What happens to the energy of photon red shifted by expansion of universe?

Question

There are many similar questions but the answers don't satisfy me.

I mean it as asked in the title. Some answers given before give the explanation based on gravitational potential energy but consider one generates photon in open space with something small like bulbs photon will still face red shift but have not to overcome any gravity. I mean only by expansion of universe. In what way energy of it will decrease.

The second thing is the expansion of the universe is an accelerated process so energy will continue to drain from photon and it should be in a continuous fashion but energy is quantized. photon wavelength is not quantized but i am talking about energy that is lost if we are considering that energy is being lost gets into space-time or anywhere else, lost energy should be quantised but as expansion is accelerative continuous energy loss appear to occur free particles have continuous energy but it have to be associated with photon all the time.

I understand that energy not considered as conserved in cosmology but based on that argument can we say the energy of photon is reduced and we have no need to give any explanation to it, just like that.

There is no tag 'forum' in physics stackexchange it is added by stack exchange — UV0, Feb 06 '21 at 04:42
The energy could be considered going into slightly slowing the rate of expansion of the universe. — shai horowitz, Feb 06 '21 at 07:48
Expansive force is considered by dark energy whoes appearance is more than energy lost by photons — UV0, Feb 06 '21 at 07:50
If the universe is expanding then it is expanding at a certain speed. If you have photons in the universe they will red shift and also slightly slow down the expansion. (This slow is mostly negligible though) — shai horowitz, Feb 06 '21 at 07:52
As i mentioned expansion is not slowing down but accelerative and force for it is by dark energy — UV0, Feb 06 '21 at 07:55
Also importantly note if we were to contract space time back to what it was pre expansion all the photons would blue shift to exactly what they were to conserve energy. — shai horowitz, Feb 06 '21 at 07:55
Also photon wavelengths aren't quantised, see eg this question. — jacob1729, Feb 06 '21 at 13:12
Yes photon wavelength is not quantized but i am talking about energy that is lost if we are considering that energy is being lost gets into space-time or anywhere else, lost energy should be quantised but as expansion is accelerative continuous energy loss appear to occur — UV0, Feb 06 '21 at 14:46

score 0 · Answer 1 · answered Feb 06 '21 at 15:22

I understand that energy not considered as conserved in cosmology but based on that argument can we say the energy of photon is reduced and we have no need to give any explanation to it, just like that.

It's not true that energy is not conserved in cosmology. Energy is locally conserved but not globally conserved.

The second thing is the expansion of the universe is an accelerated process so energy will continue to drain from photon and it should be in a continuous fashion but energy is quantized.

The energy of free particles is not quantized, it's continuous.

Some answers given before give the explanation based on gravitational potential energy

It's impossible to reply in detail because you haven't linked to the answers. However, this is wrong. There is no notion of gravitational potential energy that applies to cosmological spacetimes.

shai horowitz says in a comment:

The energy could be considered going into slightly slowing the rate of expansion of the universe.

No, this is wrong. This assumes that there is a globally conserved energy that needs to be conserved, which is not the case.

"In particular, a lot of folks would want to say “energy is conserved in general relativity, it’s just that you have to include the energy of the gravitational field along with the energy of matter and radiation and so on.” Which seems pretty sensible at face value." https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ been a while since someone called me out in an answer wrongly. — shai horowitz, Feb 06 '21 at 17:52
Then free particle would have to be associated with photon all the time — UV0, Feb 08 '21 at 04:59
This is a difficult question to answer because in Einstein's field equations energy only occurs on one side of the equation. — , Jan 21 '23 at 12:58

What happens to the energy of photon red shifted by expansion of universe?

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