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Despite the tremendous advance of observational cosmology, the value of theHubble constant ($H_0$) is still controversial (the so called ``Hubbletension'') because of the inconsistency between local/late-time measurementsand those derived from the cosmic microwave background. As the age of theUniverse is very sensitive to $H_0$, we explored whether the present-day oldeststars could place independent constraints on the Hubble constant. To thispurpose, we selected from the literature the oldest objects (globular clusters,stars, white dwarfs, ultra-faint and dwarf spheroidal galaxies) with accurateage estimates. Adopting a conservative prior on their formation redshifts ($11\leq z_{\rm f} \leq 30$) and assuming $\Omega_{\rm M} = 0.3 \pm 0.02$, wedeveloped a method based on Bayesian statistics to estimate the Hubbleconstant. We selected the oldest objects ($>13.3$ Gyr) and estimated $H_0$ bothfor each of them individually and for the average ages of homogeneoussubsamples. Statistical and systematic uncertainties were properly taken intoaccount. The constraints based on individual ages indicate that $H_0<70.6$km/s/Mpc when selecting the most accurate estimates. If the ages are averagedand analyzed independently for each subsample, the most stringent constraintsimply $H_0<73.0$ with a probability of 90.3% and errors around 2.5 km/s/Mpc. Wealso constructed an ``accuracy matrix'' to assess how the constraints on $H_0$become more stringent with further improvements in the accuracy of stellar agesand $\Omega_{\rm M}$. The results show the high potential of the oldest starsas independent and competitive cosmological probes not only limited to theHubble constant.

Revisiting oldest stars as cosmological probes: new constraints on the Hubble constant