Chameleon Theory: Detecting the Particle with Astronomical Surveys

[fresh_42]

TL;DR

Does this only sound interesting, or is it a good candidate?

I came across the following paper by Christian Arnold, Matteo Leo & Baojiu LiRealistic simulations of galaxy formation in $f(R)$ modified gravity

Future astronomical surveys will gather information that will allow gravity to be tested on cosmological scales, where general relativity is currently poorly constrained. We present a set of cosmological hydrodynamical simulations that follow galaxy formation in f(R) modified gravity models and are dedicated to finding observational signatures to help distinguish general relativity from alternatives using this information. The simulations employ the Illustris TNG model and a new modified gravity solver in AREPO, allowing the interplay of baryonic feedback and modified gravity to be studied in the same simulation, and the degeneracy between them in the matter power spectrum to be resolved. We find that the neutral hydrogen power spectrum is suppressed substantially in f(R) gravity, which allows this model to be constrained using upcoming data from the Square Kilometre Array. Disk galaxies can form in our f(R) gravity simulations, even in the partially screened regime, and their galaxy stellar properties are only mildly affected. We conclude that modified gravity allows the formation of realistic galaxies and leaves observable signatures on large scales.

https://www.nature.com/articles/s41550-019-0823-y
From what I have read on Wikipedia about the chameleon theory, it only left one question: How are the chances to detect such a particle?

 

[kimbyd]

I'm not sure such particles could ever be detected directly. I believe the primary observable impacts would all be gravitational, and the current lack of detection places very strong constraints on the properties of a hypothetical chameleon particle.

As I understand it, the principle observational evidence would come from structure formation. This may include observations of the cosmic microwave background, the frequency of galaxies of different masses at different redshifts (which I think this paper explores), and observations of the integrated Sachs-Wolfe effect (which is impacted by the rate at which large-scale structures change over time).

 

[fresh_42]

which I think this paper explores

I found the paper as scientific source of a pop science article which said, that they have used a big computer and could show galaxy formation could have taken place under the assumption of chameleon particles. This agrees with your statement about possible evidence, and made me curious. If it can explain DE and still leads to the same structures on a cosmological scale while simultaneously allowing GR to be accurate on a local scale, then it reads as if it is what we are looking for. But if it does all that and we have no chance to test it, then what is it worth?

 

[PAllen]

You might be able to test indirectly. There are no direct quark detections, for example.

 

[kimbyd]

I found the paper as scientific source of a pop science article which said, that they have used a big computer and could show galaxy formation could have taken place under the assumption of chameleon particles. This agrees with your statement about possible evidence, and made me curious. If it can explain DE and still leads to the same structures on a cosmological scale while simultaneously allowing GR to be accurate on a local scale, then it reads as if it is what we are looking for. But if it does all that and we have no chance to test it, then what is it worth?

I didn't say no chance to test it. I said little to no chance to directly measure the particle, which isn't the only way to build evidence for a theory.

If structure formation can be shown to be explainable through this theory but hard to fit with other theories, that might provide evidence in its favor. The reality is that this is often hard to do, as the details of structure formation are extremely complex and difficult to accurately model. But it's at least possible in principle for the history of structure formation to be easily fit by this kind of model but basically impossible to fit with a dark energy model that doesn't contain modified gravity.