Ok, I am not sure light is a constant. But its used as a constant because its far and a way from other things. But what if different light has different speeds. There is more than one type of light. And it seems that each has a unique speed but all of them are realistically far from everything else but not exactly the same. Like two stars are millions of light years apart but sufficiently far as to be relatively similar when used in any formula.
Light goes slower if its not in a vacuum. But I think the variance that physicists are wondering about is something else. If as the universe expands the speed of light and a vacuum has changed....but I have not even read the article yet because I am at work and need to do work stuff....just dropping a ping comment....