It's not as bold a prediction as you might think. Existing estimates have the spectral index at 0.968, which is just close enough that the scientists could be right if there's wiggle room. The challenge, of course, is collecting data with high enough accuracy that you can make a definitive call. There's no guarantee that this will happen any time soon.

If the variable light speed concept is ever proven right, though, it would change our understanding of how the universe expanded. Right now, the constant speed theory doesn't give enough time for light to have traveled to where it is in the cosmos, evening out the universe's energy. Super-fast light would fill in that gap and force scientists to rethink early existence. And if light has always traveled at the same speed, that both rules out the variability theory lends weight to an existing inflation concept where the universe briefly evened out before expanding rapidly. Either way, science wins.