A Is MOND Inadequate for Explaining Vertical Oscillations in Galaxy Motions?

[strangerep]

Stacy McGaugh's blog post of 7-Apr-2023: A Few Words About the Milky Way left me gobsmacked. A "few" words? Ha! Here follows my "short" version...

Stars in a galaxy don't have just a flat orbit around the galaxy centre. During their orbit they also do small oscillatory motions up and down in the z direction (think: cylindrical coordinates $r,\phi,z$). This means their z-acceleration is nonzero.

The stars' orbital motion involves their $r \dot\phi^2$ centripetal acceleration, which involves their tangential velocities. By red/blue-shift experimental analysis we discover that their centripetal acceleration is too high to be accounted for by Newtonian gravity and the galaxy's baryonic mass distribution. Note (for later) that this $r \dot\phi$ orbital motion is perpendicular to the gravitational force they experience, which is overwhelmingly in the -$r$ direction.

In contrast, the $z$ oscillatory motion is parallel to the $z$-component of the gravitation force generated by a disk-like galaxy.

Here's the kicker: MOND accounts well for the orbital motion discrepancies, but seems totally irrelevant to the z motion. In the z direction, Newtonian gravity seems to do just fine by itself.

McGaugh is (rightfully) very cautious about this, emphasizing that lots more work needs to be done, analyzing vast amounts of Gaia data, before this puzzle could be considered solid. (Indeed, I was hesitant whether to even mention it here in the BTSM forum, since it's probably pushing the current boundaries.)

 

[Ken G]

On the surface that claim seems inconsistent with this paper, https://ui.adsabs.harvard.edu/abs/2023MNRAS.519.4479Z/abstract, which claims that MOND can be used to understand the orbits in the Milky way very accurately. In fact, this paper claims that even if it is actually dark matter, and not MOND, that is ruling those orbits, it would still be easier (and equally accurate) to use the mathematics of MOND to describe those orbits, than to use the mathematics of dark matter. Since the data used to argue all this comes from sensitive GAIA measurements, I would have thought the vertical oscillation effect would be naturally included, whereas this paper would be consistent with that blog claim only if it is restricted to the azimuthal orbital characteristics.

 

[ohwilleke]

It is a natural result if one sees MOND as having its origins in some sort of dimensional reduction of part of the gravitational force, in the same vein as Deur's proposals, whether or not it is exactly that.

 

[kodama]

It is a natural result if one sees MOND as having its origins in some sort of dimensional reduction of part of the gravitational force, in the same vein as Deur's proposals, whether or not it is exactly that.

does some sort of dimensional reduction of part of the gravitational force occur at the scale of solar system or galaxies clusters ?

 

[ohwilleke]

does some sort of dimensional reduction of part of the gravitational force occur at the scale of solar system or galaxies clusters ?

In Deur's analysis, at least, yes, in galaxy clusters, as one dimensional flux tubes between galaxy masses that are roughly point-like at that scale, and there is some evidence that this description is a better fit to lensing data than dark matter particles as the inferred halo shapes are far too tightly clustered around the visible matter in galaxy clusters.

In solar systems, probably not because the mass of a solar system is overwhelmingly concentrated in a spherically symmetric star (although possibly in a binary or higher order star system). In our solar system, 99.8% of the mass of the solar system in concentrated in our very nearly spherically symmetric Sun which is centered very close to the center of mass of the entire solar system.