The minimally extended Varying Speed of Light (meVSL)

Understand

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 $\tilde c$.