Wednesday, January 26, 2022

Superfluid Dark Matter Has Some Issues

A recent pre-print, with two of my favorite physicist physics bloggers (McGaugh and Hossenfelder) collaborating as co-authors, examines the fit of Mistele and Hossenfelder's superfluid dark matter hypothesis to a large sample of galactic rotation curves, compares its performance to Modified Newtonian Dynamics (MOND) and finds it wanting.
We make rotation curve fits to test the superfluid dark matter model. Our aim is to investigate whether superfluid dark matter provides satisfactory fits to galactic rotation curves with reasonable stellar mass-to-light ratios. We fitted the superfluid dark matter model to the rotation curves of 169 galaxies in the SPARC sample. We found that the mass-to-light ratios obtained with superfluid dark matter are generally acceptable in terms of stellar populations. However, the best fit mass-to-light ratios have an unnatural dependence on the size of the galaxy in that giant galaxies have systematically lower mass-to-light ratios than dwarf galaxies. A second finding is that the superfluid often fits the rotation curves best when the superfluid's force does not closely resemble that of Modified Newtonian Dynamics (MOND). In that case, we can no longer expect superfluid dark matter to reproduce the phenomenologically observed scaling-relations that make MOND appealing. If, on the other hand, we consider only solutions whose force approximates MOND well, then the total mass of the superfluid is in tension with gravitational lensing data. We conclude that even the best fits with superfluid dark matter are still unsatisfactory.
Tobias Mistele, Stacy McGaugh, Sabine Hossenfelder "Galactic Mass-to-Light Ratios With Superfluid Dark Matter" arXiv:2201.07282 (January 18, 2022).

String Theory Is Still Vaporware

Woit explains convincingly that even string theorists know that string theory has no foreseeable observational basis, and that the theoretical arguments currently being advanced are wanting as well. 

Friday, January 21, 2022

What Is The Weak Gravity Conjecture?

The review article is 120 pages long, but the abstract and introduction alone are very helpful in providing a grounding in what the weak gravity conjecture is, and why it matters.

The Weak Gravity Conjecture holds that in a theory of quantum gravity, any gauge force must mediate interactions stronger than gravity for some particles. 
This statement has surprisingly deep and extensive connections to many different areas of physics and mathematics. Several variations on the basic conjecture have been proposed, including statements that are much stronger but are nonetheless satisfied by all known consistent quantum gravity theories. 
We review these related conjectures and the evidence for their validity in the string theory landscape. 
We also review a variety of arguments for these conjectures, which tend to fall into two categories: qualitative arguments which claim the conjecture is plausible based on general principles, and quantitative arguments for various special cases or analogues of the conjecture. 
We also outline the implications of these conjectures for particle physics, cosmology, general relativity, and mathematics. 
Finally, we highlight important directions for future research.
Daniel Harlow, Ben Heidenreich, Matthew Reece, Tom Rudelius, "The Weak Gravity Conjecture: A Review" arXiv:2201.08380 (January 20, 2022) (invited review, submitted to Reviews of Modern Physics).