Asked by: Jackie Taunton, Reading
Light has long been known to slow down whenever it passes through a medium such as air or glass.
Light also interacts with the molecules in its surroundings.
But over the years, some theorists have proposed that the speed of light in a vacuum may have been far higher during the Big Bang. While this would solve some of the technical problems concerning the early Universe, there’s no compelling evidence that this is the case.