The age of the Universe from a large sample of the oldest Galactic stars

Abstract

We estimate the age of the Universe using the Xiang & Rix sample of 247,103 Milky Way stars with low-resolution spectroscopy from LAMOST DR7 and $Gaia$ eDR3 parallaxes. Stellar ages were estimated using YY isochrones up to 20 Gyr. We require consistency between YY ages and photometric FLAME ages based only on $Gaia$ data. To remove a population of stars with unusually low vertical action for their age, metallicity, and distance, we require old stars to be metal-poor and $α$-enriched. Our final sample of 146,810 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 $A_\star = 13.80^{+0.20}_{-0.16}$ Gyr. We conservatively estimate a systematic uncertainty of $\pm 0.4$ Gyr due to the choice of quality cuts, isochrone model, and initial helium abundance. Allowing 0.2 Gyr for the first long-lived stars to form, our inferred $A_\star$ is consistent with the 13.6 Gyr expected in CMB-calibrated $Λ$CDM. 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.