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

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.

8 pages, 3 figures, 2 tables. Paper published in Astroparticle Physics (180, 2026, 103249)

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