The oldest Milky Way stars: New constraints on the age of the Universe and the Hubble constant

¹ Dipartimento di Fisica e Astronomia “Augusto Righi”–Università di Bologna Via Piero Gobetti 93/2 I-40129 Bologna, Italy
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna Via Piero Gobetti 93/3 I-40129 Bologna, Italy
³ Leibniz-Institut für Astrophysik Potsdam (AIP) An der Sternwarte 16 14482 Potsdam, Germany
⁴ Institut für Physik und Astronomie, Universität Potsdam Karl-Liebknecht-Str. 24/25 14476 Potsdam, Germany
⁵ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB) Martí i Franquès 1 08028 Barcelona, Spain
⁶ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB) Martí i Franquès 1 08028 Barcelona, Spain
⁷ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC 08860 Castelldefels (Barcelona), Spain
⁸ Instituto de Astrofísica de Canarias E-38200 La Laguna Tenerife, Spain
⁹ Departamento de Astrofísica, Universidad de La Laguna E-38205 La Laguna Tenerife, Spain
¹⁰ Kavli Institute for Cosmological Physics, University of Chicago 5640 S. Ellis Avenue Chicago IL 60637, USA

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Received: 29 August 2025
Accepted: 17 December 2025

Abstract

Aims. In this work, we exploit the most robust, old, and cosmology-independent age estimates of individual stars from Gaia DR3 to place a lower bound on the age of the Universe, t_U. These constraints can be used as an anchor point for any cosmological model, thus providing an upper limit to the Hubble constant H₀.

Methods. Our primary stellar age catalog comprises 3000 of the oldest and most robustly measured main-sequence turn-off (MSTO) and subgiant branch (SGB) stars, with ages older than 12.5 Gyr and associated uncertainty below 1 Gyr. Stellar ages are derived via isochrone fitting using the Bayesian code StarHorse, spanning the uniform range 0−20 Gyr, not considering any cosmological prior knowledge on t_U. By applying a conservative cut in the Kiel diagram and strict quality cuts on both stellar parameters and posterior probability distribution shapes, and filtering out potential contaminants, we isolated a final sample of 160 bona fide stars, the most numerous sample of precise and reliable MSTO and SGB stars ages available to date.

Results. The age distribution of the final sample peaks at 13.6 ± 1.0 (stat) ± 1.4 (syst) Gyr. Assuming a maximum formation redshift for these stars of z_f = 20, corresponding to a formation delay of ∼0.2 Gyr, we obtain a lower bound on t_U of t_U ≥ 13.8 ± 1.0 (stat) ± 1.4 (syst) Gyr. Considering the 10th percentile of the posterior probability distributions of the individual stars, we find that, at 90% confidence level, 70 stars favour t_U > 13 Gyr, while none exceeds 14.1 Gyr. For this upper envelope to fall below 13 Gyr, a shift of nearly the full systematic error budget would be required, indicating that such low values are only attainable under very peculiar assumptions.

Conclusions. This work presents the first statistically significant use of individual stellar ages as cosmic clocks, opening a new independent approach for cosmological studies. While this analysis already represents a significant step forward, future Gaia data releases will enable even larger and more precise stellar samples, further strengthening these constraints.