* Standard models predict about ten times as many small galaxies as we actually detect.
To be fair, the nature of MOND makes running simulations difficult. The strength of gravitational acceleration in MOND varies in a more complex way than in standard Newtonian gravity, being much more dependent on the distribution of matter. Work on this problem has been ongoing for some time, and now at last the first MONDian simulations of galaxy formation have been unleashed. And... they're eerily familiar.
I have to say I found this paper really excessively long at 58 pages so I had to skip over large parts of it, which mainly seem to consist on tedious descriptions that could be better expressed with figures alone. But the gist of it is this : they simulate a big blob of gas, let it evolve, find that it goes phwhooop and out pops a healthy spiral galaxy that spins around nicely.
To be fair, in some ways this is no mean feat. You may remember my own efforts to construct a spiral galaxy from scratch (using standard models). Without dark matter, this is extremely difficult to set up. Get anything slightly wrong and the whole thing can become horribly unstable, blasting itself apart in a whole variety of interesting ways. Dark matter is great at stabilising everything, but without it, specifying the parameters of a disc and having it remain stable is feckin' hard.
And dark matter makes it pretty easy to start off with something that looks nothing much like a galaxy and get a very convincing spiral with minimal effort. Start with a big rotating blob of gas and let it do its thing, and bam! out comes a spiral : I should know, because I've run such simulations myself. Pretty much all the major properties of a typical disc galaxy emerge quite naturally. No need to fine-tune anything very much - it just works.
Twenty years ago, this "monolithic collapse" approach was interesting, but even by then, the rival theory of hierarchical merging and more-or-less replaced it. Monolithic collapse is elegant, but nobody could see how such giant monolithic clouds could ever form. Far more natural to suppose a series of smaller objects could gradually merge, an approach which has by and large been very successful (albeit not without plenty of hiccups and occasional bouts of serious illness).
So MOND's revist of this idea looks decidedly odd. It's reasonable of them to say that the different MONDian gravity should mean we expect fundamentally different initial conditions than the standard model, though this does open a pandora's box of free parameters : not only is gravity different, but so are all the initial conditions. It would be nice if they at least postulated what the new initial conditions should be, but they don't speculate on that here.
But it's not at all reasonable to claim that these results - their major one being the formation of an exponential stellar disc - constitutes much of a success for MOND. We did this twenty years ago using dark matter and got the same thing. True, this won't work with purely Newtonian gravity without dark matter, but no-one is claiming such a scenario. Instead, since MOND mimics the effects of dark matter quite precisely, in effect all these simulations have done is recreate the dark matter's gravity by another method. Their claims that the results don't depend on the precise baryonic physics is true, but misleading : that violent relaxation phase is going to wipe out the initial conditions, and it's that which is going to set the final outcome. That realistic-looking galaxies are a "generic outcome of collapsing gas clouds" is as true for the standard model as it is for MOND. Nothing new under the Sun...
The formation of exponential disk galaxies in MOND
The formation and evolution of galaxies is highly dependent on the dynamics of stars and gas, which is governed by the underlying law of gravity. To investigate how the formation and evolution of galaxies takes place in Milgromian gravity (MOND), we present full hydrodynamical simulations with the Phantom of Ramses (POR) code.
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