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?
 A: First you need to separate clearly in your mind the terms "expansion" and "accelerating". The cosmic expansion, in the present, is simply stuff on freefall trajectories moving away from other stuff, because in the past they were launched on trajectories with velocities arranged in that way. By "stuff" here you can read, approximately, galaxy clusters. (How the velocities came to be arranged that way is, as I understand it, a great mystery which we mostly just take for granted. It is not explained by inflationary models, for example, which also have to take it for granted.)
Acceleration of the expansion, in the present, is a further feature which was quite unexpected and is not understood. Owing to the slight inhomogeneity in the universe on large scales, the evidence for the acceleration is not yet as certain as it is widely supposed. What happens is that if we assume the universe is homogeneous on very large scales then observations (chiefly supernova brightness measurements combined with CMB and other measurements of Hubble parameter) give strong evidence for accelerating expansion. But if we allow for the inhomogeneity then the evidence for accelerating expansion is still there but it is not quite so compelling. I don't have numbers to hand but this is what I understand the situation to be. For more details on this see, for example,
https://ui.adsabs.harvard.edu/abs/2022ApJ...937L..31S/abstract
https://ui.adsabs.harvard.edu/abs/2019arXiv191204257C/abstract
If the expansion is accelerating then it is physically possible that whatever is causing the acceleration could also cause other things such as photon formation. But one would expect the rate of the latter to be unobservably small.
A: I think that probably we should take into account the physical mechanism that actually causes the cosmic expansion. In the cosmology community, there are plenty of ideas about how this mechanism could work, and one of the main ideas is that we could have a scalar field, just like we have the inflaton for inflation which, till the day I am writing this, seems to have compliance between the theory and the observations but there are a lot of things that should be also tested, and especially the tensor perturbation amplitude. I am referring to inflation because since it is the era we have a rapid acceleration of the universe caused by the inflaton this could mean that the observed cosmic acceleration could also be caused by another type of scalar particle. This scalar field could have a lot of properties and obviously, we could have reactions like $\phi\rightarrow A+B+...$ or $\phi+\phi\rightarrow A+B+...$ or $\phi+\gamma\rightarrow A+B+...$, where A, B... could be detectable particles but I would be very difficult or even impossible to detect them, and even if we detect a really tiny percent of, for example, photons that come from that type of interactions then we could not really distinguish them from the CMB or reionization photons, which in fact seem to be the vast majority.
