I Distinguishing between the effects of dark matter and MOND

[ohwilleke]

"MOND" as generally used corresponds to a whole family of theories. Either side of F=ma can be modified. You might get more force at low accelerations, or you might get more acceleration for the same force (which might or might not be restricted to gravity). We could call these models MOND. MONG and MONI. But we don't.

While there are a number of MOND theories, that acronym is not generally used to refer to the whole universe of gravity based explanations of dark matter phenomena or modifications of gravity. It is one specific approach centered around a transition at the physical constant a₀.

It is not used to describe the work of Moffat or Verlinde or Deur or any other number of theories that don't have the basic outlines of Milgrom's work built into them directly. I have never seen those theories included within the term MOND in the literature.

Admittedly, there is a lot of the literature that is oblivious to the fact that there are gravitational approaches to describing dark matter phenomena other than the one that Milgrom came up with first which is the most studied one. But, that is a very different thing than saying that specific non-Milgromian gravity modification theories are part of MOND.

For example, the paper Federico Lelli, et al., "Cold gas disks in main-sequence galaxies at cosmic noon: Low turbulence, flat rotation curves, and disk-halo degeneracy" arXiv:2302.00030 (January 31, 2023) (Accepted for publication in Astronomy and Astrophysics) states:

Milgromian dynamics (MOND) can successfully fit the rotation curves with the same acceleration scale a₀ measured at z≃0.

Similarly, MOND and Moffat's theories are distinguished from each other (even though both explain DM-phenomena with gravitational modifications) in Yongda Zhu, et al., "How Close Dark Matter Halos and MOND Are to Each Other: Three-Dimensional Tests Based on Gaia DR2" arXiv:2211.13153 (November 23, 2022) (accepted for publication in MNRAS).

MOND is distinguished from Verlinde's theories here.

"Modified gravity" is a general and non-theory specific term, but MOND is not.

 

[Ken G]

Still, now that I understand you are taking a strict meaning of MOND, I can better understand how you are summarizing the landscape. In part from what I've seen, and in part from your summary, it seems that we are as far away from a new theory of gravity to add to relativistic and quantum dynamics as we are from a new set of dark matter particles to add to the particle zoo. One can see why there is general favoritism to simply finding the "dark matter particle", since then we just have the attributes and interactions of that particle to understand, much like the discovery of the neutrino, and we can otherwise maintain the existing dynamical machinery. If it's MOND, then the particles stay the same, but the complete dynamical machinery must change! That's a lot of overhead. I have faith in the process of science, that this monumental task will eventually be undertaken successfully, but I haven't much faith it will happen any time soon, so I doubt I will see it. If it takes a century, none of us will!

On the other hand, one of the things that does seem to offer promise to be resolved in the foreseeable future is "precision cosmology." Some claim it's already here, but I am not among them, not as long as their remains tension in the determination of the Hubble parameter. But if I'm an optimist, and I think the next round of cosmological observations will line everything up nicely as things appear to be turning out, then we are going to be looking at dark matter and dark energy as the key elements of cosmological dynamics. As long as that continues to be true, then dark matter is pretty much here to stay, regardless of any successes of MOND. As I said above, it may turn out that MOND is only used as a convenience, a way to parametrize the effects of dark matter in galaxies because it is easier to use than some whole new paradigm for equipping galaxies with dark matter and its interactions. Or it might turn out that dark matter is only used as a convenience in cosmology, a way to parametrize some unknown dynamical effect on the expansion of the universe. An uneasy peace, perhaps, and maybe only a period of "cheating" as alluded to above, but that does seem to be where we are, and I'm not sure I see that situation changing any time soon. But who knows?

Then of course, there is also the very nonnegligible chance that we are completely missing something, and it is neither MOND nor dark matter. That will require thinking outside the box, maybe one of the alternatives that has been mentioned or something completely new. The one thing that has always been true of scientific thinking is it tends to overestimate who close it is to reaching the complete story!

 

[ohwilleke]

Still, now that I understand you are taking a strict meaning of MOND, I can better understand how you are summarizing the landscape. In part from what I've seen, and in part from your summary, it seems that we are as far away from a new theory of gravity to add to relativistic and quantum dynamics as we are from a new set of dark matter particles to add to the particle zoo. One can see why there is general favoritism to simply finding the "dark matter particle", since then we just have the attributes and interactions of that particle to understand, much like the discovery of the neutrino, and we can otherwise maintain the existing dynamical machinery. If it's MOND, then the particles stay the same, but the complete dynamical machinery must change! That's a lot of overhead. I have faith in the process of science, that this monumental task will eventually be undertaken successfully, but I haven't much faith it will happen any time soon, so I doubt I will see it. If it takes a century, none of us will!

On the other hand, one of the things that does seem to offer promise to be resolved in the foreseeable future is "precision cosmology." Some claim it's already here, but I am not among them, not as long as their remains tension in the determination of the Hubble parameter. But if I'm an optimist, and I think the next round of cosmological observations will line everything up nicely as things appear to be turning out, then we are going to be looking at dark matter and dark energy as the key elements of cosmological dynamics. As long as that continues to be true, then dark matter is pretty much here to stay, regardless of any successes of MOND. As I said above, it may turn out that MOND is only used as a convenience, a way to parametrize the effects of dark matter in galaxies because it is easier to use than some whole new paradigm for equipping galaxies with dark matter and its interactions. Or it might turn out that dark matter is only used as a convenience in cosmology, a way to parametrize some unknown dynamical effect on the expansion of the universe. An uneasy peace, perhaps, and maybe only a period of "cheating" as alluded to above, but that does seem to be where we are, and I'm not sure I see that situation changing any time soon. But who knows?

I would say that the balance has already tipped. Lambda-CDM which is the paradigm is dying a death of several dozens serious cuts, if not the thousand of the Chinese proverb.

Lots of the barriers to acceptance of modified gravity approaches have been overcome now that some modified gravity theories can explain galaxy clusters and the Bullet Cluster, and can reproduce the Cosmic Microwave Background. The failure to dark matter particle theories to predict the early formation of galaxies observed by the James Webb Space Telescope (JWST) has also helped tip the balance.

Another factor tipping the balance is the reality that you need not just a new particle but also a new force somewhat similar to a gravity modification to describe what is observed in a dark matter particle paradigm.

One of the particularly notable efforts to root MOND's phenomenological fits into a deeper theory with a full range of applicability and not just a phenomenological fit with a limited domain of applicability, has been the work of Alexandre Deur who has devised a gravitational theory to achieve that. Deur claims (and others dispute) that it is merely a correct implementation of GR that implements it in a way the considers non-perturbative effects usually ignored. But even if Deur's wrong and his equations really do modify GR, they modify GR in a way that addresses the CMB, galaxy formation timing, clusters, the bullet cluster, the dependence of inferred dark matter amounts on galaxy shape, etc. in a way that has fewer free parameters than GR with dark matter, a single field, no dark energy, conservation of mass-energy at a global level, and a capacity to make predictions that unduly free DM theories struggle to. Deur proposes that this comes from a second order gravitational field self-interaction effect that falls off more slowly than the first order Newtonian one does with distance in mass distributions which are more disk-like than spherically symmetric or are two point systems - using the geometry of the mass distribution to bridge the issues that MOND struggles with, in close analogy to QCD (the quantum theory of the strong force that binds quarks and gluons together). In strong gravitational fields where GR deviations from Newtonian gravity are clear, these second order effects are dwarfed by the first order effects and imperceptible. But in very weak fields at very great distances, the slower falloff of the second order effects relative to the first order Newtonian-like effects in weak field GR eventually become stronger than the first order effects and become noticeable.

There is really no DM particle theory out there which fits such a broad range of data.

Neither self-interacting dark matter, nor warm dark matter (which involves collisionless DM at the keV order of magnitude mass threshold where it starts to be wave-like), which were once quite popular, solve some of cold dark matter's most serious problems. See, e.g., Mark R. Lovell, et al., "Local Group star formation in warm and self-interacting dark matter cosmologies" arXiv:2002.11129 (Feb. 25, 2020) (accepted by MNRAS); Isabel M.E. Santos-Santos, et al., "Baryonic clues to the puzzling diversity of dwarf galaxy rotation curves" (November 20, 2019) (accepted for publication in MNRAS) (also noting the failure to SIDM to reproduce what is observed); and Alyson Brooks, "Re-Examining Astrophysical Constraints on the Dark Matter Model" (July 28, 2014) (published in Annalen der Physik) (noting that SIDM and warm dark matter has most of the same problems as cold dark matter theories, as also shown in this paper by the same author together with a co-author which was published in MINRAS).

Axion-like particle (ALP) dark matter and similar ultra-low mass fuzzy dark matter theories, mostly proposing bosonic dark matter particles with masses on the order of 10^-23 eV plus or minus a couple of orders of magnitude (which approach the same ballpark as the mass-energy of a graviton of typical frequencies) seem to do better so far, but are relatively new and less well vetted at this point. CERN recaps some of the basics of these theories.

 

[Ken G]

That's quite an interesting take, you are either way ahead of the curve on this, or buying off on speculative results that are not as well vetted as you claim. I don't know which, but either is still interesting, and more promising than the perspective I expressed above!

 

[ohwilleke]

That's quite an interesting take, you are either way ahead of the curve on this, or buying off on speculative results that are not as well vetted as you claim. I don't know which, but either is still interesting, and more promising than the perspective I expressed above!

We certainly don't have a final answer yet.

But optimism that we will get there is warranted, because of the torrent of new astronomy data that we are collecting, and our increasingly computational capacity to make sense of it.

Also, importantly (because there is no saving the satisfied scientist), in astronomy, it is widely understood that we don't have a model the neatly and cleanly describes all observations the way that the Standard Model of Particle Physics (deservedly) is recognized for doing in high energy physics. Everyone agrees that we need more data and that it is challenging to make the existing data (e.g. the value of Hubble's constant) mutually consistent.

There are hundreds of new astronomy papers each week, most of which report on new data, while there are only dozens of new high energy physics experimental data papers each week, dominated by four or five major colliders. The astronomy community is also more fractured than the high energy physics community, because new data is coming from more comparatively small research groups than in HEP, so it isn't quite as vulnerable to groupthink.

There is room for challenges to the Lambda CDM paradigm to develop, and this is happening on multiple fronts. Dark matter particle theories are still in the majority, but are also fractured themselves into many competing variant theories.

The down side of the current situation, however, is that because there are so many papers out there, everyone is looking only at their own little corner of their own research interests. Few scientists are looking at the big picture. So, quite a bit of research is done on proposals that existing work in some other subfield using different methodologies already strongly disfavors. It may take a generation or so for the constraints of existing research to really cross-pollinate across the larger discipline of astrophysics.

 

[Ranku]

I mean what I wrote. The amount and distribution of DM in rotationally supported galaxies can vary, but it always varies to match MOND.

Ok, got it now.

 

[Ken G]

I hope I see significant advances in this big picture. I guess the question is, when is the appropriate time for someone to expand on what you wrote above and deliver the "current state" of that picture? If one does it now, it will probably have to end with the need for more data, but if one waits too long, the reaction will be "yeah everyone knows that except the hangers-on."

 

[Vanadium 50]

I think you need more information, and good information - not just nibbling at the edge cases.

Suppose one of the supercomputing modeling types were to say that her DM modes reproduce MONDy behavior. Too much DM and you get pressure-supported galaxies not rotationally-supported. Too little and you never trigger star formation. Oh. and by the way, Tully-Fisher pops out too. That would be another,

Or suppose an LHC experiment - or LZ - detects DM. That would be another thing.

Guessing "when" sort of requires guessing "what".

 

[Ken G]

Yes that's another way to frame it, we could ask the question, what is the discovery that would settle the matter?

If someone detects dark matter decay of some kind, then the MOND world can just say, all that has happened is some new particle has been discovered, not the first time that's happened. It doesn't resolve anything unless the amount of it can be connected with the right amount required, and how does one do that when all one sees is decay without knowing the decay rate?

And if someone shows that a particle can be postulated that would do what dark matter needs to do, but there is no evidence that such a particle actually exists, then we have Dirac's neutrino suggestion without the Cowan-Reines experiment. The inability to manipulate astrophysical sources in the laboratory will likely continue to present significant challenges to resolving the issue.

After all, Aristarchus proposed the heliocentric model before 200 BC, and it wasn't until Galileo,18 centuries later, that scientists found a way to confirm it!

 

[ohwilleke]

Yes that's another way to frame it, we could ask the question, what is the discovery that would settle the matter?

No one discovery will "settle the matter" and even several significant discoveries taken together will only greatly winnow the field of possibilities.

But various kinds of discoveries would rule out lots of theories and narrow the parameter space dramatically for other kinds of theories (as would simply having wider diffusion of existing experimental results that aren't universally known in a systemic way).

Often different tests rule out different parts of the parameter space of particular theories.

Examples of Existing Often Piecemeal Constraints

For example, in the case of primordial black hole dark matter candidates several different constrains are pieced together to get our current parameter space, (1) Hawking radiation and the age of the universe is used to theoretically rule out very small primordial black holes, (2) the absence of gravitational lensing in seemingly empty space constrains how much dark matter can be in the form of large primordial black holes, and (3) the dynamics of rocky objects in the solar system is an important tool to constrain how much dark matter can be in the form of asteroid mass primordial black holes.

For collisionless dark matter candidates, heavy dark matter particles are ruled out by (1) the fact that galaxies don't universally have cuspy inferred dark matter halos and (2) wave-like dynamics in inferred dark matter.

The existence of observed levels of galaxy structure rules out collisionless thermal freeze out dark matter particles of less than about ca. 10 eV (which would be "hot dark matter").

Dark matter candidates that interact via the weak force with the same "weak force charge" as all other particles that interact via the weak force is ruled out (1) from about 1 GeV to 1000 GeV by direct dark matter detection experiments (which themselves combined several different kinds of parameter space exclusions from different kinds of direct dark matter particle detection experiments), and (2) for all masses below about 45 GeV (half of the Z boson mass) by W and Z boson decays. In a somewhat smaller mass range near the electroweak scale (tens and low hundreds of GeVs), dark matter particles with "milli" and "micro" weak force charges are also pretty much ruled out.

Dark matter particle candidates that interact with the Higgs boson with a Yukawa coupling proportional to rest mass (like the Standard Model interactions of Standard Model particles with the Higgs boson) are strongly disfavored in a mass range of about 2 GeV to 62 GeV because such a particle would throw off the predicted decay fractions of Standard Model particles that are reasonably close to the predicted values by easily detectible amounts.

Dark matter candidates in the meV to 10 eV mass range or so, are disfavored by astronomy based N_eff measurements (i.e. estimates of the effective number of neutrino species).

The Bullet Cluster did not, as some people claim, rule out all modified gravity theories, but it is a big problem for many simple, single field, spherically symmetric modified gravity theories like bare bones toy-model MOND theories where the effect is unrelated to the geometry of the matter distribution. It also poses different kinds of problems for dark matter particle theories because galaxy cluster collisions like the Bullet Cluster are far too common in the observable universe relative to LambdaCDM cosmology predictions of their relative speeds and frequencies.

Potential New Discoveries

As noted above, more precise data on wide binary star dynamics can provide a key generic constraint on lots of possible dark matter and modified gravity theories.

An observation of a "no dark matter" low surface brightness dwarf galaxy isolated from other galaxies, if made, would be an important constraint on many possible theories (both dark matter particle theories and modified gravity theories). Dark matter particle theories and MOND-like theories, however, both have plausible explanations for "no dark matter galaxies" near other more massive galaxies (tidal stripping for dark matter particle theories, and the external field effect in MOND).

Better modeling of the gravitational feedback of ordinary matter on dark matter distributions (which is just on the brink of being precise enough to be useful) is another method by which a lot of theories could be favored or ruled out, and by which parameter spaces could be greatly constrained for the remaining theories. This is one of the main fudge factors in existing supercomputer modeling of dark matter theories.

Larger data sets of the alignment of satellite galaxies in the plane of their primary galaxies for spiral galaxies would be useful in discriminating between theories that are spherical symmetric and those that can have asymmetrical effects.

Currently, the observed scatter in the Tully-Fischer relationship has a magnitude consistent with being entirely due to measurement error. As the measurements used to make those comparisons grows more precise with more data from better telescopes, the extent to which there is or is not intrinsic scatter as opposed to mere measurement error is important. Less scatter favors modified gravity theories. More scatter favors dark matter particle theories.

More data from the James Webb Space Telescope and instruments like EDGES (21 cm wavelength observations) of the very early universe is going to tightly constrain structure formation models which will require a major purge of dark matter particle theories.

If someone detects dark matter decay of some kind, then the MOND world can just say, all that has happened is some new particle has been discovered, not the first time that's happened.

While not strictly speaking impossible, this possible experimental breakthrough is way down my list of likely possibilities in the foreseeable future.

Dark matter particle candidates that have any kind of significant Standard Model particle interactions (which it would have to in order to have detectible decay products or to be found in a collider) are pretty much ruled out up to the mid-hundreds of GeVs to the low single digit TeV mass scale depending upon the candidate from collider factors, cosmic ray observations, and direct dark matter detection experiments. But, as noted above, a variety of factors disfavor dark matter particle candidates heavier than the ranges that are already ruled out by these means.

There are a variety of reasons (beyond the scope of this thread) to think that new undiscovered particles are unlikely to be found in the next generation of colliders or astronomy "telescopes" (using the term broadly).

The True Nightmare Scenario

One very real possibility, more likely than a dark matter particle candidate showing up at a particle collider, is that our observations will become over constrained.

In other word, we might end up with data that rule out every dark matter particle theory and every modified gravity theory and every hybrid of the two that we can imagine that work with no limitations on their domain of applicability.

If this happened, we'd have to figure out which constraint we thought was a real constraint actually has some kind of loophole.

 

[Ken G]

Maybe that last possibility is not really a nightmare after all. The piecemeal approach to winnowing the theories that you describe above is certainly good science, but is a kind of nightmare scenario of its own, because how many great discoveries in the history of science occurred that way? We could say that the quest to know the neutrino mass, or find the Higgs particle, were winnowing processes, but in both cases we had good reason to believe those particles would be the solutions and indeed they were, pinning down their masses and patting ourselves on the back for anticipating that success is not really the kind of breakthrough discovery we are talking about.

Instead, the most significant breakthroughs tend to happen when we realize that something we thought we could take as secure turned out to be the thing that was wrong. In other words, there was some kind of blockage that, once removed, created an opportunity for a much simpler solution to the problem! I'm sure you can think of examples of "outside the box breakthroughs" as easily as I can, but I would throw out some of the biggest in the history of astronomy (the heliocentric model, unblocked by realizing that stars should not show parallax because they are ridiculously far away, the Big Bang model, unblocked by realizing that the universe could have an origin that was outside of our physics, the age of the Sun, unblocked by realizing matter could contain a vast source of fusion energy, etc.). Maybe your nightmare scenario is just what we need right now!