The Age of the Universe: From Globular Clusters to Cosmological Parameters
Dhruv Punjabi
09/09/2026
The age of the Universe is a fundamental cosmological parameter that can be inferred independently from stellar astrophysics and from theoretical cosmology, and comparing the two provides a stringent test of the CDM model. Here we determine the absolute ages of seven Milky Way globular clusters from Hubble Space Telescope Advanced Camera for Surveys (ACS) photometry, and separately compute the age of the Universe by integrating the Friedmann equation with the Planck 2018 cosmological parameters. Cluster membership is established from the HUGS proper-motion catalogue, and the age, distance modulus, reddening, metallicity and /Fe of each cluster are inferred jointly from the full colour-magnitude diagram using Dartmouth Stellar Evolution Database (DSED) isochrones within a Bayesian framework, following the likelihood of Valcin et al. (2020). The resulting ages range from 12.62 to 14.35 Gyr, with a statistical uncertainty of order 1 Gyr per cluster and a common systematic uncertainty of 0.5 Gyr from stellar modelling, consistent with previously published determinations for the same clusters. Given our considerably smaller sample than that of Valcin et al. (2020), we treat these ages as an independent, proof-of-concept check on the cosmological result rather than a population-level measurement. The Friedmann integral, evaluated with the Planck 2018 TT,TE,EE+lowE+lensing+BAO parameters with full propagation of their uncertainties, yields an age of the Universe of 13.792 0.111 Gyr, in excellent agreement with the value tabulated directly by the Planck Collaboration for the same parameter combination, 13.787 0.020 Gyr. The persistent tension between the CMB-inferred and distance-ladder-measured values of the Hubble constant is discussed as motivation for possible extensions to the standard model.