Distinguishing between the effects of dark matter and MOND

[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!