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Dark matter theory in trouble?
I have been seeing a few papers and articles pointing to problems with dark matter theory.
*Survey of 400 red giants finding no dark matter within 13k lightyears of us.
*Large scale structures around the Milkyway.
Do these results also cause problems with MOND?
Following through looking for recent comments on these results always seems to arrive at
"Important if true"
How much doubt is there about these results?
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An excellent and unanswered question. At the very least it should put some hard-to-work-with constraints on MoND.Originally Posted by PetTastic
Lots of doubt. The paper laid the groundwork for a kind of calculation that can be done with much smaller error bars when the Gaia data becomes available in a few years. It also showed not that there was no dark matter but that if their lines through the dots were about right that the distribution of dark matter near us doesn't drop off as fast as expected assuming the Milky Way has one large unified halo. It found the derivative of the distribution curve.How much doubt is there about these results?
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Hah. seriously. MOND, i thought that was dead and burried like 5+ years ago.
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The term gets broadly applied to similar ideas that do and don't include relativity. If the poster had said "f(R) Gravity", who'd have read the post?
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Order of Kilopi
I'm with Tobias. MOND already had problems and had been largely discounted for decades. Dark matter has remained mysterious for some 80 years. The longer the mystery, the more alternative possibilities are considered. No alternatives are yet particularly compelling.
Everyone is entitled to his own opinion, but not his own facts.
Established Member
There was an interesting three-way discussion on MOND and the Lambda-CDM models at Cosmic Variance recently:
http://blogs.discovermagazine.com/co...y-a-trialogue/
Established Member
Oh, thanks for sharing that, StupendousMan. Very interesting.
I do wonder whether any of the MOND proponents have successfully predicted the BAO peak in the galaxy correlation function (don't judge: I talk about what I know!), which is a direct prediction of Lambda-CDM. I've never seen such a prediction, but I have seen a few statements that it is explicitly not predicted by MOND. But I'm not familiar enough with the MONDian literature to be certain.
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I found it very interesting too (including the comments which followed). Stacy makes a very important point. Whatever MOND means, it probably means something. The theory itself could be wrong, but what is compelling is that a quite simple formula describes the motion of a variety of galaxies based only the apparent baryonic mass distribution. So before you toss it altogether for not predicting the CMB correctly, try to explain it. New theories are built on such clues, e.g. Kepler's laws lead to Newtonian gravity. Don't throw the baby out with the bathwater.
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Most of the MOND die hards seem to be keeping a low profile on this.
I don't fully understand their maths myself, but I can't find anybody highlighting an error in their work.
Am I correct in thinking that their results within the limits of their stated errors, rule out a dark matter density gradient strong enough to explain the observed rotation curve?
Order of Kilopi
That's neither surprising nor compelling since that's exactly what the simple formula was designed to do. That's like watching someone design and build a doghouse, and then being surprised (compelled?) when the result is a small shelter for a dog.
Everyone is entitled to his own opinion, but not his own facts.
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No. If they are correct in their measurements, then the distribution of dark matter in the Milky Way doesn't drop off as rapidly out by us as the usual model would suggest... but the rotation curve issue for our galaxy has enough free parameters that its not really possible for such a local look to say much concrete about confirmation or falsification from this. The paper is merely comparing observed velocities of a few bright stars to values expected if the DM density follows a straight power-curve... and again this can only reveal the first derivative of DM density as a function of radius from the MW center. It can't show DM density.
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Established Member
You are completely missing the point. The formula is based only on the apparent distribution of baryonic matter but it predicts the rotation curves accurately in a variety of cases. This implies that the dynamics somehow follow the distribution of visible matter. The implication is that the distribution of any DM and baryonic matter have a tight relationship in the various cases. That is hard to explain with CDM that has a profoundly different behavior from baryonic matter.
So it is telling us something!
From Pavel Kroupa's comment in that discussion:
Dr. McGaugh is a true scientist, unafraid to accept the truth when he encounters it. (This does not necessarily mean that MOND is the correct theory of gravity!)In about 1997 I heard a brilliant presentation by Stacy McGaugh at Harvard about his PhD research. I remember that he began by stating that his original aim had been to falsify MOND using disk galaxies. Instead, he apologised many times to the audience (according to my memory) that he could not falsify MOND and that instead every prediction made by MOND was verified.
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That's interesting. Frankly I didn't understand the paper well enough to get this. So it does not place a limit on DM density around here.
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It only places a limit if you accept a pretty wide collection of assumptions. It's worth noting that they aren't individually BAD assumptions, but you can't easily say which one(s) are wrong.
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Order of Kilopi
When people say this, it is usually incorrect.
AIUI, that is also incorrect. The formula is designed to fit the rotation curve, but using only baryonic matter. And the method for fitting is completely unnatural: When the acceleration due to gravity falls below a certain threshold, then the effect of gravity is claimed to gain strength. In other words, when the gravity from the baryonic matter becomes too small to explain the rotation curve, then, at that point only, but not before, let's increase the strength of gravity to match the rotation curve. That's not only unnatural, it's tautological.
Everyone is entitled to his own opinion, but not his own facts.
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I am not saying I disagree with your assessment, but the wording in their paper is quite strong on the matter. (or Dark matter)
http://www.eso.org/public/archives/r...17/eso1217.pdf
What concerns me the most about their results is that it even puts a hard cap on the conventional dark matter in the plane of the disk, that many people seem to think is too low.We extrapolate a dark matter (DM) density in the solar neighborhood of 0±1 mM⊙ pc−3, and all the current models
of a spherical DM halo are excluded at a confidence level higher than 4sigma.
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So they don't set hard limits. They extrapolate based on some assumptions they lay out. That extrapolation is based only on the the first derivative of density of gravitating matter, as calculated by the collective velocities of certain bright stars going through that gravitating matter at different radii from the center of the galaxy.We extrapolate ...
Post Edit: Let me add that as dismissive as my above statement sounds, the idea they are trying to work with is very cool, and I *am* looking forward to new better results about the distribution of Dark Matter. /Post Edit
Even THEY say that followup measurements with more precise measurements (from the ESA Gaia probe) is required... at the end of the paper.
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Ok, thanks.
I think I have a better handle on the degree of the problem now.
Established Member
It seems to be correct in this case.
It is correct that the formula they use assumes a change in the behavior of gravity at a specific acceleration (the same in all cases). That this assumption leads to a detailed correct match with a variety of baronic matter distributions is not a tautology. This is precisely what makes it interesting. Do you think that the people who study this are fools? They can match the rotation curves in great detail using this assumption. Why it predicts the curves so well is not known with any certainty, but it does. Certainly it could be that there is another explanation, so that it is not really because gravity actually follows this rule. However, in order to derive this rule from LCDM theory, there is a need for a strange conspiracy (over a variety of galaxies) between the distribution of baryonic matter and CDM. Such a conspiracy is extremely unlikely given the huge difference in the dynamics and interactions of baryonic and CDM.
I'm sorry that you cannot appreciate the importance of this discovery. It is a consistent observation that requires an explanation. Denying that it exists or claiming it is a tautology is to deny the observations. Scientists cannot simple ignore relationships because they do not know how to predict them with the theory they prefer.
Established Member
Yes, what they actually suggest is that the leading theories of how non-baryonic dark matter is distributed (based on the assumed properties) are inconsistent with their results. Popular news articles based on this paper pretty much fail to make this clear, making it sound like that were claiming that no DM (of any kind) is there.
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One thing I noticed in their paper is the assumption that the galaxy is not growing or shrinking, and that radial velocity is constant with time.
http://www.eso.org/public/archives/r...17/eso1217.pdf
But they also say this:
As far as I understand; their use of the Poisson equation is totally dependent on the gravitational system not changing size.Recent investigations detected a non-null mean radial motion
of stars (Casetti-Dinescu et al. 2011; Moni Bidin et al. 2012). However, this is not a consequence
of a global motion of disk stars, but it is rather a signature of local kinematical substructures, so
that assumption (V) is still valid as a general property of disk stars.
A local kinematical substructure the size of their sample area would be a big problem.
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Note: A rebuttle paper has been published. It is referenced in Fun Papers in the Astronomy section, and there is a Universe Today story describing it today as well.
Short version, the original paper did some odd things calculating the predicted motions of stars above and below the plane of the galaxy, using a cylindrical mapping where a spherical one would have been more appropriate. Final result: there is about 300 MeV of dark matter per cubic centimeter in our part of the galaxy, which is 20% more than expected ... but there is about 20% uncertainty in the figure too.
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Thanks.
This is exactly what I was looking for.
Fascinating how just changing assumptions without any changes to the raw data modifies the published result so much.
Under such circumstances, I do not understand how either team can claim such low uncertainty in the value of the result.
One point in the second paper does look a bit odd to me, related to the change to spherical coordinates.
If the density of dark matter was very low, one possibility for the dominant motion of stars is to oscillate up and down through the plane of disc faster than their orbital period.
In this type of motion in spherical coordinates the distance from the centre of the galaxy is not constant, but in cylindrical the distance from the centre axis of the galaxy is nearly constant.
Does this make the second paper biased towards seeing dark matter, and the first paper biased towards not seeing it?
Last edited by PetTastic; 2012-May-24 at 02:01 PM. Reason: with --> without
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