Evidence for a Lower Value for $H_0$ from Cosmic Chronometers Data?
  An intriguing discrepancy emerging in the concordance model of cosmology is
the tension between the locally measured value of the Hubble rate, and the
'global' value inferred from the cosmic microwave background (CMB). This could
be due to systematic uncertainties when measuring $H_0$ locally, or it could be
that we live in a highly unlikely Hubble bubble, or other exotic scenarios. We
point out that the global $H_0$ can be found by extrapolating $H(z)$ data
points at high-$z$ down to $z=0$. By doing this in a Bayesian non-parametric
way we can find a model-independent value for $H_0$. We apply this to 19
measurements based on differential age of passively evolving galaxies as cosmic
chronometers. Using Gaussian processes, we find $H_0=64.9 \pm 4.2$ km s$^{-1}$
Mpc$^{-1}$ $(1\sigma)$, in agreement with the CMB value, but reinforcing the
tension with the local value. An analysis of possible sources of systematic
errors shows that the stellar population synthesis model adopted may change the
results significantly, being the main concern for subsequent studies. Forecasts
for future data show that distant $H(z)$ measurements can be a robust method to
determine $H_0$, where a focus in precision and a careful assessment of
systematic errors are required.
