`c' is the speed of light, isn't it?

TL;DR

This paper reviews theories proposing a varying speed of light as an alternative solution to cosmological problems, discussing observational hints and theoretical frameworks including scalar-tensor theories.

Contribution

It provides a comprehensive analysis of varying speed of light theories, connecting them with scalar-tensor gravity models and recent observational data.

Findings

Varying speed of light theories are motivated by cosmological problems.

Observational hints suggest possible variation in fundamental constants.

Scalar-tensor theories offer a framework for varying speed of light models.

Abstract

Theories proposing a varying speed of light have recently been widely promoted under the claim that they offer an alternative way of solving the standard cosmological problems. Recent observational hints that the fine structure constant may have varied during over cosmological scales also has given impetus to these models. In theoretical physics the speed of light, $c$, is hidden in almost all equations but with different facets that we try to distinguish. Together with a reminder on scalar-tensor theories of gravity, this sheds some light on these proposed varying speed of light theories.