[eru]

> Is it possible that another turn of the crank or two takes dark matter to zero?

Not sure about the Milky Way. But we already found galaxies out there that don't seem to have any dark matter; ie where their rotation curves match what you would predict from the ordinary matter alone.

Interestingly enough that is evidence in favour of dark matter: it's easy to conceive of mechanisms that can separate dark matter from visible matter with some low probability in a galaxy merger or near-merger. (Similarly for galaxies having more than the average amount of dark matter.) But alternatives like Modified Newtonian Dynamics (MOND) would have a much harder time accommodating those differences.

[throwawaymaths]

No. MOND predicts a large external field effect in the milky way due to satellite galaxies (like the large magellanic clouds). This would manifest itself in as "a reduced DM halo", and an keplerian decline (see figure in TFA). LCDM typically has a difficult time with this because in order to have a flat curve, the dark matter must be distributed in a halo that is MUCH bigger than the galaxy, and that leaves no explanation for the decline.

Not having run the numbers, at first glance it looks like these new data are STRONG support for MOND.

MOND also generally predicts that denser fast rotating galaxies should appear to have ~no dark matter (in these cases centripetal acceleration is > a0 constant, so it's in the Newtonian regime) , which is highly consistent with observations of galaxies.

Naively (I have not run simulations) thinking about it it seems puzzling that the denser galaxies would have no DM -- since DM is supposed to be the nucleus for galactic formation in the early universe.

Conversely, the galaxies that are highly diffuse (UDGs) seem to have extra dark matter, which is very consistent with MOND (since being diffuse, gravitational acceleration a < a0). Again, this makes little sense with LCDM as you would expect a pocket of extra DM to attract mass and nucleate the formation of a very dense galaxy.

> separate dark matter from visible matter with some low probability in a galaxy merger

Generally these observations have been done with weak lensing, and unfortunately there is not really yet a good model that reconciles GR with MOND. Apparently the math is hard. It is entirely possible that when you combine the two the calculated spacetime curvature solves to what you would expect from the observed weak lensing effects.

[misja111]

Did you read the article? It explicitly states

> .. at the outskirts of the disk of the Galaxy, this [rotation] curve begins to decrease rapidly, following the prediction known as the Keplerian decline

[throwawaymaths]

Yes. MOND predicts the keplerian decline, due to the EFE, hence:

> these new data are STRONG support for MOND

In standard cosmology keplerian decline is predicted because other galaxies have it. It makes no sense at all from the structure of DM halos that galaxies are supposed to have.

[misja111]

You have it completely backwards. Keplerian decline, i.e. the decline of the rotation speed the further away you get from the center of your solar system, galaxy or whatever, is predicted by traditional cosmology. What is detected in other galaxies, and what was thought to be detected in our own galaxy is well, is a lack of Keplerian decline. That is where both dark matter and MOND stepped in trying to explain. See e.g. https://link.springer.com/article/10.12942/lrr-2012-10

What was detected now, is that our galaxy shows Keplerian decline after all. Which, at least for our own galaxy, removes the need for introducing MOND or dark matter.

[throwawaymaths]

No. Keplerian decline doesn't mean that the entire rotation curve is keplerian. It means that it regresses to keplerian after a certain radius. You cannot explain the plateauing part within that radius without MOND or LCDM.

[username332211]

Without knowing anything about MOND, this never made sense to me.

If MOND can explain away all cases where m > 0[*], it'd be a bit strange it can't explain m = 0.

If MOND can only accommodate a certain range, wouldn't that also present a problem for dark matter? If all previously known galaxies had dark matter in that range, why would every galaxy have roughly the same amount of dark matter except the one that doesn't have any?

[*] Where I guess m would be the ratio of dark to ordinary matter.

[eru]

MOND can most easily explain a certain fixed value of what you call m. It has a much harder time explaining lots of different m at the same time.

> If all previously known galaxies had dark matter in that range, why would every galaxy have roughly the same amount of dark matter except the one that doesn't have any?

They don't all have the same amount of dark matter. There's a distribution.

You were right, that a weird distribution would invite investigation.