[Sniffnoy]

My understanding is that, though commonly repeated, this is not actually true. The thing is that modified gravity (MOND) theories modify gravity by introducing a new field(s); this means that, as with dark matter, it is possible for this new field to be concentrated away from the normal matter. Quoting from here: https://backreaction.blogspot.com/2017/01/the-bullet-cluster...

> Isn’t it obvious the visible stuff is separated from the center of the gravitational pull? But modifying gravity works by introducing additional fields that are coupled to gravity. There’s no reason that, in a dynamical system, these fields have to be focused at the same place where the normal matter is. Indeed, one would expect that modified gravity too should have a path dependence that leads to such a delocalization as is observed in this, and other, cluster collisions.

Indeed since any new dark matter particle would also be a new field, dark matter theories vs modified gravity theories are actually not that different -- both add new fields, the difference is just what kind. I mean OK obviously what kind matters a lot, but the point is that the difference isn't quite as large as it's currently made out to be, and things like the Bullet Cluster that show a matter-vs-gravity distinction aren't the slam-dunk evidence against modified gravity that they're commonly claimed to be.

[mannykannot]

I may well be mistaken, but I don't think MOND started off as being a theory in which "there’s no reason that, in a dynamical system, these fields have to be focused at the same place where the normal matter is." In fact, I thought the original motivation for MOND was to preserve just that, and if so, then if it has become, as a result of challenges such as this, unable to preserve that simplicity, then it has, in one sense, already failed.

More significantly, however, and regardless of whether the point about some galaxies behaving as expected is a problem for MOND, the issue here is whether they are a problem for the resolution posited in the paper under discussion.

[Sniffnoy]

Sure, original MOND no, but original MOND is not an actual theory of gravity. People try to make actual theories of modified gravity that replicate MOND's predictions, but you can't actually just use MOND as a theory of gravity. These MOND-like theories of modified gravity then include extra fields.

[mannykannot]

I am not sure I get your drift, as your post prompted some questions: What's the motivation for developing a theory that replicates MOND's predictions? Isn't the goal of science to produce theories that predict or explain what is observed? And if original MOND did not go so far as to be an actual theory of gravity, what predictions was it making?

My guess is that your point here is something like this: MOND did not start as a specific theory, but perhaps something more like a principle, or a question along the lines of "can the anomaly in galaxy rotation be explained without introducing a new type of matter?" (Though by the time I heard of MOND, it took the form of a specific proposal that made specific predictions.)

If this was any other topic than gravity - electrodynamics, for example - the introduction of extra fields, would, I believe, automatically imply additional particles, but I suppose that, without a quantum theory of gravity, this is not necessarily the case here.

So, one can certainly say that modified gravity too could have a path dependence that leads to the delocalization observed in cases like the Bullet Cluster, but MOND still needs a theory which predicts (or at least explains) those observations. This does not go away just because it is facing even greater challenges in getting the early universe right.

None of the above should be construed as a claim that dark matter exists. Also, the more relevant point that I made in the second paragraph of my original post here is independent of any of this, I think.

[Sniffnoy]

I think you're just mistaken as to how complete of a theory MOND proper has ever been. See e.g.: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Ov...

Like, if you pick a specific interpolation function and value of a_0, and say OK this only applies to gravity (and rewrite the equation appropriately), then I guess this would be a theory of gravity, but it's one we'd know to be wrong, as it's just a simple modfication of Newtonian gravity that doesn't account for relativistic effects at all. It would presumably also violate conservation laws all over the place. The problem then is to come up with a proper theory of gravity that replicates both GR and MOND (and presumably have giant noticeable failures of conservation all over the place). There have been a number of attempts at this.

Or just see Wikipedia: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Ov...

> By itself, Milgrom's law is not a complete and self-contained physical theory, but rather an ad hoc empirically motivated variant of one of the several equations that constitute classical mechanics. Its status within a coherent non-relativistic hypothesis of MOND is akin to Kepler's Third Law within Newtonian mechanics; it provides a succinct description of observational facts, but must itself be explained by more fundamental concepts situated within the underlying hypothesis. Several complete classical hypotheses have been proposed (typically along "modified gravity" as opposed to "modified inertia" lines), which generally yield Milgrom's law exactly in situations of high symmetry and otherwise deviate from it slightly. A subset of these non-relativistic hypotheses have been further embedded within relativistic theories, which are capable of making contact with non-classical phenomena (e.g., gravitational lensing) and cosmology.[13] Distinguishing both theoretically and observationally between these alternatives is a subject of current research.

[mannykannot]

Well, I guess I did think there is more to MOND than this. As you present it here, it does not seem to advanced to the point of being an evaluable theory, despite being first proposed almost forty years ago.

I'm still confused by your statement of the goal: "The problem then is to come up with a proper theory of gravity that replicates both GR and MOND." Surely the problem is to come up with a theory - any theory - that is a) consistent with observations, and b) makes testable predictions?

[mannykannot]

Ah, I think I get it: The problem in advancing MOND is to come up with a proper theory of gravity that is compatible with all the evidence supporting GR and which gives rise to the dynamics as hypothesized in MOND to explain the observed anomalies?

If that's the point then I agree, but isn't MOND having some difficulty coming up with a single dynamics that works in all cases?

Meanwhile, dark matter is having some difficulty in finding a particle that works at all.

[andrewflnr]

> The real challenge is to get the early universe right, to explain the particle abundances and the temperature fluctuations in the cosmic microwave background.

This has been my impression for a while. I won't be surprised if dark matter stops being necessary for galaxy rotation curves but sticks around to explain the CMB spectrum.

[mr_toad]

> as with dark matter, it is possible for this new field to be concentrated away from the normal matter.

That sounds like dark matter with extra steps. If normal matter isn’t producing the field it’s just another unknown phenomenon.

[Sniffnoy]

In what way is it extra steps, as opposed to the same number of steps?