Modern science assumes that the speed of light has always been the same. Researchers have suggested that this seeming constant might have changed over time, however, and they now have a way to find out whether or not that's true. Professors João Magueijo and Niayesh Afshordi have developed a prediction that should test for changes in light speed. They've given the fluctuations in density the early universe, detectable through cosmic background radiation, an exact spectral index number based on the theory that light was much faster in the first seconds following the Big Bang (0.96478, if you're curious). If future measurements of the index line up with this number, they'll support the notion that light speed has shifted.