The Horizon Problem
Cosmic microwave background (CMB) radiation left a fossilized imprint of the universe. By that time the universe was already quite spread out, to put it mildly.
The CMB indicates that the temperature and other fundamental physical properties of the universe were largely uniform then. Such consistency should not be possible. There is no mechanism to explain how the universe had a consistent temperature long before heat-carrying photons had time to scour the cosmos and deliver uniformity.
American physicist Charles Misner considered this conundrum in the late 1960s and termed it the horizon problem. Alan Guth dreamed up cosmic inflation to address this inscrutable homogeneity. But there is another possible explanation: an inconstant light speed; or, as it is commonly called, a varying speed of light (VSL).
Einstein first mentioned VSL in 1907 and seriously considered it until propounding general relativity in 1915. His conclusion was that light speed was subject to gravity (by warping spacetime).
French astrophysicist Jean-Pierre Petit first proposed VSL in 1968 to solve the horizon problem. Others have since variously modeled how VSL might work.
The CMB reflects the speed at which light and gravity propagate as the temperature of the universe changes. Following the ΛCDM model, some astrophysicists propose VSL in the feverous early universe, with light outracing gravity by exceeding the blazing speed it now travels. Though merely speculative, VSL is not contradicted by evidence like cosmic inflation is.
Varying light speed would invalidate Einstein’s relativity theories, which anyway were inapplicable in the infant universe before matter took form and mass had much meaning, which meant that gravity would have been nebulous. The physics of the early universe are not understood.