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If the paper is accepted for publication, does it mean we don't need the dark matter theory anymore? Or, should I say, it is not dark anymore?

"The Radial Acceleration Relation in Rotationally Supported Galaxies" Stacy McGaugh, Federico Lelli, Jim Schombert

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    As a general rule, the acceptance of a single paper for publication doesn't itself spell the end of anything – AGML Sep 29 '16 at 16:45
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    @AGML: Except for the author's time composing it :D – Lightness Races in Orbit Sep 29 '16 at 18:16
  • One paper alone doesn't matter yet. – Turion Sep 29 '16 at 22:46
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    "We report a correlation between the radial acceleration traced by rotation curves and that predicted by the observed distribution of baryons... Consequently, the dark matter contribution is fully specified by that of the baryons... The dark matter needs to respond to the distribution of baryons (or vice-versa) in order to give the observed relation. This is not trivial to achieve, but the observed phenomenology might emerge if dark matter behaves as a fluid." How is this the "end" of dark matter? – Conifold Sep 30 '16 at 01:26

2 Answers2

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I'll answer the question I believe to be implied: do the results in this paper mean that the rotation curves are explained solely by the visible matter?

And the answer is that no, visible matter alone cannot account for the rotation curves. Figure 3 in the paper plots the observed centripetal acceleration against the centripetal acceleration and this does not follow the line $x=y$. Instead the relation between observed and calculated centripetal acceleration is well fitted by (equation 4 in the paper):

$$ g_\text{obs} = \frac{g_\text{bar}}{1 - e^{-\sqrt{g_\text{bar}/g_\dagger}}} $$

where $g_\text{obs}$ is the centripetal acceleration calculated from the rotation curves and $g_\text{bar}$ is the centripetal acceleration calculated from the observed matter (baryonic) distribution. The parameter $g_\dagger$ is experimentally fitted and the authors do not suggest a physical interpretation for it.

So to explain the rotation curves still requires some extra effect whether it be dark matter or some as yet unidentified effect.

What is surprising is that the result means that if dark matter is present it must have a distribution closely correlated with the distribution of visible matter. This is surprising because there's no obvious reason why this should be so, and it's quite a strong constraint on the distribution of dark matter. Arguably it makes dark matter a less natural explanation of the rotation curves, but it certainly does not rule it out.

I note with some amusement that the result do seem to be naturally explained by the MOND theory, though the authors are at pains to say they are not suggesting that MOND is the explanation.

Community
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John Rennie
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    To say that it is "closely correlated" is not quite right. It depends on the size of $g$. It is well established that the distributions of visible matter and dark matter are very different in many galaxies. I think this paper argues they are different in a way that can be predicted from the distribution of visible matter. – ProfRob Sep 29 '16 at 19:57
  • It is not quite clear how what the authors showed in their paper can be explained by having the distribution of DM follow that of baryons. They seem to be correlated, but their analysis is not totally clear that it is not as Jeffreys says. And for such an important conclusion they do not discuss how it compares with the many observations of different distributions of the two, such as in galaxies vs halos. Hopefully there will be a follow up paper by others. Either way, you still need the dark matter, or some other way of explaining the the rotation curves, as both Rennie and Jeffreys indicate – Bob Bee Sep 30 '16 at 04:18
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That perhaps depends what you mean by dark matter. If by that you mean non-baryonic matter, then no, not by a long way. The rotation curves of galaxies are only one of many pieces of evidence for non-baryonic dark matter, including the dynamics of galaxy clusters, gravitational lensing, the cosmic microwave background, baryon acoustic oscillations, the primordial abundances of He and D and so on.

The rotation curves of galaxies only indicate (assuming our understanding of gravity were correct) large quantities of unaccounted for, unseen mass; and actually, that is completely unchanged by this paper.

ProfRob
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    As far as I know, the calculations at the local level, meaning solar system or nearby don't deviate from GR predictions. So if there is additional gravitional mass correlated to the location of observed matter, wouldn't you expect some discrepency at the local level? – Peter R Sep 29 '16 at 19:44
  • @PeterR The density of dark matter in the solar vicinity cannot be constrained by planetary motions. It would be about 1% of the interplanetary medium density. Actually there is no evidence for much dark matter in the disk of the Milky Way. It is more widely distributed and does not directly follow the visible matter at all. – ProfRob Sep 29 '16 at 19:49