The Oldest Stars in the Galaxy Just Weighed In on One of Cosmology's Biggest Arguments
How do you check the universe's age without a birth certificate? One clever answer is to find its oldest surviving residents and see how old they've had time to become. A team led by Indranil Banik at the University of Portsmouth has done exactly that, combing through the ages of nearly a quarter of a million stars scattered across the Milky Way. The result lands squarely in the middle of one of modern cosmology's most heated disputes — and delivers a verdict that some theorists may find uncomfortable.
The study draws on a remarkable catalogue of 247,103 Milky Way stars, each analysed using high-resolution spectroscopy from the LAMOST telescope at Xinglong Station in Hebei Province, China, alongside extraordinarily precise distance and motion measurements from the European Space Agency's Gaia satellite. Together, these two instruments form a powerful tandem: LAMOST dissects the chemical fingerprints embedded in starlight, while Gaia maps the cosmos with an accuracy that would have seemed miraculous to astronomers even a generation ago.
Reading the Ages of Ancient Stars
Stellar ages were determined using detailed stellar evolution models — sophisticated computer simulations describing how stars change over billions of years as they burn through their nuclear fuel. In this sense, astronomers read a star's age much like a geologist reads rock strata: the composition, luminosity, and temperature of a star encode its entire life history, if you know the language. Because ageing stars change most rapidly in their final stages of life — as they swell into subgiants and red giants — the team strategically focused on stars approaching the end of their main-sequence lifespans, where age estimates are most precise and reliable.
Getting a trustworthy answer also meant being ruthless about data quality. The team applied a rigorous series of cross-checks, insisting that any genuinely ancient star also be metal-poor — that is, containing very low abundances of elements heavier than hydrogen and helium — and enriched in particular chemical signatures, exactly what stellar nucleosynthesis theory predicts for objects born in the universe's earliest chapters. In the cosmic lexicon, "metals" refer to all elements beyond hydrogen and helium, and ancient stars formed before successive generations of stars had time to forge and scatter heavier elements across the galaxy. The team also cross-checked their results against a completely independent set of stellar ages calculated using Gaia data alone. After all that careful filtering, a final, high-confidence sample of 155,600 stars remained — one of the largest and most rigorously vetted datasets of ancient stars ever assembled.
"The oldest stars in our galaxy serve as cosmic fossils — living records of the universe's earliest epoch that no telescope pointed at the distant cosmos can replicate."
A Universe 13.8 Billion Years Old
The headline result is a figure for the oldest star in the sample of roughly 13.73 billion years, with only a narrow margin of uncertainty on either side. Adding a small but physically motivated allowance for the time it likely took the universe's very first long-lived stars to form after the Big Bang — a period of perhaps 50 to 100 million years known as the cosmic dawn — points to a total cosmic age of approximately 13.8 billion years. This figure comfortably matches the independent estimate derived from the cosmic microwave background (CMB), the faint afterglow of the Big Bang itself, as measured with extraordinary precision by missions such as ESA's Planck satellite and NASA's WMAP mission.
The CMB encodes a snapshot of the universe when it was only about 380,000 years old — the moment when the cosmos cooled enough for atoms to form and light to travel freely for the first time. By analysing the subtle temperature fluctuations in this ancient light using the ΛCDM model (Lambda Cold Dark Matter), cosmologists have long pegged the universe's age at 13.787 ± 0.020 billion years. The new stellar archaeology result is a striking, fully independent confirmation of that figure from an entirely different domain of astrophysics.
The Hubble Tension: Cosmology's Most Stubborn Dispute
To appreciate why this result matters so profoundly, it is necessary to understand the Hubble tension — a persistent and increasingly worrying disagreement that has occupied cosmologists for well over a decade. The Hubble constant (H₀) describes the rate at which the universe is expanding today, expressed in kilometres per second per megaparsec. Two fundamentally different methods of measuring this quantity keep returning stubbornly different answers.
- Early-universe methods — anchored to the CMB and large-scale structure of the cosmos — consistently return a Hubble constant of approximately 67–68 km/s/Mpc.
- Late-universe methods — based on the cosmic distance ladder using Cepheid variable stars, Type Ia supernovae, and other local distance indicators — persistently yield values closer to 72–74 km/s/Mpc.
- The discrepancy between these two camps now stands at a statistical significance of roughly 5 sigma, far exceeding the threshold typically required to claim a genuine inconsistency in physics.
- Neither observational errors nor straightforward systematic biases appear sufficient to fully explain the gap, suggesting that something fundamental may be missing from our understanding of the cosmos.
Among the most actively discussed proposed solutions are models invoking Early Dark Energy (EDE) — a hypothetical form of energy density that briefly dominated the universe in its first few hundred thousand years, subtly altering the sound horizon imprinted in the CMB and effectively inflating the inferred Hubble constant. Such models are mathematically elegant and observationally testable, but they carry a significant physical consequence: they predict a noticeably younger universe, with some variants placing cosmic age as low as 12.9 billion years.
Stars That Shouldn't Exist — But Do
This is precisely where the new stellar chronology becomes so consequential. If Early Dark Energy or similar early-universe modifications were correct, and the universe is only around 12.9 billion years old, then the stars identified in this study simply should not exist. Ancient stars aged at 13.73 billion years would be older than the universe itself — a physical impossibility as glaring as finding a grandmother younger than her grandchild.
Instead, the ancient stars of the Milky Way agree emphatically with the standard cosmological picture. Their existence casts serious doubt on proposed solutions to the Hubble tension that invoke exotic new physics in the universe's earliest moments, while leaving the underlying mismatch itself stubbornly unresolved. The findings do not solve the Hubble tension — that puzzle endures — but they meaningfully narrow the solution space, ruling out an entire class of otherwise promising theoretical fixes.
The stars agree with the standard picture, casting real doubt on early-physics explanations for the Hubble tension, while leaving the underlying mismatch itself unresolved. It is a striking reminder that some of the universe's oldest inhabitants can still settle arguments among the astronomers studying them.
The Power of Stellar Archaeology
What makes this study particularly compelling is its methodological independence. Rather than probing the distant universe with telescopes pointed billions of light-years away, it turns inward — to the ancient stellar populations orbiting quietly in our own galaxy — and asks them to serve as cosmic timekeepers. Stellar chronometry, the art of determining stellar ages from spectroscopic and photometric observations, has advanced enormously in recent years thanks to missions like Gaia and large-scale spectroscopic surveys such as SDSS and LAMOST, enabling astronomers to build the kind of statistically robust sample that was unthinkable even a decade ago.
The convergence of stellar ages with CMB-derived cosmic chronology also reinforces the internal consistency of the Standard Model of Cosmology. A universe 13.8 billion years old, dominated by dark energy and cold dark matter, remains — despite the Hubble tension — the most observationally well-supported framework humanity has yet devised to describe the cosmos from its first moments to the present day.
Looking Ahead
The Hubble tension will not be resolved by a single study, however elegant. Future surveys — including the ESA Euclid mission, the Vera C. Rubin Observatory's Legacy Survey of Space and Time, and the Nancy Grace Roman Space Telescope — promise to deliver even more precise measurements of cosmic expansion at multiple epochs, potentially revealing whether the discrepancy is a harbinger of new physics or a subtle systematic effect yet to be identified. Meanwhile, studies like this one, grounded in the patient observation of the Milky Way's oldest inhabitants, remind us that the universe's own history is encoded not only in the distant cosmos but in the ancient stars shining quietly overhead.
It is a striking testament to the power of modern astrophysics that a quarter of a million stars, observed with telescopes on Earth and in orbit, can speak to questions about the birth of the universe itself — and that, when asked their age, these cosmic elders give an answer that is, quite literally, as old as time allows.
Source: The age of the Universe from a large sample of the oldest Galactic stars — Banik et al., University of Portsmouth.
