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

Understand

Despite the tremendous advance of observational cosmology, the value of the Hubble constant ($H_0$) is still controversial (the so called ``Hubble tension'') because of the inconsistency between local/late-time measurements and those derived from the cosmic microwave background.

• As the age of the Universe is very sensitive to $H_0$, we explored whether the present-day oldest stars could place independent constraints on the Hubble constant.
• To this purpose, we selected from the literature the oldest objects (globular clusters, stars, white dwarfs, ultra-faint and dwarf spheroidal galaxies) with accurate age estimates.
• Adopting a conservative prior on their formation redshifts ($11 \leq z_{\rm f} \leq 30$) and assuming $\Omega_{\rm M} = 0.3 \pm 0.02$, we developed a method based on Bayesian statistics to estimate the Hubble constant.