I Can there be some kind of photon emission caused by space expansion?

AI Thread Summary
Photon emission is not caused by the expansion of spacetime, as spacetime expansion is merely a geometric change without affecting the objects within it. In their own frames of reference, objects are not moving apart in a way that would lead to photon emission. The concept of expansion is a coordinate effect rather than a physical motion. While there is a positive expansion scalar related to comoving observers, this does not lead to any emission processes. Overall, spacetime is not expanding; it simply exists as a framework for understanding distances and motion.
Suekdccia
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Can there be some kind of photon emission caused by cosmological expansion?
Are there any kind of observed and experimentally verified processes or mechanisms where photon emission occurs and which are directly cause by spacetime expansion in some way?
 
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No. Why would there be? Spacetime expansion is just geometry changing. Nothing is happening to the things that are moving apart. In their own frames of reference they are not even moving (other than whatever motion they have aside from expansion).

Put another way, expansion is coordinate motion, not proper motion.
 
Suekdccia said:
spacetime expansion
Spacetime is not expanding. Spacetime just is.

"Space" is expanding if we choose standard FRW coordinates for our universe, but as @phinds says, this is really a coordinate effect and can't cause anything.

There is a physical invariant that can also be described as "expansion" in our universe, namely the positive expansion scalar of the congruence of comoving observers, but these observers are in free fall and free fall can't cause any emission process either.
 
PeterDonis said:
scalar of the congruence
Just a sanity check please: Is this what an ordinary person would call "distance apart"? I really need to learn this stuff...
 
hutchphd said:
Just a sanity check please: Is this what an ordinary person would call "distance apart"? I need to learn this stuff...
The expansion scalar is the coordinate-free generalisation of the naive "how fast the volume this stuff occupies is increasing".
 
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