The Age of the Universe from the Galaxy's Oldest Stars ✛ ✦
Context
The age of the Universe is a fundamental constant, but its precise value depends on the cosmological model. Direct measurements of redshift via Hubble's law (discovered by Edwin Hubble) conflict with estimates from the cosmic microwave background (CMB). This 'Hubble tension' motivates the search for alternatives to the standard ΛCDM cosmology. 'Early' solutions, which modify physics before the recombination epoch, are of particular interest. They predict a younger Universe—about 12.9 billion years. This can be tested through 'galactic archaeology': the study of the most ancient stars in our Milky Way, whose ages serve as cosmic clocks, almost independent of theoretical models.
Methods
The foundation was a catalog of 247,103 stars, for which ages were determined from LAMOST spectroscopy and Gaia eDR3 astrometry using YY isochrones (up to 20 Gyr) and likelihood functions. To filter out stars with unreliable estimates (e.g., inexplicably small errors for ages >16 Gyr), additional population criteria were introduced: 'old' stars must be metal-poor ([Fe/H] < 2.2 – 0.21×age) and enriched in α-elements ([α/Fe] > –0.5 + 0.05×age); their ages must agree with independent FLAME estimates from Gaia within 3σ. After selection, 155,600 stars remained, with age uncertainties rising monotonically toward high values. To determine the true age of the oldest star (A★), a non-parametric MCMC reconstruction of the hidden population age distribution was performed in 100 bins, using a gradient penalty for smoothing. Then, by analyzing the decline of the probability-to-uncertainty ratio (P/σ) with age, a break was identified and interpreted as the A★ boundary.
Results
The reconstructed distribution shows an almost linear decline in P/σ over 12–14 Gyr, followed by a 'noise plateau' beyond 14 Gyr. The intersection of these two regimes gives an oldest star age of A★ = 13.73 (+0.18/–0.15) Gyr. Adding 0.2 Gyr for the formation of the first long-lived stars after the Big Bang, a concept pioneered by Georges Lemaître, yields a cosmic age of 13.93 (+0.18/–0.15) Gyr, which agrees with the ΛCDM prediction of 13.8 Gyr (calibrated by the CMB) within 1σ. Meanwhile, a model with an 'early' solution to the Hubble tension, using Friedmann's equations and modern parameter values, gives a Universe age of 12.91 ± 0.18 Gyr and consequently A★ = 12.71 ± 0.18 Gyr. Our result rules out this scenario at >3σ tension, even with the most stringent sample cuts. Analysis of 1,556 stars with extreme metallicity ([Fe/H] ≤ –0.9) and high [α/Fe] (≥0.2) confirms a close age of 13.85 (+0.05/–0.04) Gyr, almost independent of assumptions about the initial helium abundance.
Implications
This age limit calls into question a whole class of ΛCDM modifications that attribute accelerated expansion to new physics in the early Universe. If the expansion rate had been 9% higher throughout most of cosmic history—as required by such models—the age of the cosmos would have dropped by ~1 Gyr, incompatible with the observed age of the oldest stars. This strengthens the case for 'late' solutions (e.g., a local void or modifications of gravity) and points to the need to reinterpret redshifts in the nearby Universe without revising general relativity A. Einstein.
Future development
Further progress requires improvements in five directions: larger samples and finer age grids; reducing systematics in stellar models (especially regarding helium enrichment and the mixing length parameter αML); refining the time of first star formation (tf); increasing the precision of cosmographic parameters from low-redshift observations; and more fully accounting for uncertainties in the MCMC reconstruction method.
Impact
The results impact cosmology (Hubble tension, nature of dark energy), galactic archaeology, and the physics of stellar evolution.
Next steps
The next step is to repeat the analysis with LAMOST DR8 and Gaia DR4 data, and to apply more modern isochrone grids that account for individual α-element abundances.
Key open problems
The work is directly linked to unsolved problems: the nature of dark energy and dark matter, the first moments after the Big Bang (tf), and the origin of stellar populations in the early Galaxy.
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 authors determined the age of the oldest stars in the Milky Way using data from LAMOST and Gaia, employing YY isochrones and Bayesian analysis. After selecting 155,600 stars based on metallicity and alpha-enhancement, they reconstructed the age distribution and found the boundary of the oldest star to be approximately 13.73 billion years, which is consistent with ΛCDM and rules out 'early' solutions to the Hubble tension.
Where its strength is
The strength of the work lies in its direct empirical constraint on the age of the Universe, independent of cosmological models, and in the careful filtering of the sample to suppress systematic errors.
Where it could go next
- It might be interesting to compare the obtained age with estimates from independent methods, such as horizontal branch stars or white dwarfs, to check the robustness of the result to isochrone systematics.based on: The Universe Isn't Getting Younger: Ancient Stars Rule Out "Early" Solutions arXiv:2607.00764v1
- Related work exists on how rotation and binarity can bias age estimates; testing the impact of these effects on the selection of old stars could refine the A★ boundary.based on: How Spin and Hidden Companions Confuse Stellar Ages arXiv:2606.09485
- To further refine the cosmological age, one could consider combining with other local indicators, such as gravitational wave estimates, allowing a cross-check of the 'Hubble tension'.based on: Gravitational waves measure the expansion rate of the universe arXiv:2605.27227
Worth trying
- Apply a Bayesian hierarchical model that accounts for uncertainties in isochrones and metallicity to more rigorously determine the age of the oldest star.
- Investigate the sensitivity of the result to the choice of metallicity and alpha-enhancement thresholds by varying them within reasonable limits.
- Use stellar kinematics data to isolate the halo population, which could improve the selection of ancient objects.
Why this may matter
Precisely determining the age of the Universe is important for resolving the 'Hubble tension' and testing alternative cosmological models, while the age of the oldest stars serves as a model-independent anchor.
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Original abstract
as written by the authors arXiv:2607.00764
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 the selected sample, Gaia DR3 3656896440426302592, has an observed age of ~13.5 Gyr — only 0.3 Gyr younger than the entire Universe. If its age were underestimated by the typical 7.5%, the true value could reach 14 Gyr!
Key numbers
- oldest star (A★): 13.73 (+0.18/–0.15) Gyr
- Universe age from CMB (ΛCDM): 13.8 Gyr
- Universe age from 'early' solutions: 12.91 ± 0.18 Gyr
- final sample size: 155,600 stars
- initial catalog size: 247,103 stars