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 H0 and the b/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 CLASH, CCCP, and Planck-based estimates. We find no deviation from a constant c when adopting CLASH or CCCP priors, while Planck-based calibration yields a mild tension, with the hypothesis of constant c being only marginally consistent at the 2σ level, indicating a non-negligible sensitivity of the results to the adopted calibration scheme.
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