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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Here’s a Veritasium video on it [https://www.youtube.com/watch?v=9DrBQg_n2Uo](https://www.youtube.com/watch?v=9DrBQg_n2Uo)
He says it is 100% doppler. Well, really that there is no difference between doppler, gravitational, and cosmological redshift. They are all the same effect due to changing relative velocities of source and observer / accelerating reference frames. Energy and wavelength aren’t intrinsic properties of photons; and it depends on observers. In an expanding universe, every reference frame is changing at far enough distances; and the observers start having high relative velocities.
Photon energy/wavelength being observer dependent is also what causes you to see red (and blue) shift when you travel at relativistic speeds. You’d see blueshift in front, and red in back. But a non relativistic observer would see the same photons as *not shifted*; it is observer dependent. And in an expanding universe; *all directions* are accelerating away from you, so everything is red.
idk why so many posts here are saying energy is lost. I could be wrong and am deferring to the video; I don’t understand these other posts. But, based on this video, no energy is “lost”, because its all relative. If you had something moving at the same relative velocity as a distant galaxy; it would not view its light as red shifted.
Edit: another good point in the video:
People often confuse the fact that “space expands” and “wavelength gets longer”, and associate the two to mean “space stretches photons”. This isn’t true, because, again, wavelength isn’t a property of a photon. And, if it were true, space would stretch everything else as it expands, like atoms. It doesn’t.
Edit 2: here’s another good analogy, I think.
Imagine you’re in deep space, and friend traveling as the same relative velocity as you (i.e. doesn’t appear to be moving) throws something at you. Fast. Say, a baseball at 100 mph. It hits you, and hurts like hell. The ball had a lot of energy.
Now, do the same scenario. But this time, when they throw the ball at 100 mph *relative to themselves*, you start moving away from your friend. By the time the ball catches up to you, it is moving at 5 mph *relative to you*, and when it hits you, it doesn’t hurt. The ball “had less” energy.
The ball “lost energy” according to you, but not your friend. And not according to the universe. The ball didn’t “slow” down, and nothing acted upon it to “slow down”. But the energy it imparted upon you was less than the first scenario.
Same for photons, except they don’t slow down. Their “energy” is still source-observer velocity dependent, like the ball. Instead of “seeing the ball slow down”, you see “the wavelength increase”. Both cause “less relative energy”.
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