2021MNRAS.505.2764B

Planck data provide precise constraints on cosmological parameters when assuming the base ΛCDM model, including a 0.17 per cent measurement of the age of the Universe, t_0=13.797±_0.023 Gyr_. However, the persistence of the 'Hubble tension' calls the base ΛCDM model's completeness into question and has spurred interest in models such as early dark energy (EDE) that modify the assumed expansion history of the Universe. We investigate the effect of EDE on the redshift-time relation z-t and find that it differs from the base ΛCDM model by at least ~4 per cent at all t and z. As long as EDE remains observationally viable, any inferred t - z or z - t quoted to a higher level of precision do not reflect the current status of our understanding of cosmology. This uncertainty has important astrophysical implications: the reionization epoch - 10 > z > 6 - corresponds to disjoint lookback time periods in the base ΛCDM and EDE models, and the EDE value of t₀ = 13.25 ± 0.17 Gyr is in tension with published ages of some stars, star clusters, and ultrafaint dwarf galaxies. However, most published stellar ages do not include an uncertainty in accuracy (due to, e.g. uncertain distances and stellar physics) that is estimated to be ∼7-10 per cent, potentially reconciling stellar ages with t_0, EDE_. We discuss how the big data era for stars is providing extremely precise ages (< 1 per cent) and how improved distances and treatment of stellar physics such as convection could result in ages accurate to 4-5 per cent, comparable to the current accuracy of t-z. Such precise and accurate stellar ages can provide detailed insight into the high-redshift Universe independent of a cosmological model.