[podiki]

In classic internet fashion I only did a quick skim/search of the article and preprint of the paper (linked in this article). However, neither make mention of the bullet cluster [0] or "particle dark matter" and seem to rely more on galaxy rotation curves.

While the bullet cluster may not be as definitive as we would like, it is at the very least very strongly suggestive of particle dark matter or something similar. As far as I remember modified gravitational theories without particle dark matter don't offer a good explanation of the bullet cluster (in short, evidence of a non- or weakly interacting but gravitational matter).

I won't argue with schemes like MOND or this proposal that can explain rotation curves, but the bullet cluster at the very least is a more interesting test these days. Without even a mention of some alternative explanation here (from my quick look), it is a non-starter for a dark matter-less theory. And a pretty glaring omission (again, maybe I missed it) for what is a well-known observation.

Still, alternatives for quantum gravity, and anything that might say something about dark energy, are exciting. Falsifiable ones especially. But if you didn't address the bullet cluster then...

[0] https://en.wikipedia.org/wiki/Bullet_Cluster

[podiki]

After a little more of a skim: the paper seems mostly to be about reproducing an equation like that of MOND (modified Newtonian dynamics). Which, fine, some people are fans of MOND (not most, and for good reason from the theoretical which this might address, to the practical like previous comment), but there's a whole lot more than just the dynamics of rotating galaxies to consider. Without even going into that (bullet cluster, lensing, n-body simulations, galaxy formation, etc.) it doesn't (yet?) replace anything.

[sigmoid10]

Most important is still the CMB. Back then everything was ionized and yet we have extremely good evidence of matter that only contributed gravitationally to density perturbations and not via the ordinary electromagnetic path. There is no way to explain this merely by modifying the law of gravity. So even if MOND or similar approaches turn out to be correct, they can never explain all aspects of dark matter. Only particles can.

[raattgift]

Sure, but the origin of this preprint is in the stochastic-classical/uncertain-quantum "postquantum-classical" theory the Oppenheim group is working on. Their theory is premised on a weakening of the gravitational interaction at short distances (it's "asymptotically free" and thus renormalizable) compared to perturbative quantum gravity (which is non-renormalizable by power counting). However, they discovered that the gravitational interaction in their theory can be too strong. More technically, in their diffusion limit they extract a different scalar potential than from GR's weak field limit, with the former growing (statistically) stronger with increasing radius. This in practice means even with a highly similar source, their theory predicts significantly different geodesics compared to General Relativity.

Rather than abandon the theory or complicate it such that it becomes compatible with fully classical GR, they solved a Schwarzschild-de Sitter (static spherical symmetry and an expansion term which "washes out" the extra-strength gravitational potential before it gets too strong) spacetime with a galactic mass (described simply and with some restrictions) and found that the geodesics are within a standard deviation or so of stable circular MONDian orbits. That result would relieve some of the immediate concern that their theory is unphysical, and drive them towards further study of its large-length-scale behaviour.

(In fact their result is better than some earlier attempts to make a generally covariant MOND by adding auxiliary gravitational fields to GR (see the overview by Famaey & McGaugh 2012). Postquantum-classical gravity was on a relativistic footing from the start, although exactly how diffeomorphism invariance and manifest covariance of formulations works in postquantum-classical gravity is really interesting, see <https://arxiv.org/abs/2402.17844> if you want details)

It would be interesting to see how their theory does does -- it would have to be studied numerically -- with any sort of CDM halo. Various stellar binaries are also an obvious place to look, since for sufficiently wide ones, you can't "wash out" the extra strength of the scalar potential with an expansion term. (Wide binaries are really rough on MOND too, for comparable reasons).

> it doesn't (yet?) replace anything

The authors know that. From just after Eqn (23) in the preprint <https://arxiv.org/abs/2402.19459>:

"While this study demonstrates that galactic rotation curves can undergo modification due to stochastic fluctuations, a phenomenon attributed to dark matter, it is important to acknowledge the existence of separate, independent evidence supporting ΛCDM. In particular, in the CMB power spectrum, in gravitational lensing, in the necessity of dark matter for structure formation, and in a varied collection of other methods used to estimate the mass in galaxies"

[naasking]

Milgrom's perspective on the Bullet Cluster: overblown and completely expected.

http://astroweb.case.edu/ssm/mond/moti_bullet.html

[podiki]

I won't get into an argument about MOND. Don't take my word for it, take the vast majority (all but a few?) of anyone working in the field. And don't take Milgrom's word for it (or the one reference given) either, for that matter, as clearly he has a horse in the game. Sure you can continue to make MOND work to some degree, but is it really predictive if you keep adding things to make it work? What is the underlying theory? (The proposal in this linked article is interesting in that regard.)

Okay, so maybe I did continue the argument a little :) I love for people working on alternative less mainstream ideas. But MOND was never very alive to begin with, let alone decades later. Let's move on.

[zmgsabst]

> Sure you can continue to make MOND work to some degree, but is it really predictive if you keep adding things to make it work?

Do you hold dark matter theories to this standard?

[podiki]

Absolutely. I'm out of the field these days, but e.g supersymmetric WIMP dark matter was the most popular but got more and more clugely with low energy supersymmetry seeming less likely (or not as useful in solving the problems it meant to solve). Everyone would come up with some tuned ad hoc model for whatever dark matter "signal" was in fashion (before being ruled out); none of those were compelling. Fun to play with maybe, but didn't really tell us anything.

[naasking]

> What is the underlying theory?

Who cares? This has literally never been a requirement of science and I don't know why people bring it up for MOND. What was Isaac's Newton's underlying theory when he proposed his law of gravitation? No explanation for why two masses attract, therefore we should reject an effective description? That's nonsense.

A theory is scientific if it describes what we observe, period. If you have a deeper explanation right off the bat, that's a bonus, otherwise that's the goal of further research.

The other poster adequately addressed the implicit hypocrisy applied to MOND vs. LCDM, so I'll just leave this reference for a proper analysis of who has been making predictions vs. tweaking their theory to fit observations:

From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity, https://arxiv.org/abs/2110.06936

[podiki]

Fine, rather than "underlying theory" use some other words, basically does any new proposal give us predictions, solve some unexplained phenomenon, do something simpler and more compelling (sure, a matter of taste, but so is all of this to some degree), in short, does it tell us something new. Newton certainly did as he gave a law to describe and predict successfully the motions of the planets, for instance. I don't think we are saying anything different really. There wasn't a predicitve framework before (as far as I know), but then there was, so that is certainly progress. But the bar gets ever higher as we know more for what a theory should do. Just writing down an equation that can fit some data and can make predictions is great but is not the end of the story. We could just have some arbitrary functions that fit the data we have and call it a day.

[throwawaymaths]

Yes. Sone researchers went looking for the absence of EFE to disprove MOND and wound up finding it instead. MOND has predictive power. Moreover, we keep finding that lenticular and dense elliptical galaxies have "no dark matter", which is a prediction of MOND.

One of the reasons why things "keep getting added" to MOND is that it's known to be incomplete. The equations to reconcile MOND with relativity are not known and are tougher (but not necessarily impossible) than the Newtonian equations.

So it's expected that things will have to be added. That the equations are tricky for us mere mortal humans should not be held against it; the universe has no requirement that equations must be easy to solve.