Conservation of energy and space expansion

[Lopes1801]

This had me thinking for a while. Imagine a photon emitted by a very distant object at a redshift of z = 2.0 for example. As the photon travels through space, due to space expansion the photon's wavelength will shift towards red. With an increase in the wavelength there must come a decrease in the frequency. With that, the energy of the photon also decreases. Therefore, the photon must lose energy. And by the law of conservation of energy, this amount of energy cannot disappear. So where does the energy go to?
(I don't know whether the question should be labeled as Basic or Intermediary, so I arbitrarily chose one of the options)

 

[andrewkirk]

Energy conservation only applies within inertial frames. Within a non-inertial frame - which is what is needed for any thought experiment big enough to incorporate galactic redshift - energy is only defined locally, and there is no principle of global energy conservation.

 

[marcus]

I agree with Andrew. The cosmologist Sean Carroll has a blog post about this called "Energy is not conserved in an expanding universe"
If you google the title you will, I think, get it. A lot of fairly intuitive discussion. Non conservation of energy in GR is well known, just a bit unintuitive when first encountered.

 

[Chronos]

The problem with energy conservation is not energy, the problem is in defining how you define energy.

 

[bcrowell]

We have a FAQ on this: https://www.physicsforums.com/threads/what-is-the-total-mass-energy-of-the-universe.506985/