[WeniTheElder]

We observe from a far distant star the red shifted spectrum. This is explained by the expansion of the galaxy and the expanding room between us and this star. So the photons coming from there have a red-shifted and hence lesser engery. What has happend to the energy of the photon? Where's it gone to?

[greendestiny]

I think the theory is suggesting that the photon was emitted with less energy.

[jajag]

I think the question is that if the current accepted theory for the redshift is correct, then where has the energy gone?

[michaelcampbell]

Would the star then appear to be a different color to "someone" its own solar system than it does us? Does OUR star thus appear to be moving TOWARD them since it's "our" color vs what "they" would consider to be the correct color for a star our size?

[FakeComments]

Doesn’t this require the photon to be emitted differently depending on where it ends up, as it would experience different amounts of redshift depending on the length of flight?

That’s some serious retro-causality if a photon from 13B ly hits us instead of Andromeda.

[aroberge]

Forget about the expansion of the universe; you could ask the exact same question for an observer moving away from a source and detecting a different frequency (hence energy) of the light emitted due to "normal" Doppler effect in a non-expanding universe. Energy (on its own) is not an invariant in (special/general) relativity. You have the same situation for measurements of lengths (i.e. Lorentz contraction). Energy is one coordinate of a four-vector whose "length" is the true invariant.

This is similar to a situation where you draw a set of perpendicular axis on a plane and use them to give coordinates (x, y) to a point. You then rotate the set of axis, keeping the point fix. Your question about the energy amounts to asking "what happens to the x-coordinate of that point?".

[Geee]

The energy of a photon doesn't go anywhere, it just seems to us to be lower. The observed energy of a photon depends of the relative motion. If you move at the same velocity as the observed object, the redshift disappears and the energy of photon is there.