The local and distant Universe: stellar ages and $H_0$

Understand

The ages of the oldest stellar objects in our galaxy provide an independent test of the current cosmological model as they give a lower limit to the age of the Universe.

• Recent accurate parallaxes by the Gaia space mission, accurate measurements of the metallicity of stars, via individual elemental abundances, and advances in the modelling of stellar evolution, provide new, higher-precision age estimates of the oldest stellar populations in the galaxy: globular clusters and very-low-metallicity stars.
• The constraints on the age of the Universe, $t_U$, so obtained are determined from the local Universe and at late time.
• It is well known that local and early-Universe determinations of another cosmological parameter closely related to the age of the Universe, the Hubble constant $H_0$, show a $\gtrsim 3 \sigma$ tension.