This is rather odd, since there's supposedly much more dark matter than normal matter in the Universe, with its total mass being responsible for forming the large-scale structures (filaments of galaxies and galaxy clusters) within the lifetime of the observable Universe. Without that extra mass it's hard to get these structures to form in the given time. Galaxy formation is often envisaged as being a process of how normal matter gets into dark matter halos and starts forming stars. Since those halos can merge over time this becomes tremendously complicated. So it's not outside the realm of possibility that earlier galaxies would have less dark matter, but it is odd.
Even odder is that the claims of the paper appear, for once, appear to be even stronger than in the press release : "we find that the dark matter fractions near the half-light radius for all our galaxies are modest to negligible." This does not necessarily mean they have no dark matter, just less than normal matter, but that's in complete contrast to nearby galaxies which are dominated by dark matter by a typical factor 5-10.
I'm less than convinced by this.
Below I reproduce the main part of figure 1 from their paper. Column a shows their observations of H-alpha (ionized hydrogen). Panel b shows the velocity of the H-alpha, which can be directly measured. Panel c shows the rotation curve of the galaxy - taking that direct velocity measurement and converting it into rotation. The points show the data, the red curves show their best fit to the data.
Galaxies which are dominated by dark matter have flat rotation curves at large distances. Their claim is that the galaxies in their sample show declining rotation curves, which is what you'd expect if there wasn't any dark matter. They have a larger sample of galaxies but they claim the six in the figure are their best candidates.
None of them look particularly convincing to me. The first one blatantly shows a flat curve, how they managed to fit a decline to it I don't know. The second has a clear asymmetry and looks like it might be interacting, with one side of the curve declining much more steeply than the others. The third is again flat, with the only hint of a decline being a single data point on the right hand side - again, how on earth they fitted such a nice symmetrical, steeply declining curve I don't know. Similarly the fourth and sixth appear to be dominated by individual data points. Such "wiggles" are well-known; to extrapolate like this is very dangerous. See also point 2 here, and note that other galaxies are well-known to show slight declines which then flatten off. Only their fifth galaxy looks at all plausible to me, and even that one hardly looks convincing.
They say :
We find interacting low mass satellites in three of our six sources and evidence for some tidal stripping in one, but the rotation curve is symmetric, even near the satellite.
Wait, what ? No it isn't ! Any fool can that just from looking at the second galaxy.
They also try "stacking" the rotation curves in their larger sample. This is essentially just adding them all up, which can give a much stronger signal (at the expense of losing information about individual objects). This produced a very nice, steeply declining curve. The problem is that if there are systematic errors in this they will only get larger, not weaker. Their model fits rely on some extremely questionable assumptions (neglecting atomic gas completely), the data is clearly messy (column a), and their rotation curve fit relies on a complicated model with many free parameters that seems so blatantly to be doing such a bad job in their best cases... sigh.
As a rule of thumb, if you can see a pattern visually it's worth investigating further to see if you can verify it rigorously. If you see a pattern, it might be real but it might not - your brain loves patterns in order to avoid getting eaten by stripy tigers in dark forests. It's had millions of years of practise over far longer timescales than any mathematical techniques humans have invented to detect trends. So if you can't see a trend in the data, chances are, there isn't one. I can't see a trend in this data, only dodgy fitting and rather extreme claims.
But heck, this is published in Nature, which is supposed to be about important, exciting results. Maybe their full, longer paper will be better, but I haven't read that yet .
https://arxiv.org/abs/1703.05491
Thanks for the sanity check, Rhys Taylor
ReplyDeleteWhile on the topic, there was a much hyped story a while ago about an "alternative to dark matter" relying on only gravity.
Is that bunk or under evaluation?
Andreas Geisler You mean Verlinde gravity ?
ReplyDeletephys.org - Verlinde's new theory of gravity passes first test
The maths is beyond me. People who do seem to understand it think it's not crazy, but it's still very early days. There are a whole bunch of reasons both observational and theoretical in support of dark matter. Showing that an alternative theory can do better in one or two aspects is easy; showing which is better overall is a huge task. MOND's been struggling with this for 30 years. None of it's bunk, but I wouldn't hold your breath.
Extraordinary claims, unconvincing evidence eh.. One expects better from ESO and Nature.
ReplyDeleteThis is exactly the post I needed to save me from a day spent scratching my head. 😶
Thanks Rhys Taylor for the further sanity. The way things are reported, it's very difficult to tell the difference between sensationalist claims and sensationalist coverage of legit science.
ReplyDeleteI echo Andreas Geisler's point. Indeed, even some more reputable sources at times seem to adapt relatively inflated titles or claims these days. This makes it somewhat difficult for the more "uninitiated" or for the public at large to discern facts from factoids or outright inaccurate claims. It is indeed even more difficult for those of us that attempt to communicate STEM information as being the conduit of such information to the public, ends up making us partially "guilty" by proxy..
ReplyDeleteRhys Taylor From what I've been able to grasp (extremely limited) Verlinde Gravity as an emergent, rather than a fundamental force, is a useful way of squeezing the problem. Well, I call it squeezing, when I face a problem containing a preposterous unknown - and let's face it, Dark Matter is getting harder and harder to defend with the current explanations.
ReplyDeletearxiv.org - arxiv.org/pdf/1702.04355.pdf
Think of Einstein stubbing in the Cosmological Constant to make his equations work: to his dying day, Einstein thought it was a huge mistake, the biggest he ever made. It wasn't, of course. Einstein was just squeezing the problem.
Thus with Verlinde Gravity. Here's the deal as I see it: until the James Webb Telescope starts producing data in the infrared, giving us a better picture of the early universe - and until someone gives us a better and simpler String Theory, the cosmologists are sitting there like Alfonso the Wise, who commissioned the best astronomical tables of the time, grumbling about the Ptolemaic system: "If the Lord Almighty had consulted me before embarking on creation thus, I should have recommended something simpler."
Dan Weese Rumours of the death of dark matter are greatly exaggerated. On the radial acceleration mentioned in that paper you reference, see this and links therein :
ReplyDeletehttp://astrorhysy.blogspot.cz/p/q-a.html#hyperboleandahalf
Rhys Taylor Yes, I read that blog entry some while back. Oh it's all Squeezing at this point. Dark Matter is as good an explanation as any, at this point, by my (uninformed) guess. These cosmological Beds of Procrustes are ugly as home-made sin at a mathematical level: I like none of them especially well. The equations are hideous.
ReplyDelete