DM versus alternative gravity

[notFritzArgelander]

In a study of the radial acceleration relation of 259,000 galaxies DM and alternative gravity are close competitors except that the amount of DM present appears to vary. This supports the reality of particle DM and weighs against alternative gravity.

https://phys.org/news/2021-06-dark-real ... avity.html

[turboscrew]

The plot thickens...

[helicon]

Indeed! I'm reading this article twice.

[Michael131313]

This is very interesting and clearly written. Thanks nFA.

[SkyHiker]

I think the article says that so long as we don't know the amount of gas that surrounds various types of galaxies, we cannot decide between DM or alternative gravity models as it pertains to the RAR problem. And we won't be able to do so any time soon because those measurements are difficult. One of two DM models is incompatible with the measurements, MOND requires the gaseous differences between red and blue galaxies, Verlinde's theory does not yet include those types of galaxies. Sounds like it's still up in the air, not surprisingly. Nice read, nice to see some RUG researchers involved, my alma mater.

[notFritzArgelander]

I think the article says that so long as we don't know the amount of gas that surrounds various types of galaxies, we cannot decide between DM or alternative gravity models as it pertains to the RAR problem. And we won't be able to do so any time soon because those measurements are difficult. One of two DM models is incompatible with the measurements, MOND requires the gaseous differences between red and blue galaxies, Verlinde's theory does not yet include those types of galaxies. Sounds like it's still up in the air, not surprisingly. Nice read, nice to see some RUG researchers involved, my alma mater.

I think that is a side consideration. The paper (https://www.aanda.org/articles/aa/full_ ... 08-20.html) makes clear that corrections due to gas can only be moderate. Your statement that "One of two DM models is incompatible with the measurements" only applies to the simulation codes, not to DM models. There is a difference between particle DM as implemented in the MICE simulator and the BAHAMAS simulator. From the caption to figure 5 of the paper

The discrepancy relative to MICE is caused by a bias in the stellar-to-halo-mass-relation (SHMR) of isolated galaxies in BAHAMAS, which is absent in MICE: BAHAMAS galaxies have stellar masses typically three times higher at fixed halo mass than their non-isolated counterparts.

So the problem isn't even traceable to a difference in DM modeling, it's a difference in how BAHAMAS handles stars.

The conclusion of the paper is notable:

... we find that the lensing RAR is a promising method to be used by future cosmological surveys to distinguish between MG and DM models. This can be done by measuring the RAR including large-scale baryonic mass observations; by simply performing the same comparison with even more accurate lensing and stellar mass measurements; or by further exploring the offset that we have found between the RARs of different galaxy types. All these options require that systematic biases in the stellar and other baryonic mass measurements be reduced.

So they are not proposing a decisive result. yet. Nevertheless the fact that they find a split (based on color) between galaxies that need more or less DM, in conjunction with the other research reporting galaxies which are DM free (and thus falsify alternative gravities) does not bode well for MOND fans.