I have a feeling I know just enough about this to be confused, but here goes.
From my understanding the speed of light in a vacuum is constant in all reference frames, at 299,792,458m/s. The only way for something to move faster than this is for the space between two reference frames to expand, increasing the amount of distance between them that light has to travel (the two objects on a partially inflated balloon analogy
). Such an expansion happened immediately after the big bang, and the universe has been expanding (albeit slower) ever since.
What I don't really understand is why the vacuum of today should be the same as the vacuum of 5 billion years ago. Sticking with (and probably breaking) the balloon analogy, as the balloon is inflated the properties of the rubber change. A wave moving from one point on the inflated balloon to another should move faster than before, due to the tension. For light through a vacuum, I guess this would mean the vacuum is 'more empty'? It has less vacuum energy
Either way, it seems like light should be moving faster as the universe expands. The change in speed would be incredibly minor on any timescale we can measure, but over billions of years significant. Since I haven't seen this theory anywhere outside of my head, I assume I'm missing something.
What am I missing? Is there some compensatory change in the flow of time, or am I misunderstanding relativity?