Moon, and other physical measures seem to place strict bounds on this. Complicating the picture is the possibility that other fundamental parameters could alter in tandem, masking the impact of a single changing value. Thus, one needs to think hard before ruling out any given effect.

Since Einstein’s day, physics has embraced the mantra that the speed of light in a vacuum cannot vary. No moving object, according to theory, can exceed this universal limit. This assumption serves as the underpinning for special relativity, which is supported by a wealth of laboratory data. Thus, most physicists take it as sacrosanct that light always moves at the same velocity.

In 1999, theoreticians Andreas Albrecht and João Magueijo, then at Imperial College in London, offered the radical proposition that the speed of light has varied over time. Calling this theory the Varying Speed of Light (VSL) hypothesis, they asserted that it would serve as well as inflation in solving the horizon and flatness problems and could also explain astrophysical data on the cosmological constant.

If signals once traveled from one part of the universe to another faster than they do now, that could explain why space is so uniform. Through a rapid and far-reaching process of thermal equilibrium, temperatures in the early universe would have had ample opportunity to even out. Also, any significant pockets of high- or low-density matter would tend to even out over time through either the release or the accumulation of energy. Although such processes would violate standard conservation laws, they would be permitted if the speed of light could vary. This leveling out would lead automatically to a flat cosmos. Hence, the horizon and flatness issues would vanish, with no inflationary smoothing needed to accomplish these feats.

Furthermore, alterations in the speed of light would affect astronomers’ measurements of the velocities of distant galaxies. The