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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2$\begingroup$ Particle creation by the expansion of the universe definitely occurs, but it is a very small effect at our stage of cosmic evolution, in which the expansion is very slow compared with the size of the observable universe. So it is unlikely to be directly observed, ever. $\endgroup$– Buzz ♦Commented Oct 21, 2022 at 1:38
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$\begingroup$ @Buzz but how can we be so sure about the particle creation if it has never been directly observed? $\endgroup$– vengaqCommented Oct 21, 2022 at 12:30
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$\begingroup$ If you try to describe quantum fields in a time-varying spacetime background, the particle creation is an automatic consequence. It would break all kinds of things we understand to be fundamental (like local energy-momentum conservation) if it didn't happen. $\endgroup$– Buzz ♦Commented Oct 21, 2022 at 18:29
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$\begingroup$ @Buzz Sorry but particle creation-annihilation is just a mathematical construct of qft, why should we expect that such a phenomena should actually occur physically? There can be other approaches to describe same observations which on a fundamental scale doesn't rely on particle creation-annihilation interpretation $\endgroup$– paul230_xCommented Oct 23, 2022 at 14:16
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2 Answers
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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.
answered Oct 21, 2022 at 13:18
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$\begingroup$ +1 Just to add, if our universe has inhomogeneities, it's back-reaction can even behave as an effective cosmological constant term (arxiv.org/abs/2002.10831v2). Thus, even if our universe is effectively FRW on a cosmological scale, there can be a significant contribution to $\Omega$ parameter due these inhomogeneities $\endgroup$ Commented Oct 23, 2022 at 14:51
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$\begingroup$ Might the possible inhomogeneity take the form of the larger objects being further out, while remaining similar to the smaller objects in shape? (I'm a layperson, and, given that relativity has both "scale invariance" and provisions for spatial expansion, am uncertain about whatever independence those aspects of GR might have from each other.) $\endgroup$– EdouardCommented Oct 24, 2022 at 2:45
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1$\begingroup$ @Edouard sorry I don't know about that $\endgroup$ Commented Oct 24, 2022 at 7:40
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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.
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$\begingroup$ My request for clarification from Andrew Steane was intended to establish whether Nikodem Poplawski's torsion-based inflationary (i.e., expanding at a quasi-exponential rate) cosmological model (which relies on 1929's Einstein-Cartan Theory, rather than the older GR) might be the valid one, as it doesn't require such hypothetical components as an inflaton field or particle: It does, however, require that fermions have spatial extent (which, in our locality, would be a few orders of magnitude above the Planck length). The model's on Arxix & can be found by Poplawski's name, 2010-2021. $\endgroup$– EdouardCommented Oct 24, 2022 at 2:31