The Age of the Universe from the Oldest Stars: A New Blow to 'Early' Solutions of the Hubble Tension ✛ ✦
Context
The age of the Universe is a fundamental quantity that defines all cosmic history. Georges Lemaître first proposed that expansion began from a super-dense state, Edwin Hubble experimentally confirmed it by discovering the redshift of galaxies, and Vera Rubin found compelling evidence for dark matter in spiral arms. Today, the standard cosmological model ΛCDM, incorporating dark matter and dark energy, predicts an age of 13.8 billion years from cosmic microwave background data. However, there is the Hubble tension—a discrepancy between the Hubble constant derived from the CMB and local measurements from supernovae. One class of solutions involves increasing the expansion rate in the early Universe, which inevitably reduces its age to about 12.9 billion years. An independent age determination from the oldest stars in our Galaxy can settle this debate. After all, if stars over 13 billion years old have managed to be born and survive to the present day, then the Universe cannot possibly be younger than that limit—the effect of cosmological time is relentless. Adding the typical time (~0.2 billion years) from the Big Bang to the formation of the first long-lived stars gives a lower bound on the age of the entire cosmos.
Methods
The scientists started with a catalog of 247,103 stars for which high-quality spectra were obtained in the LAMOST survey, and the Gaia space telescope measured precise parallaxes. The age of each star was determined by Bayesian comparison of its temperature and luminosity to evolutionary tracks (isochrones) for different abundances of helium and carbon. To filter out stars with erroneous estimates, clever population filters were applied: for example, by chemical composition—ancient stars should be iron-poor and enriched in α-elements, born in supernova explosions. Ages were also compared with independent determinations from Gaia data. The final sample included 155,600 stars within 5,000 light-years of the Sun. Then, the hidden true age distribution was reconstructed using an iterative method and Markov Chain Monte Carlo (MCMC) to account for unavoidable measurement errors.
Results
The reconstructed distribution showed a sharp drop in probability at an age of around 13.73 billion years. This value was taken as the age of the oldest star in the sample: A★ = 13.73+0.18/–0.15 billion years. Adding 0.2 billion years for the formation of the first stars gives a Universe age of roughly 13.93 billion years. This number matches with high precision the standard ΛCDM prediction: 13.8 billion years. Meanwhile, the value expected in 'early' new physics models (12.9 billion years) differs from the measured one by almost 0.8 billion years, a discrepancy of more than 3σ. Even with deliberately relaxed selection criteria, the minimum age of the oldest star was 13.31 billion years—still too high for the 'early' solutions of the Hubble tension. Analysis of a control sample of 1,556 stars with chemical signatures of the very first generations gave a similar estimate: 13.85 billion years. Thus, the result is robust to methods and criteria, and the Universe has clearly had time to grow old by our era.
Implications
This age milestone casts doubt on a whole class of explanations for the Hubble tension that invoke new physics before the recombination epoch—for example, extra relic particles or variable gravitational coupling. To simultaneously make the Universe 'young' at 12.9 billion years and avoid conflict with our data, one would have to assume a too-short interval between the Big Bang and the emergence of the first stars, which poorly matches observations of the most distant galaxies. Therefore, the resolution of the Hubble paradox likely lies in the late Universe—for instance, in local inhomogeneities ( galactic voids) or modified gravity. For the standard model ΛCDM and its components—dark matter and dark energy—the result serves as additional confirmation.
Future development
Further refinement of the oldest stars' ages will come from improved stellar evolution models, especially regarding internal mixing and helium abundance, and from new observational data. Future surveys, including the James Webb Space Telescope, will discover even older low-mass stars, while high-resolution spectroscopy will refine chemical compositions. Additionally, advances in asteroseismology will allow direct age measurements of individual 'Methuselah' stars. All this will reduce systematic uncertainties and perhaps help distinguish between competing late-time cosmological scenarios.
Impact
The results directly impact early Universe cosmology and fundamental questions about the nature of dark matter and dark energy. They are also important for galactic archaeology, which studies the formation history of the Milky Way, and for supernova theory, which determines chemical evolution.
Next steps
The researchers plan to re-analyze the data using the latest isochrones (e.g., MIST II instead of YY) and incorporate asteroseismic ages to reduce systematic uncertainties. They also intend to expand the sample with more distant stars having reliable parallaxes from future Gaia data releases.
Key open problems
The measured age of the oldest stars directly relates to the Hubble tension and constrains theories of dark energy and dark matter, since they dictate the expansion rate of the Universe throughout its history. It also sheds light on processes in the era after the Big Bang, when the first stars ignited, enriching the cosmos with helium and carbon.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study uses a catalog of 155,600 stars with data from LAMOST and Gaia, determining their ages via Bayesian comparison with isochrones and applying population filters. Reconstructing the age distribution with MCMC yields an age of 13.73 billion years for the oldest star, and accounting for the time until the first stars formed gives an age of the Universe of 13.93 billion years, consistent with ΛCDM and contradicting models of early new physics.
Where its strength is
A major strength is the large dataset and strict filters, which provide an independent lower limit on the age of the Universe, directly constraining a class of solutions to the Hubble tension.
Where it could go next
- It might be interesting to compare the obtained age with estimates from stellar archaeology studies that use different methods and samples.based on: Stellar Archaeology: The Age of the Universe from the Oldest Stars arXiv:2607.00764
- A natural next step is to account for systematic effects, such as the influence of rotation and binarity, which could bias stellar age estimates.based on: How Spin and Hidden Companions Confuse Stellar Ages arXiv:2606.09485
- The analysis could be extended to other populations or galaxies to test the universality of the derived age limit.based on: The Mature Bar: How a Galaxy 7.6 Billion Years Ago Outpaced the Universe arXiv:2607.00982
Worth trying
- Test how including corrections for rotation and binarity would alter the reconstructed age distribution.
- Compare the derived age of the oldest star with estimates from other surveys, for example, using chemical composition data.
- Apply the method to stars in other galaxies where data are available to check consistency.
Why this may matter
Independent constraints on the age of the Universe narrow the parameter space for models of early dark energy and other solutions to the Hubble tension, which is important for understanding fundamental physics.
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Original abstract
as written by the authors arXiv:2607.00764v1
We estimate the age of the Universe using the Xiang & Rix sample of 247,103 Milky Way stars with high-resolution spectroscopy from LAMOST DR7 and $$Gaia$$ eDR3 parallaxes. Stellar ages were estimated using YY isochrones up to 20 Gyr. To remove stars with unusually high and precise ages, we require old stars to be metal-poor and $$\alpha$$-enriched. We also require consistency between YY ages and those obtained with FLAME based only on $$Gaia$$ data. Our final sample of 155,600 stars within 5 kpc provides consistent cosmic age estimates using several techniques of increasing rigour. Our main results use an MCMC reconstruction of the latent age distribution, though our iterative reconstruction is very similar. Applying an innovative approach to our MCMC reconstruction and its uncertainties, we find that the oldest star has an age of $$A_\star = 13.73^{+0.18}_{-0.15}$$ Gyr. Varying the quality cuts can at most reduce this to $$A_\star = 13.31^{+0.21}_{-0.18}$$ Gyr or raise it to $$14.02^{+0.18}_{-0.15}$$ Gyr using a much lower or higher age-dependent metallicity ceiling, respectively. Our inferred $$A_\star$$ is consistent with the 13.6 Gyr expected in CMB-calibrated $$\Lambda$$CDM, assuming the first long-lived stars formed when the Universe was 0.2 Gyr old. This agreement casts doubt on solutions to the Hubble tension solely through new physics prior to recombination, which generally imply a cosmic age of $$12.9 \pm 0.2$$ Gyr to match low redshift probes. It is difficult for stellar modelling uncertainties to reconcile such a low age with our result given the low metallicities of the oldest stars in our sample and independent asteroseismic constraints.
The oldest star in this study is a typical subgiant of about 0.8 solar masses, which spent over 13 billion years in a calm 'middle age' (on the main sequence) and has only recently begun to swell, preparing to become a red giant. You could say we caught it just as it was about to 'retire'!
In Paul Anderson's novel 'Tau Zero,' a spaceship accelerates to near-light speed, and the crew watches as billions of years flash by outside — time nearly stops for the travelers while the Universe ages. Our work reminds us that, even without leaving the solar neighborhood, we can peer into the deep past of the cosmos, and all it takes is a careful study of stellar 'fossils'.
Key numbers
- A★: 13.73 +0.18 / -0.15 Gyr
- Age of Universe (ΛCDM): 13.8 Gyr
- Age of Universe (early solutions): 12.9 ± 0.18 Gyr
- Sample size: 155,600 stars
- Distance to stars: < 5 kpc (about 16,000 light-years)