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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Let’s say you and a friend are standing next to a conveyor belt moving at a constant rate.
Every second, your friend takes a cookie and puts it on the belt. The cookies travel down the belt a ways until they reach you. Every time you see a cookie, you take it off the belt and into your cookie jar. With this setup, you are receiving 1 cookie per second.
Now, let’s say your friend gets up and starts walking away from you, against the stream of the belt. But as they walk, they continue to place cookies on the belt. Since they walk away a little further along the belt between cookies, the cookies are now spaced further apart. So you are now getting less than 1 cookie per second.
But hang on. Your friend is still dumping 1 cookie per second onto the belt. But you’re getting less than 1 cookie per second at the other end. Where did all that extra cookie go?
As your friend continues to walk away, there is more belt between you to fill with cookies. That’s where the supposedly missing extra cookie is. All that extra belt your friend is creating by walking away from you is able to store extra cookies. You’re getting less of them per second because some of them are going towards filling that extra belt space.
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This is only part of the puzzle, though. What I’ve described to you is called *non-relativistic* Doppler redshift. It’s decent enough for explaining phenomena like the way an ambulance siren increases in pitch as it’s moving towards you and decreases in pitch as it moves away. But it doesn’t quite cut it for rays of light. For that, we need *relativistic Doppler redshift*.
The relativistic version is the same as the non-relativistic version, but with an extra thing we have to consider. When two things are moving with respect to one another (e.g. your friend is moving away from you), the way the two of you perceive time won’t actually sync up. One of you is going to perceive time passing more slowly from the perspective of other. This is called time dilation.
By convention, we’d say that your friend, since they’re the one emitting the cookies, is the one experiencing “normal time”. From your friend’s perspective, they are still, and *you* are the one moving away. Since you’re moving, that means your perception of time slows down relative to your friend. Because of that, you will be receiving cookies *even slower* than you otherwise would be if we didn’t take relativity into account. That is, this effect stacks with the previous effect. You receive fewer cookies per second because some of them are going towards filling up the extra belt between you, *and* because the very concept of what a “second” is to you has changed due to relativity.
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**EDIT:** Apparently I’ve neglected yet another mechanism that is unique to redshift caused by the expansion of space. Please read the replies.
The explanation above is adequate for relativistic effects in *non-expanding* space, but expanding space is probably what OP actually wanted to know about.
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