Tuesday, March 17, 2020

Sabine Hossenfelder On MOND And Dark Matter

The line between modified gravity and dark matter particles isn't always a clean one. This approach splits the difference.

The Milky Way's rotation curve with superfluid dark matter

Recent studies have shown that dark matter with a superfluid phase in which phonons mediate a long-distance force gives rise to the phenomenologically well-established regularities of Modified Newtonian Dynamics (MOND). Superfluid dark matter, therefore, has emerged as a promising explanation for astrophysical observations by combining the benefits of both particle dark matter and MOND, or its relativistic completions, respectively. We here investigate whether superfluid dark matter can reproduce the observed Milky Way rotation curve for R<25kpc and are able to answer this question in the affirmative. Our analysis demonstrates that superfluid dark matter fits the data well with parameters in reasonable ranges. The most notable difference between superfluid dark matter and MOND is that superfluid dark matter requires about 20% less total baryonic mass (with a suitable interpolation function). Our analysis further allows us to estimate the radius of the Milky Way's superfluid core and the total mass of dark matter in both the superfluid and the normal phase.
Comments:19 pages, 5 figures
Subjects:Astrophysics of Galaxies (astro-ph.GA)
Cite as:arXiv:2003.07324 [astro-ph.GA]
(or arXiv:2003.07324v1 [astro-ph.GA] for this version)
UPDATE March 24, 2020:

A Critique of Covariant Emergent Gravity

I address some problems encountered in the formulation of relativistic models encompassing the MOND phenomenology of radial acceleration. I explore scalar and vector theories with fractional kinetic terms and f(R)-type gravity, demanding that the energy density be bounded from below and that superluminal modes be absent, but also that some consistency constraints with observational results hold. I identify configurations whose energy is unbounded from below and formulate some no-go statements for vector field models and modified gravity theories. Finally, I discuss superfluid dark matter as a hybrid theory lying between CDM and modified gravity, highlighting some difficulties present also in this case, which appears preferable to the others.
Subjects:General Relativity and Quantum Cosmology (gr-qc)
Cite as:arXiv:2003.10410 [gr-qc]
 (or arXiv:2003.10410v1 [gr-qc] for this version)

A visual illustration (from here) of the circumstances under which galaxy dynamics differ from Newtonian gravity without dark matter (which starts to be observable at a Newtonian gravitational acceleration expectation of approximately 1.2 * 10-10 m/s2, which is the MOND constant a0The quantity D is the amplitude of the discrepancy, basically the ratio of total mass to that which is visible.


neo said...

I found 2 recent papers,

A fundamental test for MOND
Valerio Marra, Davi C. Rodrigues, Álefe O. F. de Almeida
(Submitted on 10 Feb 2020)


Galaxies with Declining Rotation Curves
D. I. Zobnina, A. V. Zasov
(Submitted on 19 Mar 2020)


"We directly estimate, using a Monte Carlo simulation, that the existence of a fundamental acceleration is incompatible with the data at more than 5σ. We also consider quality cuts in order to show that our results are robust against outliers. In conclusion, the new analysis further supports the claim that the acceleration scale found in the RAR is an emergent quantity. "

" For seven galaxies with the longest rotation curves, the disk mass was estimated to be with the dark halo (Newtonian model) and without the halo (modified Newtonian dynamics (MOND) model). In four of the galaxies, the MOND model encounters difficulties in interpreting the rotation curve: in order to be consistent with the observations, the MOND parameter a0 should differ significantly from the expected value a0∼10−8 cm/s2, while the disk mass exceeds the value based on IR photometry and maximum disk model. The conflict with MOND is the most significant for NGC 157. "

that MOND is ruled out, which of course would present problems for example with Deur's.

these papers affirm dark matter as best expanation and that Stacy McGaugh radial acceleration relation can be explained as emergent from dark matter

I wish Stacy McGaugh would comment.

andrew said...

Valerio Marra, Davi C. Rodrigues, and Álefe O. F. de Almeida's paper is not a solid analysis and contrary to results from multiple other authors in multiple other papers. I think that I discuss in a prior blog post. I think that Stacy has discussed it somewhere.

I have not yet read D. I. Zobnina and A. V. Zasov, which just came out on Thursday, and will do so when I get a chance. Thanks for the links.