Exploring the Troubles with Dark Matter and the Quest for a Theory of Everything

[strangerep]

[Moderator's note: Spin off from another thread due to topic change.]

[...] I believe that relativistic QFT augmented by canonically quantized gravity (with its infinitely many regularization parameters, which are probably an artifact of perturbation theory), and possibly another scalar field for dark matter, is the theory of everything.

You need to get out more.

The observationally confirmed existence of a universal acceleration scale $a_0$ (Milgrom's constant) in various disparate situations shows that we are still very far from a satisfactory understanding of astrophysics on galactic and cosmological scales, and hence very far from a theory of everything.

As time goes on, DM grows more and more ridiculous. Take a look at Stacy McGaugh's recent summary on his blog Checking-In on Troubles With Dark Matter.

 

[PeterDonis]

The observationally confirmed existence of a universal acceleration scale

I think this is a little strong. This is certainly a claim made by MOND advocates, but I don't think it is generally accepted by non-MOND advocates.

 

[strangerep]

I think this is a little strong. This is certainly a claim made by MOND advocates, but I don't think it is generally accepted by non-MOND advocates.

It [$a_0$] is in the data. Nature doesn't care about one's religion.

(The sociology surrounding this is just sad. Delusions are preferred because they're more soothing and easier to understand.)

 

[A. Neumaier]

You need to get out more.

Outside I find lots of speculation and little substance, except for those who try various semiclassical field models to account for the phenomena. But semiclassical field theory is just the lowest nontrivial approximation to a quantum field theory.

The observationally confirmed existence of a universal acceleration scale $a_0$ (Milgrom's constant) in various disparate situations shows that we are still very far from a satisfactory understanding of astrophysics on galactic and cosmological scales, and hence very far from a theory of everything.

But this is unrelated to QFT. There is no strong argument that modifying the action and adding one or more new fields could not account for the missing matter and forces.

 

[PeterDonis]

It's in the data.

Again, MOND advocates claim that this is in the data, but non-MOND advocates disagree. The data is things like galaxy rotation curves, which cannot directly tell you a "universal acceleration scale" because the distribution of matter is a free parameter in the models.

I agree that if you refuse to include any matter in the matter distribution in your model that is not visible, then you can infer a "universal acceleration scale" from the data (at least for many galaxies--but AFAIK not all, there are some whose rotation curves can be predicted from the visible matter alone without requiring MOND), as MOND advocates do: but whether that refusal is correct is precisely the point at issue in the debate.

 

[Vanadium 50]

The data allows extraction of a parameter with dimensions of acceleration, and all rotationally supported galaxies have similar values of this parameter. That's a fact.

"confirmed existence of a universal acceleration scale" seems to me (and apparently other participants) to be a stronger statement than that above. It suggests a more causal relationship.

We would not say that the luminosity of Cepheid variables is caused by their pulsation rate.

 

[PeroK]

It's in the data. Nature doesn't care about one's religion.

(The sociology surrounding this is just sad. Delusions are preferred because they're more soothing and easier to understand.)

Re the Stacy McGaugh piece you linked to:

At the moment, MOND advocates can readily point out every flaw in the preferred/established theory. And highlight things that are generally not well understood about cosmology, that on closer inspection have nothing to do with dark matter. You are free to mock and ridicule the established theory. And call mainstream cosmologists sad and deluded.

The real test would be if MOND were established as the primary, mainstream theory and DM was out. Then the onus would be on MOND to explain everything. All the data. No cherry picking. And ever flaw in cosmological theory would be laid at the MOND door. The question is whether then there would be fewer anomalies in the data and fewer things that still required an explanation.

I have no view either way - I don't know cosmology in anything like enough detail to take sides. My perspective is that in cosmology (and particle physics) we've gone so far but progress now is theoretically and experimentally difficult. Perhaps we've reached the limits of what humans have the ability to discover - at least for the time being. It's ridiculous that we can't find DM - but that doesn't mean it's not there. And if MOND were really the simple answer, then (like SR and QM 100 years ago), it would have forced its way through by application of the scientific method. It would be irresistible.

Perhaps this question will be resolved in the next 100 years and perhaps it won't. Perhaps you'd be happy to bet your house on MOND. And others would bet their houses on DM. All I know is I wouldn't bet my house on either.

 

[strangerep]

You are free to mock and ridicule the established theory.

That's not what I'm doing. I refer specifically to the empirically verified presence of the universal acceleration constant $a_0$ in the data. Failure to acknowledge this fact is what I consider "sad".

 

[PeroK]

That's not what I'm doing. I refer specifically to the empirically verified presence of the universal acceleration constant $a_0$ in the data. Failure to acknowledge this fact is what I consider "sad".

That's exactly what you're doing:

As time goes on, DM grows more and more ridiculous.

And from the McGaugh piece:

The Local Void challenge
Peebles has been pointing out for a long time that voids are more empty than they should be, and do not contain the population of galaxies expected in LCDM. They’re too normal, too big, and gee it would help if structure formed faster. In our review, we pointed out that the “Local Void” hosts only 3 galaxies, which is much less than the expected ∼ 20 for a typical similar void in ΛCDM.

I am not seeing much in the literature in the way of updates, so I guess this one has been forgotten and remains a problem.

Are you and McGaugh saying that a MOND model automatically explains this local void challenge? Show me why this particular problem in cosmology is solved by MOND.

 

[Motore]

I mean... there are papers that show absence of the acceleration scale, so the premise of the OP that a universal acceleration scale $a_0$ is observationally confirmed cannot be true, as it's not confirmed by everybody.
For example: https://www.nature.com/articles/s41550-018-0498-9.epdf (not paywall: https://arxiv.org/pdf/1806.06803.pdf)
or (not a quality paper, but still) https://arxiv.org/pdf/2205.07515.pdf

 

[Fra]

I refer specifically to the empirically verified presence of the universal acceleration constant $a_0$ in the data.

By using alternative models, wether those include mysteries unknown fields or new dynamics with new parameters, any model would infer the preferred parameters... each model would still be imperfect and cause issues elsewhere, which one is the overall best option? From a data-fitting perspective there is possibly an objective answer here if we defined the score for the fitting.

But what explanatory values does it add in any case?

I personally see MOND like an ad hoc tweak, motivated by trying to fit data. I have absolutely not problem to think that gravitational thery may need modification, on the contrary, but ad hoc tweak based on observational deviation for me adds no more "explanatory power" than does bookkeeping the deviations in the other model by adding ad hoc, "dark energy" or "dark matter".

I think, when we can explain what the dark energy and dark matter terms come from or are, or when we understand gravity enough to understand why it behaves differently at deep MOND regimes, then we have an improved explanatory value.

I an inclined to think that deeper answers will have to await a better unification of foundations of QM and GR. If conceptually spacetime is some sort of "relation" between masses, which is what "observers" are made of. Then it seems to me that the regime where MOND speculates about deviation, is the region where two masses (or observers) are very very far away, and who can barely "sense" each other gravitationally because their mutual interaction is so weak. As one could ask, how could one go about to _define_ a relation (spacetime) under such sparse conditions.

related paper...

MOND as a regime of quantum gravity​

"We propose that there is a regime of quantum gravity phenomena, for the case that the cosmological constant is small and positive, which concerns phenomena at temperatures below the deSitter temperature, or length scales larger than the horizon. We observe that the standard form of the equivalence principle does not apply in this regime; we consider instead that a weakened form of the equivalence principle might hold in which the ratio of gravitational to inertial mass is a function of environmental and global parameters. We consider possible principles to determine that function. These lead to behaviour that, in the limit of hbar to zero and the speed of light is taken to infinity, reproduces the modifications of Newtonian dynamics first proposed by Milgrom. Thus MOND is elucidated as coding the physics of a novel regime of quantum gravity phenomena..."
--https://arxiv.org/abs/1704.00780

/Fredrik

 

[Vanadium 50]

there are papers that show absence of the acceleration scale

I think the first paper overstates its case.

Figure 1 is highly, highly misleading. The x-axis is the author-assigned galaxy number. The trend is a creation of the authors, not nature. I can do the same thing with a box of resistors and disprove Ohms law.

What we actually have is a distribution of a0 that is about twice as wide as one would expect a priori. They still see a narrow distribution of a centered on a0, just one less narrow than expected. To take a historical example, we had a similar situation iwth the Hubble constant some time back - the correct conclusion was most assuredly not to toss the Big Bang cosmology.

As an aside, even if MOND were 100% correct, you'd expect some dispersion in the measured a0 distribution. From...dark matter. Baryonic dark matter, i.e. gas. There is nowhere near enough to explain rotation curves, but there is enough to alter rotation curves.

 

[Vanadium 50]

As a PS, when I read the title, I thought at was about the particle a0(980), which all right-thinking people (but not the majority) believe it to be a KKbar bound state.