Back in 2017 there was a Nature paper claiming to have detected declining rotation curves in galaxies at high redshift. This would mean that galaxies in the distant early Universe have significantly less dark matter that contemporary nearby galaxies, whose flat rotation curves are one of the principle signatures of dark matter in the first place.
I was rather skeptical of this. None of their individual measurements looked the least bit convincing to me, with the fitted curves highly dependent on single data points : the slightest error could have thrown them off (and some curves just don't go through the points at all). True, the stacked curve was much more convincing, but any systematic error in estimating the rotation velocity will only compound this error rather than averaging it out.
On the other hand none of this actually would be evidence against dark matter. As with the now-plethora of Ultra Diffuse Galaxies and the like (in the nearby Universe, with relatively precise, sensitive data) which seem to have a significant and sometimes total deficit of dark matter, all this really indicates is that dark and visible matter can be separated. This is very much harder to do with modified gravity theories. If the rotation curve arises only as a result of baryonic matter, then if you have two systems in which the baryons have similar distributions (and are suitably isolated), then they should always have similar rotation curves. The only reasonable way in which they can differ is if one has dark matter and the other doesn't.
The real question is whether, according to cosmological theory, we expect galaxies in the early Universe to be less dark matter-dominated than today's. Ethan Siegel simply says "yes", that these declining rotation curves are indeed expected in standard models of galaxy formation.
The author's of today's paper, however, say No. And this is certainly more intuitive. If dark matter is mass-dominant, then it seems odd that it would actually become more important over time. Surely it should be gas falling into dark matter haloes that describes the process of galaxy assembly, and if that's the case – if dark matter makes up the bulk of the mass of galaxies from the word go – then they should always have similar (though not necessarily identical) rotation curves to those of the present day.
Now I should mention that the first author was my Master's project supervisor. You can read about this in some detail here, but in brief, we ran a galaxy formation simulation without dark matter and found that it just didn't work (see also my latter efforts failures to build a stable disc without dark matter). Anyway, the arguments that seemed quite compelling to younger me no longer have the same appeal; I'm delighted to see that others are still pursuing this line of inquiry and I wish them well, but I don't see this as likely to be anything more than a dead end. An interesting one to be sure, but I'm not convinced there's light at the end of the tunnel, so to speak.
What they do is run a series of simulations of the monolithic collapse scenario of galaxy formation. This is far simpler than the standard paradigm of hierarchical merging, in which galaxies assemble from a multitude of mergers in the early Universe (which can be a surprisingly efficient process, but then the early Universe was a lot smaller than the modern one). They consider different initial geometries, in which the initial dark matter halo is the same size, smaller, differently-structured, or spatially separated from its associated gas cloud.
They get the same result in all cases. The collapsing monolith experiences violent relaxation which essentially wipes out its initial conditions : the collapse proceeds at ever-increasing speed, the gas shocks and triggers star formation, and the galaxy forms during the re-expansion phase. This means that it doesn't really matter how you start, you get the same thing regardless.
And in this scenario you always get a galaxy with a flat rotation curve. The only way they could get a declining curve is to take out the dark matter altogether*.
* I'm intrigued that this is much more successful than my attempts at the same scenario, which gave something completely unphysical. I'll have to try and follow up on that.
This, as they've argued before, suggests that the initial conditions are irrelevant. But as I understand it, the hierarchical merging scenario is just more radically different than they give it credit for. There's just no reason in modern cosmology to assume the presence of any sort of collapsing monoliths, certainly at the very least not at all as the norm for galaxy formation in the early Universe. I don't think you even get much or any in the way of violent relaxation in this scenario, but rather a continuous assembly of tiny, already stabl-ish- proto-galaxies. So I really don't think it's fair to compare the failure of reproducing declining rotation curves in a monolith collapse scenario with the expectations from standard cosmology; this is comparing apples with oranges.
I also wonder just how similar these objects are to the observations. They don't compare the gas masses at all. Their gas radii are in some gases huge at 70 kpc : not outlandish by any means, but definitely on the larger side. They don't give this comparison to the observations, preferring to normalise their results to more accurately compare the shape rather than the size of the rotation curves.
This isn't a mistake in and of itself. But as in the press release that accompanied the original claim, there are a multitude of different factors in play. The galaxies detected might not actually be the progenitors of modern-day spirals but rather more spheroidal systems, which are anyway known to have less dark matter. There may indeed be significantly less dark matter in early galaxies overall, and those earlier systems might be more dominated by turbulence than rotation – which significantly affects the interpretation of the rotation curve.
This is not to say that there aren't questions to answer. The authors of this study point to other findings of more typical simulations saying that the turbulence isn't enough to account for all this, but they also note other observations of individual galaxies showing flat rotation curves (though they dispute this result) at earlier epochs. And they note that some galaxies might form in dark halos and others not, which seems very likely given all the hoo-hah about UDGs.
All this is very reasonable. The whole thing is a mess with many different variables to juggle. It just seems to me that we're a very long way indeed, further than ever in fact, for needing to invoke other physics like magnetic fields (as the authors do) to explain modern flat rotation curves. It seems far more likely that a combination of different effects are likely to explain early declining rotation curves than it is that they undermine the whole paradigm, which is otherwise supported by a vast array different and independent considerations, both in theory and observation, on a whole set of enormously different scales.
Do we fully understand galaxy assembly ? Absolutely not. But it's way to drastic to point to a complex result like this and say it calls the whole basis of modern theory into question, and it's just not fair to compare the results of a monolithic collapse scenario with the radically different processes postulated by mainstream cosmology.
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