>The issue is that if a photon slows down in its, I suspect that when it is absorbed and re-emitted that it is a "fresh" photon, with the same energy as the old one, but now moving at c.
>
>That means when old light enters a lens, and interacts with the electrons in the lens, it is absorbed and re-emitted and thus becomes new light again.

So basically it looks as if you have a theory that can't be tested.

There are some other issues, on a more theoretical level.

Olber's Paradox, I think, is taken care of - if light gradually gets old and dim, there is no such paradox. (Olber's Paradox is the question, which may seem trivial at first sight, "Why is the sky dark".)

More serious is the issue of entropy. How would your theory take care of that? I.e., the Universe can't last forever, because there are irreversible processes; or put another way, useful energy gets converted into non-usable energy.

Also, there are observations of distant galaxies, indicating that in the distant past, the universe was different that it is now. For example, currently there don't seem to be any quasars - they all belong to an early stage of galactic evolution.