: How old is the universe? While cosmologists have studied this question for decades, different methods have produced conflicting results. A new study suggests that the answer may lie much closer to home—inside the oldest stars in our own galaxy.
Researchers from the University of Bologna and the Leibniz Institute for Astrophysics Potsdam (AIP), together with collaborators from other institutions, have proposed a new approach to estimating the age of the universe. Instead of focusing only on how fast the universe is expanding, they examined the ages of the oldest known stars in the Milky Way. Their results suggest that the universe is most likely about 13.6 billion years old.
The findings provide an important new perspective in one of the biggest debates in cosmology today, known as the Hubble tension.
The Puzzle of the Universe’s Expansion
At the center of the debate is the Hubble constant, a number that describes how fast the universe is expanding. The expansion of the universe was first discovered in the 1920s, and measuring its rate has since become one of the main goals of cosmology.
However, scientists have encountered a frustrating problem. Different methods used to measure the Hubble constant produce different results.
One method examines relatively nearby objects in the universe, such as Cepheid variable stars and distant supernova explosions. These observations suggest that the universe is expanding faster, which implies a younger universe—around 13 billion years old.
Another method studies the cosmic microwave background, the faint radiation left over from the Big Bang. Observations of this ancient light indicate a slower expansion rate and therefore an older universe—closer to 14 billion years.
These two estimates do not agree, and scientists still do not fully understand why. This disagreement has become known as the Hubble tension.
A Different Way to Approach the Problem
Instead of directly measuring the expansion rate of the universe, the new study approached the problem from a different angle. Researchers asked a simple but powerful question: What if we measure the age of the universe by studying the oldest stars it contains?
The logic is straightforward. The universe cannot be younger than the stars that exist within it. If astronomers can determine the age of the oldest stars with high precision, they can establish a reliable minimum age for the universe itself.
This idea brought together two research fields that rarely collaborate closely: cosmology and stellar archaeology, the study of ancient stars that preserve information about the early universe.
Using the Milky Way as a Cosmic Laboratory
The researchers focused on stars within the Milky Way galaxy, which has become an increasingly valuable laboratory for cosmological research thanks to the European Space Agency’s Gaia mission.
Launched in 2013, Gaia is mapping the positions, distances, and motions of more than a billion stars in the Milky Way. Its data releases provide incredibly precise measurements of stellar properties, including brightness, distance, and chemical composition.
For this study, scientists used data from the third Gaia data release, which offers extremely accurate parallaxes and stellar spectra. These measurements allowed researchers to estimate the ages of a vast number of stars with unprecedented precision.
The team began with a catalog of more than 200,000 stars whose ages had been estimated using detailed stellar models. These models combine information about a star’s brightness, position, and distance to determine how long it has been evolving.
Finding the Oldest Stars
From this large dataset, the researchers carefully selected a much smaller sample of stars that had the most reliable age estimates. Rather than simply choosing the largest number of stars possible, they prioritized data quality and accuracy.
Using a sophisticated analysis tool known as the StarHorse code, they filtered out stars with uncertain measurements and potential contaminants. The final sample included about 100 extremely old stars in the Milky Way.
These stars are considered some of the oldest “fossils” of our galaxy, formed shortly after the universe itself came into existence.
When the researchers calculated their ages, the result was striking. The most probable age of these stars was approximately 13.6 billion years.
What the Result Means
This estimate has important implications for the Hubble tension debate.
If the oldest stars in the Milky Way are about 13.6 billion years old, the universe must be at least that old—likely slightly older. This finding aligns well with the age predicted by observations of the cosmic microwave background, which suggest a universe roughly 13.8 billion years old.
However, it creates a potential problem for the measurements based on Cepheid stars and supernovae, which imply a younger universe. According to those results, the universe might be too young to contain stars that are already 13.6 billion years old—unless certain assumptions in cosmological models are adjusted.
In other words, the new findings add another piece of evidence suggesting that the universe may be older than some measurements indicate.
A Step Toward Solving the Hubble Tension
Although the results are promising, the researchers emphasize that the study does not completely resolve the Hubble tension. Measuring the ages of stars is extremely complex, and uncertainties still remain.
Nevertheless, the study demonstrates the growing power of near-field cosmology—using nearby objects within our own galaxy to answer fundamental questions about the universe.
With future Gaia data releases, scientists expect even more precise measurements of stellar ages. These improvements could allow researchers to refine the timeline of our galaxy and place stronger constraints on the true age of the universe.
Ultimately, the oldest stars in the Milky Way may help solve one of the biggest mysteries in modern cosmology—and reveal just how long the universe has existed.
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