A pair of interesting papers: have counterarguments been published?

[PeterDonis]

TL;DR Summary

Two interesting papers by Pavel Kroupa give arguments that dark matter is not present in galaxies based on fairly simple tests. Have any papers giving counter arguments been published?

I know there have been a number of previous threads discussing the work of Pavel Kroupa, and this work has generally been categorized as far out of mainstream because it deals with his views on alternative gravity theories and so forth. However, I have come across two of his papers that describe what seem to me to be fairly simple tests for the presence of dark matter, both of which would seem to indicate that dark matter is not present in our galaxy or in other galaxies that we observe. I am wondering if any papers giving counter arguments have been published?

The two papers are:

(1) Galaxies as simple dynamical systems: observational data disfavor dark matter and stochastic star formation.

https://arxiv.org/abs/1406.4860

The basic argument in this paper is that if galaxies we observe have dark matter halos, then smaller galaxies orbiting them should experience Chandrasekhar dynamical friction (basically, they should slow down and we should see this in their orbits). However, we do not observe this.

(2) Effect of the Solar dark matter wake on planets.

https://arxiv.org/abs/1907.07130

This paper argues that as our solar system moves through our galaxy's dark matter halo, the Sun should leave a dark matter "wake" that affects the orbits of the planets in a way that is not observed.

 

[mathman]

Basic argument for dsrk matter is to explain why galaxies do not fly apart.

 

[PeterDonis]

Basic argument for dsrk matter is to explain why galaxies do not fly apart.

I'm not asking for general arguments for the dark matter hypothesis; I am already familiar with those. I am asking if there have been any specific counter arguments published regarding the specific claims made in the two papers I linked to. Those claims basically amount to pointing out predictions of the dark matter hypothesis that appear to conflict with observation; so a counter argument would either be to show how the dark matter hypothesis does not actually make those predictions, or showing how those predictions are not actually conflicting with observation.

 

[Vanadium 50]

If you linked to the arXiv page and not the PDF, we could easily all check to see which papers that cited these. There are 78 that cite the first and 2 on the second.

The second paper, however, says the opposite of your summary. It says "Over the nite (<= 30 year) period covered by accurate space-age observations, the DM wake of the Sun has an imperceptible effect on the motion of Solar System planets." It is critical of a 2019 paper by Hernandez that suggests that the solar DM "wake" would be visible in the motion of the planets, especially Saturn.

The 78 papers that reference the first paper often try and place it in context, but have a broader scope than "Why Paper 1 is Wrong (Or Right)."

 

[PeterDonis]

If you linked to the arXiv page and not the PDF, we could easily all check to see which papers that cited these.

Fair point, I've edited the OP to fix the links.

 

[Vanadium 50]

FWIW, I am not sure I buy the Hernandez argument. Suppose DM was very, very cold - i.e. ends up in the sun's frame. I don't think we would see an effect at all. So there is surely some dependence on the exact DM model you assume.

 

[PeterDonis]

Suppose DM was very, very cold - i.e. ends up in the sun's frame.

If it were very, very cold, wouldn't it be approximately at rest in the galaxy rest frame? The halo is around the galaxy not the solar system. The sun is certainly not at rest in the galaxy rest frame.

 

[Vanadium 50]

I was thinking at rest in the sun's frame - that is, the sun drags the nearby DM to a "halt". So it surely depends on the "effective viscosity". You can't simulaneously say that DM is "non collisional" and talk about its wake post-collision (a near miss is kinematically the same as a elastic collision)

 

[PeterDonis]

You can't simulaneously say that DM is "non collisional" and talk about its wake post-collision

DM is only "non collisional" with respect to non-gravitational interactions. The "wake" interaction is gravitational, and a "drag" interaction that allowed the sun to drag nearby DM to a halt would be also. As I understand it the calculations in the papers are assuming a gravitational interaction.

 

[Vanadium 50]

Yes, but Hernandez' point is that the deviations from collisionlessness is observable. I think this depends on the details of your model. How collisionless is collisionless?

 

[PeterDonis]

Hernandez' point is that the deviations from collisionlessness is observable. I think this depends on the details of your model.

Yes, agreed. I have not tried to look at the Hernandez calculation in detail or parse out all of its assumptions.