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I thought I'd ask this question here because why not?

This is something that has bugged me for a while and I've been unable to find an answer. So here it is:

The wavelength of light is stretched by the expansion of space, i.e. its energy will decrease over time, so where does that energy go?

Obviously a doppler redshift is light overcoming the velocity of the object emitting it, and gravitational redshift is the enregy of the photon being converted to gravitational potential energy. But what about the cosmological redshift? Is it just a form of gravitational redshift, produced as massive objects move away from the light (relatively speaking of course)? Or is it something else?

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You might find this thread on the physics forum useful:
Don't think of redshift as a photon losing energy. Redshift is entirely due to a photon being observed in a different frame from where it was emitted. In relativity everything, such as times, lengths and energy are relative quantities that depend on the observer. The same is true for redshift. Nothing 'happens' to the photon, what matters are the frames of the emitter and the observer.
Energy of Photon/Red-shift, inflation, acceleration
http://www.physicsforums.com/showthread.php?t=195279
 

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