On the use of galaxies as clocks and the universal expansion

TL;DR

This paper critically examines the use of galaxy age data to estimate the Hubble parameter, revealing that claimed precisions are unrealistic and that the data provide limited constraints on cosmic expansion.

Contribution

The study reanalyzes galaxy-based Hubble parameter estimates, highlighting the impact of assumptions and correlations on uncertainty and questioning the utility of such data for constraining cosmic expansion.

Findings

Unrealistically small galaxy age uncertainties are needed for claimed precision.

Correlations affect the parameter estimates and uncertainties.

Galaxy data do not provide useful constraints on the Hubble parameter.

Abstract

We set out to rederive the 8 Hubble parameter values obtained from estimated relative galaxy ages by Simon et al. [Physical Review D, 71, 123001 (2005)]. We find that to obtain the level of precision claimed in $H(z)$, unrealistically small galaxy age uncertainties have to be assumed. Also, some parameter values will be correlated. In our analysis we find that the uncertainties in the Hubble parameter values are significantly larger when 8 independent $H(z)$ are obtained using Monte Carlo sampling. Smaller uncertainties can be obtained using Gaussian processes, but at the cost of strongly correlated results. We do not obtain any useful constraints on the Hubble parameter from the galaxy data employed.