Revisiting the Constancy of the Speed of Light: Galaxy Cluster Mass Bias Implications

TL;DR

This study tests whether the speed of light varies over cosmic time using galaxy cluster and supernova data, finding no deviation with some calibration schemes but mild tension with Planck-based calibration.

Contribution

It combines multiple observational datasets with different calibrations to assess the constancy of the speed of light independently of specific cosmological models.

Findings

No deviation from constant $c$ with CLASH or CCCP calibrations.

Mild tension with Planck-based calibration at 2 sigma.

Results are sensitive to the choice of cluster mass calibration.

Abstract

In recent years, improvements in galaxy cluster observations have enabled a variety of tests of fundamental physics using these systems. In this work, we test the constancy of the speed of light, $c$, by combining X-ray gas mass fraction measurements from galaxy clusters with SNe Ia luminosity distance measurements from Pantheon+. We adopt the SH0ES prior on $H_0$ and the $\Omega_b/\Omega_m$ ratio from galaxy clustering observations, thereby minimizing the dependence of our analysis on any specific cosmological model. We explore different assumptions for the cluster mass calibration (mass bias), including \textsc{CLASH}, \textsc{CCCP}, and Planck-based estimates. We find no deviation from a constant $c$ when adopting \textsc{CLASH} or \textsc{CCCP} priors, while Planck-based calibration yields a mild tension, with the hypothesis of constant $c$ being only marginally consistent at the $2\sigma$ level, indicating a non-negligible sensitivity of the results to the adopted calibration scheme.