The minimally extended Varying Speed of Light (meVSL)

TL;DR

This paper introduces a minimally extended Varying Speed of Light (meVSL) cosmological model where the speed of light varies with cosmic time, preserving key physics laws, and explores its implications on cosmological observations and the Hubble tension.

Contribution

It proposes a new VSL model that maintains fundamental physics laws and derives its cosmological implications, providing constraints from current observations.

Findings

The Hubble parameter in meVSL is rescaled, potentially resolving the Hubble tension.

Most cosmological distances in meVSL match those of GR, except luminosity distance.

Constraints on the variation of the speed of light are obtained from observational data.

Abstract

Even though there have been the various varying speed of light (VSL) cosmology models, they remain out of the mainstream because of their possible violation of physics laws built into fundamental physics. In order to be the VSL as a viable theory, it should inherit the success of special relativity including Maxwell equations and thermodynamics at least. Thus, we adopt that the speed of light, $\tilde c$ varies for the cosmic time not for the local time, i.e., $\tilde c[z]$ where $z$ is the cosmological redshift. When one describes the background FLRW universe, one can define the constant-time hypersurface by using physical quantities such as temperature, density, and $\tilde c$. It is because they evolve in time, and the homogeneity of the Universe demands that they must equal at the equal cosmic time. The variation of $\tilde c$ accompanies the joint variations of all related physical constants in order to satisfy the Lorentz invariance, thermodynamics, and Bianchi identity. We call this VSL model as a "minimally extended VSL (meVSL)". We derive cosmological observables of meVSL and obtain the constraints on the variation of $\tilde c$ by using the current observations. Interestingly, $z$ and all geometrical distances except the luminosity distance of meVSL are the same as those of general relativity. However, the Hubble parameter of meVSL is rescaled as $H = (1+z)^{-b/4} H^{(\rm GR)}$ which might be used as a solution for the tension of the Hubble parameter measurements. In this manuscript, we provide the main effects of meVSL on various cosmological observations including BBN, CMB, SZE, BAO, SNe, GWs, H, SL, and $\Delta \alpha$.