Since photons are redshifted as the universe expands, this is offered to support the claim that energy is not conserved in GR. When I asked where the lost energy goes, the answer offered is that it "disappears". I find this answer woefully insufficient for the following reason: if energy in GR is not conserved, it just means that a photon's energy changes as the universe expands. No matter at what time in the evolution of the universe, the photon's energy remains unchanged* if it's conserved*. That is, conservation of energy means the photon's energy remains unchanged. It doesn't say the lost energy "disappears". So the open question IMO is this; where does the lost energy go, and GR does not answer this question. AG
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