Space

Huh I never thought about photons not aging. I guess since they're traveling at the speed of light it means they don't experience any time between being emitted and when they collide with something.

That theory on the big rip sounds fascinating, I'm going to have to look that up! Thanks for the info.

As for the redshifting... as far I understand it you can't see a photon until it actually hits something anyway, at which point it is no longer an unresolved particle of light? My immediate thought on this subject is that you would need to calculate the theoretical maximum width of the universe in light years when the last bit of energy is used up, and that should give a maximum value for how long a photon can exist beyond that period. However then you start bringing in other theories like the torus universe that wraps around on itself, and now you have the potential for photons to travel forever until they do hit something so there could be the possibility of photons existing indefinitely after the heat death. And you can't really call the universe "dead" if there is still some energy tied up in photons.

I mean it's all just a fun exercise of the mind, there may not be an actual answer, but I love thinking about what happens at the extreme ends of things.