To recap, this is the discovery that the expected acceleration predicted by the visible material in a galaxy correlates extremely well with its observed acceleration. Because most of the mass of a galaxy is invisible dark matter, there isn't a direct one-to-one relation between the two. What seemed interesting to some people was the fact there was a correlation at all. It seemed weird that if dark matter was really so dominant that you should be able to use the visible matter to predict what it was doing.
Other people just shrugged their shoulders and said, "we see that in our simulations all the time". Later on they were able to show why it happened : it's a perfectly natural result of selection effects. You can't form galaxies in dark matter halos of any old mass, and the standard model makes very specific predictions for the distribution of dark matter and thus the resulting acceleration. To oversimplify (see first link for details), in effect the acceleration of baryons depends on the acceleration from the dark matter, rather than it being the other way around (which would be really weird). You can find a lengthy write-up of the whole sorry business here, including why lots of supporters of modified gravity got very cross.
While the main form of the relation appears to already have been entirely explained within the standard model, the last vestige for an interesting possibility was the stronger scatter at very low accelerations. It wasn't really clear if this was simply due to bad data, stronger intrinsic random variations, or if the slope of the relation hadn't been correctly measured. One intriguing effect noticed by the modified gravity camp was that if you accounted for the acceleration from the bigger parents of satellite galaxies, the deviant points seemed to move back to the standard relation, albeit with more scatter than the rest. But this scatter also appeared to be explicable in standard models simply because the scatter is intrinsically higher at lower accelerations.
Everyone agreed that the key place to look to settle the matter was the low acceleration regime. That's what this latest paper does, using very high quality data so that any deviation can't be attributed to measurement errors. And they do find that the deviation is real... but it's not random. Instead, there's a third parameter at work - the distance of each point from the centre of the galaxy. What's so far been plotted as a 2D relation with high scatter is, in fact, a 3D relation with very low scatter. What we've been seeing isn't a simple linear relation after all, it's actually a plane. And this is exactly consistent with the varying distribution of dark matter in different galaxy types.
Although the results don't exactly rule it out, there's just no need to invoke different physics to explain this relation. With the main relation was predicted by modified gravity theories, this particular aspect was predicted by dark matter. This won't be the final nail in the coffin (it will be interesting to see how the modified gravity community respond to this), but to my mind it further reinforces the uselessness of this relation as a way to distinguish modified gravity theories from dark matter. In my view, modifying gravity remains an unnecessarily drastic step that raises far more questions than it solves. What's so bad about dark matter anyway ?
The Radial Acceleration Relation (RAR): Crucial Cases of Dwarf Disks and Low-sur
The Radial Acceleration Relation (RAR): Crucial Cases of Dwarf Disks and Low-surface-brightness Galaxies
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