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

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Discussion Overview

The discussion centers around the question of whether photon emission can be caused by the expansion of spacetime. Participants explore theoretical implications and interpretations of spacetime expansion, particularly in relation to photon behavior and emission processes.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant inquires about any observed processes where photon emission is directly caused by spacetime expansion.
  • Another participant argues that spacetime expansion is merely a change in geometry and does not result in any physical processes, asserting that objects in their own frames of reference do not experience motion due to expansion.
  • A different participant clarifies that while "space" is expanding in a coordinate sense, this is a coordinate effect and does not lead to photon emission.
  • It is noted that the expansion scalar of comoving observers is a physical invariant, but these observers are in free fall, which cannot cause emission processes.
  • One participant seeks clarification on whether the expansion scalar corresponds to what is commonly understood as "distance apart."
  • Another participant confirms that the expansion scalar represents a generalization of how the volume occupied by matter is increasing, independent of coordinates.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between spacetime expansion and photon emission. There is no consensus on whether spacetime expansion can lead to photon emission, with some asserting it cannot and others questioning the implications of expansion.

Contextual Notes

Participants highlight the distinction between coordinate motion and proper motion, as well as the need for clarity in definitions related to expansion and emission processes.

Suekdccia
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TL;DR
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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