Astrophysics of Galaxies
The Age of the Universe with Globular Clusters IV: Multiple Stellar Populations
We revisit the determination of the age of the Universe from galactic globular clusters, extending previous analyses by explicitly accounting for the presence of multiple stellar populations within each cluster. Using high–quality Hubble Space Telescope color–magnitude diagrams for 69 globular clusters, we relax the standard single–population assumption, and model two stellar populations with independent ages, metallicities, helium abundances, and population fractions. The inference is performed using the full color–magnitude diagram morphology, an explicit treatment of field contamination, and a hierarchical framework that propagates non–Gaussian age posteriors. Allowing for multiple stellar populations has a negligible impact on globular cluster age estimates. The ages of the oldest populations remain fully consistent with those obtained under the single–population assumption, with differences at the $0.6\sigma$ level. Restricting to the metal–poor subsample ([Fe/H] $< -1.5$), we infer a dominant old component with mean age $t_{\rm GC}=13.61\pm0.25\,\mathrm{(stat)}\,\pm0.23 \mathrm{(sys)}\,\mathrm{Gyr}$. Adopting a conservative delay between the Big Bang and the formation of the first globular clusters, we obtain an age of the Universe of $t_{\rm U}=13.81\pm0.25\,\mathrm{(stat)}\,\pm0.23 \mathrm{(sys)}\,\mathrm{Gyr}$. In addition to age constraints, our analysis yields simultaneous measurements of metallicity and helium content for the different populations, including constraints on helium enrichment and population fractions which are consistent with independent determinations from the literature. These results demonstrate that globular–cluster–based cosmic chronometry is robust to stellar population complexity, reinforcing its role as a precise and largely cosmological model–independent probe of the age of the Universe.