It's supposed to be the most fundamental constant in physics, but the speed of light may not always have been the same. This twist on a controversial idea could overturn our standard cosmological wisdom. In 1998, Joao Magueijo at Imperial College London, proposed that the speed of light might vary, to solve what cosmologists call the horizon problem. This says that the universe reached a uniform temperature long before heat-carrying photons, which travel at the speed of light, had time to reach all corners of the universe. The standard way to explain this conundrum is an idea called inflation, which suggests that the universe went through a short period of rapid expansion early on – so the temperature evented out when the cosmos was smaller, then it suddenly grew. But we don't know why inflation started, or stopped. So Magueijo has been looking for alternatives. Now, in a paper to be published 28 November in Physical Review, he and Niayesh Afshordi at the Perimeter Institute in Canada