Where does photon energy go in universe expansion?

[samsara15]

[Moderator's note: Spun off from another thread, where it was off topic. Please start a new thread when you have questions on a new topic.]

Photons lose energy when they travel long distances. Where does that energy go? What happens to that energy? Is there any theory regarding this?

 

[bapowell]

The short answer: the energy is simply lost. There is no global conservation of energy in general relativity, the theory that forms the basis of modern cosmology. Check out http://www.physics.uq.edu.au/download/tamarad/papers/SciAm_Energy.pdf.

 

[PAllen]

I would add that photons losing energy as they travel long distances is a dubious description of the relation between CMB frequency today and versus when the universe was very young. In fact many completely wrong theories are build on concepts of 'tired light' which it almost sounds like you are describing. In fact, as the article bapowell links notes, it is not even necessary to view any photon, ever, as losing energy to correctly explain all observations, including that energy conservation does not apply to the whole universe, or the current CMB frequency distribution.

 

[PAllen]

Note, in relation to the OP, that the article Bapowell linked says the following:

"According to the author, the
proper interpretation shows
that the energy of individual
photons is conserved. And
phenomena taking place
inside the galaxy generally
conserve energy."

I would add that while I lean towards Tamara's viewpoint, many cosmologists would say the photon really does lose energy, but not from traveling a long distance - which smacks of discredited tired light theories.