Astronomy

So pretty much all of the calculations have been based on observed light, right? Is there a reason why the same observations wouldn't hold true if light itself simply slows down over time? If you've been traveling for 13 billion years through space, isn't there a chance you might have interacted was some random hydrogen atoms along the way? And if those photons have lost some of their energy wouldn't that mean they also appear dimmer when we detect them? Seems like it makes a tidy little package, now the older stars are dimmer and redder. Of course I have no idea about the math behind all this but it seems like you could set up an experiment to measure the speed of light coming from a truly ancient source. Wouldn't that be something if it were found that the "speed of light" isn't the constant that we thought it was?