Perturbation Theory in the Minimally Extended Varying Speed of Light (meVSL) Model

Abstract: Cosmological perturbation theory provides a fundamental framework for analyzing the evolution of density fluctuations and gravitational potentials in the Universe. It plays a crucial role in understanding large-scale structure formation and cosmic microwave background (CMB) anisotropies. In this study, we apply perturbation theory to the minimally extended varying speed of light (meVSL) model to investigate the effects of a varying speed of light on the matter density contrast and the Newtonian gravitational potential. Unlike conventional models with a constant speed of light, the meVSL model introduces modifications to the cosmological evolution equations, leading to potential deviations in structure formation and gravitational interactions. By deriving and analyzing the perturbed equations within this framework, we explore how a varying speed of light affects the growth of density perturbations and the evolution of gravitational potentials. Compared to the standard constant speed of the light model, we find deviations of approximately $2$\% in the subhorizon modes of both quantities. Although detecting these effects observationally remains a significant challenge, our results provide new theoretical insights into the meVSL model and its potential observational signatures, such as the integrated Sachs-Wolfe effect and gravitational lensing of the CMB.