In cosmological redshifting, photons lose energy because of the expansion of the universe.
Bur what actually happens to this energy? Is it just “lost” or is it physically destroyed (which would seem impossible)? If it is lost, what is it lost to or what does it become?
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>Bur what actually happens to this energy? Is it just “lost” or is it physically destroyed (which would seem impossible)?
There was a brilliant mathematician around the late 19th and early 20th centuy, called Emmy Noether.
She proved that conservation laws come from symmetries in physics. For example:
* Physics is the same if you move to a different place. This leads to “conservation of momentum”.
* Physics is the same if you face a different direction. This leads to “conservation of angular momentum”
* Physics is the same if you move to a different time. This leads to conservation of Energy.
She even gave an explicit method to translate symmetries into conservation laws.
However, over cosmological timescales, physics isn’t exactly the “same” at different times. At least, not in every way possible. For example, space becomes more stretched out. Since stretching of space represents a breakdown of the time-invariance of normal, classical physics, it means normal classical energy doesn’t have to be strictly conserved. There’s no problem with the stretching of space turning high-energy photons into low-energy ones by stretching them out.
There would be a more sophisticated symmetry *like* time-invariance that is preserved all the way back to the big bang, and that would give (via Emmy Noether’s methods) a conservation law that’s *like* conservation of energy. As another commenter put it, the “energy” goes into the stretched space itself – but then this isn’t classical energy any more, but the more sophisticated “energy” that her maths says is conserved in an expanding universe.
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