The local and distant Universe: stellar ages and $H_0$
  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. In the standard
cosmological model, $\Lambda$CDM, $t_U$ and $H_0$ are related by the matter
density parameter $\Omega_{m,0}$. We propose to combine local $t_U$ constraints
with late-time $\Omega_{m,0}$ estimates in a $\Lambda$CDM framework, to obtain
a low-redshift $H_0$ determination that does not rely on early Universe
physics. A proof-of-principle of this approach with current data gives
$H_0=71\pm2.8$ ($H_0= 69.3 \pm 2.7$) km s$^{-1}$ Mpc$^{-1}$ from globular
clusters (very-low-metallicity stars) with excellent prospects for improved
constraints in the near future.
