A New Combatant Joins The Cat Fight And Tries Very Hard To Be Nice To Everyone

Well, at least this one is a respectable paper (submitted but not yet accepted) and not an angry rant.

To recap, recently it was shown that there's a very strong relation between the density of normal matter in a galaxy and its rotational speed. This has made a lot of people very angry, and was widely regarded as a bad move.

... or for those of you not following this regularly, it's a problem because the dynamics of galaxies - how fast they rotate and suchlike - should be dominated by their dark matter. Normal matter only makes up a few percent of their mass, so it shouldn't be able to affect their rotation speed much at all. That is, if you accept dark matter in the first place. The main alternative is that our understanding of gravity (or even dynamics in general) is fundamentally flawed. Modified gravity theories like MOND can explain this "mass discrepancy relation" very well without dark matter, whereas it's not obvious if the standard model can account for it.

Recently there was a paper submitted showing that actually, when you account for all the complex physics of the normal matter, yes you can. But there was a problem. That paper only used 18 simulations of galaxies which were all of similar mass, so it didn't show if this relation could also be explained for smaller galaxies as the observations show. It also didn't really refute MOND much, which also works over a huge range of masses. Unfortunately Milgrom, who came up with the idea of MOND, took the results very personally and wrote a nasty public letter pointing out the flaws with rather more force than is generally necessary.

This new paper improves things with a much larger sample of simulated galaxies, 150-200 or so which are re-simulated using varying parameters for the so-called "sub-grid physics". Essentially some aspects of the physics (like how much energy stars inject into the gas) occur on scales too small to resolve directly in the simulation, so they have to be manually calibrated. Since this calibration is uncertain, they perform simulations with a range of possible values. And their dark matter masses vary by a factor of at least 1000, so it should be good enough to address Milgrom's, err, concerns. Though I'm not sure if the baryon/dark matter ratio varies sufficiently, but I suspect it does.

What they find is that yes, again, you can reproduce this relationship just fine with standard cosmology. Varying the complex sub-grid physics doesn't make much difference - in this paper it's just a natural scaling relation rather than anything more complex. True, this doesn't rule out MOND (and this paper is much nicer in tone, stating that the possibility of MOND should be taken seriously), but it seems that this test just isn't useful to choose between MOND and the standard model.

It will be interesting to see McGaugh's response to all this, who first raised the issue with his silly claim of having discovered "a new law of nature". Now it looks increasingly that there's nothing particularly deep or profound about this observation after all.
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