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Some physicists prefer to explain the problem of conservation of energy in General Relativity by considering the gravitational potential energy of the universe that would cancel all the other energies and therefore the energy in the universe would be conserved this way.

However, many other physicists opt to just say that energy is not conserved 1. If we take this explanation, we can conclude that energy can be created or destroyed in cosmological scales, as in 1 says:

"In general relativity spacetime can give energy to matter, or absorb it from matter, so that the total energy simply isn’t conserved"

The photon redshifting due to the expansion of the universe is usually given to indicate that energy can be lost in General Relativity. But I cannot find anything that would be an example of spacetime creating energy or giving it to matter (as it says in the reference 1 that I quoted). Therefore, if the conservation of energy is not well defined in General Relativity, can energy be created?

vengaq
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    Does this answer your question? Does the expansion of spacetime add energy to matter? – Níckolas Alves Apr 04 '22 at 17:29
  • @NíckolasAlves I was looking for a more realistic example in the sense that can be observed by our telescopes (for example something like the redshift that photons suffer when travelling through space but a process that would give energy instead of erasing it) – vengaq Apr 06 '22 at 12:31
  • My answer to the linked question concerns precisely that. In the cosmological model I gave as en example, the Universe would eventually start to recollapse into a Big Crunch. In this era, the photons would be blueshifted. That model does not apply to our Universe (it has a different matter content), but shows that energy can be created in General Relativity (as long as the spacetime is not stationary, for otherwise conservation of energy is well-defined) – Níckolas Alves Apr 06 '22 at 19:29
  • @NíckolasAlves sorry I should have indicated in both the question and the comments that I was looking for something that would give energy while space is expanding, not contracting. I have found a few gedankenexperiments like this one (https://physics.stackexchange.com/q/668248/) but nothing specific that can be observed in actual experiments – vengaq Apr 06 '22 at 19:39

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