Stacy McGaugh, an astronomer, writes about dark matter versus modified Newtonian dynamics (MOND), galaxy rotation curves, and JWST predictions.
Commenters find his MOND arguments convincing and clear, and cite his prediction of what JWST would see.
Famaey & McGaugh's review <https://arxiv.org/abs/1112.3960> (Stacey McGaugh is <https://tritonstation.com/>, a frequent critic of particle dark matter) §7 is all about adding whole new fields ("parameters") to standard General Relativity in order to capture higher-order terms in post-Newtonian expansions (in 1/c^n or in the metric tensor) of theories that preserve the central characteristics of Milgrom's MOND.
Those characteristics are: no "non-luminous" or "hidden" matter sources -- the glowing, refracting, and obscuring dusts are the sole sources of gravitation -- and in circumstances where post-Newtonian corrections are vanishingly small, strong concordance with Newton and Kepler except at the lowest accelerations.
The orbits of galactic blobs of slowly-moving molecular gas that produce characteristic spectral lines amenable to Doppler redshift study are almost always in those special circumstances, and they have higher accelerations "corewards" in galaxies and low accelerations "outwards" in galaxies. These gas blobs are most interesting at the leading and trailing limbs of edge-on discoid/spiral galaxies, or face-on in large elliptical galaxies with negligible bulk rotation (the Doppler-shifting blobs move in and out, almost entirely radially), precisely where the accelerations are smallest.
The question we then ask is: what if we have a blob or some other spectrally well-defined object in a galaxy is on a very fast orbit? We can no longer ignore post-Newtonian…
> It's quite tiresome to read the same objections as if physicists somehow hadn't thought of them.
Okay now do climate change and climatologists.
And most people just don't bother to read the article at all it seems. No better than reddit.
Changing gears, though, and trying to be more useful, if you want more coverage on this topic there's a note in Peter Woit's blog:
https://www.math.columbia.edu/~woit/wordpress/?p=13232
Points to another blog which attacks the "nightmare scenario":
https://tritonstation.com/2022/11/29/the-angel-particle/
I don't particularly buy that a gravity-only interacting particle "is not physics". If that is what exists, that is what exists. And we've already detected it via dark matter observations. So it is physics. What we can't do is ever really prove it other than by exclusion, but if we racked up a thousand years of failure, we'd have to accept that it was real (hopefully by then not dogmatically and we'd let any kid who wanted to tilt at windmill to go for it).
And Woit links to a paper on dark matter being only a right handed majorana neutrino, requiring no real physics beyond the standard model (plus three right handed non-interacting neutrinos): https://arxiv.org/abs/1803.08930
And links to a comment on that in John Baez's blog:
https://golem.ph.utexas.edu/category/2022/12/neutrino_dark_matter.html
> "Couldn't there exist different laws of gravity for big things like planets and small things like rocks, which explains why I've never seen rocks attracted to each other?"
Something very related is a topic that is seriously discussed among physicists:
> https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
The problem with the existing theories of gravitation which seem to work quite well on the scale of solar systems is that there does seem to exist evidence that some observed data on the scale of galaxies might not be well-explainable by these theories. See
> https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Observational_evidence_for_MOND
Very recently new evidence for MOND came up:
> https://phys.org/news/2023-08-smoking-gun-evidence-gravity-gaia-wide.html
> https://news.ycombinator.com/item?id=37051241
See also:
> https://tritonstation.com/2023/08/02/is-ngc-1277-a-problem-for-mond/
> https://news.ycombinator.com/item?id=37009037
Fascinating! I dug into the Wikipedia article, which cites a Scholarpedia article; the LLM answer seems to originate from a reference to this sentence [1]:
> So, MOND reduces the discrepancy in clusters at these radii to only a factor of ∼2−3 (Sanders, 1999; Ettori, et al., 2019)
So I think you're right, and today I learned something! I also checked if Stacy McGaugh had weighed in on this particular subject, and it seemed like there is still an issue for clusters [2], although interestingly the issue isn't mentioned in his latest blog post that summarizes the strengths/weaknesses with MOND [3]. Anyway, thanks for humoring me for a bit.
[1] http://www.scholarpedia.org/article/The_MOND_paradigm_of_modified_dynamics#Galaxy_clusters
[2] https://tritonstation.com/2021/02/05/the-fat-one-a-test-of-structure-formation-with-the-most-massive-cluster-of-galaxies/
[3] https://tritonstation.com/2023/06/27/checking-in-on-troubles-with-dark-matter/
I follow the lead author, Stacy McGaugh, via his blog where he posts discussions and musings about the latest research into the dark matter vs MOND debate: https://tritonstation.com/new-blog-page/
His arguments are very convincing and relatively clear. I am not an astrophysicist but I have two degrees in physics and have always found the dark matter theory to be lacking -- in absence of any evidence of causation whatsoever, dark matter can only be described trivially as "where we would put matter if we could to make our theory of gravity make sense," which is totally backwards from a basic scientific perspective.
Predictions based on modern MOND postulates are shown to be more and more accurate as our observational instruments continue to improve in sensitivity.
My hangup with MOND is still general relativity. We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold. Any model of gravity based on an inverse law is simply wrong.
Another comment linked to https://tritonstation.com/new-blog-page/, which is an excellent read. It makes the case that GR has never been tested at low accelerations, that is might be wrong. But we know for a fact MOND is wrong at high accelerations. Unless your theory can cover both, I don't see how it can be pitched as an improvement to GR.
Edit: this sounds a bit hostile. to be clear, I think modified gravity is absolutely worth researching. but it isn't a silver bullet