New research both proves and disproves ... wait, what?

[unixronin]

Here’s an interesting one for the books.  In the continuing debate over the widely-accepted cold-dark-matter theory, a new study of the dwarf galazies surrounding the Milky Way has found that their arrangement appears to simultaneously require and refute the presence of dark matter.

More exactly, the orbital dynamics of the dwarf galaxies are consistent with what would be predicted by the cold-dark-matter theory ... but their arrangement indicates that they were formed by collision with other galaxies, a mechanism that — according to dark matter theory’s predictions of the properties of dark matter — precludes them containing any dark matter.

“The fragments produced by such an event can form rotating dwarf galaxies,” Metz said.  But there is an interesting catch to this crash theory, “theoretical calculations tell us that the satellites created cannot contain any dark matter.”  This assumption, however, stands in contradiction to another observation.  “The stars in the satellites we have observed are moving much faster than predicted by the Gravitational Law.  If classical physics holds this can only be attributed to the presence of dark matter.”

Doubt has been shed on the cold dark matter theory before.  This is the first time, however, that an observation has been found for which assuming dark matter creates a paradox.  If the dark matter theory is assumed to be correct, then dark matter is the only explanation for their orbital dynamics, yet their arrangement precludes the possibility of dark matter being responsible for their orbital dynamics.

How to resolve this?

Or one must assume that some basic fundamental principles of physics have hitherto been incorrectly understood.  “The only solution would be to reject Newton’s classical theory of gravitation,” adds Kroupa.  “We probably live in a non-Newton universe.  If this is true, then our observations could be explained without dark matter.”  Such approaches are finding support amongst other research teams in Europe, too.

Mordechai Milgrom’s MOND — Modified Newtonian Dynamics — theory has long been proposed as an alternate explanation for the observations used as evidence for cold dark matter.  But there are hard scientific reasons why MOND cannot be correct if the rest of our understanding of the Universe is correct — specifically, MOND is not consistent, and cannot be made consistent, with relativity.  A greatly more complex approach to the problem is Jacob Bekenstein’s TeVeS, tensor-vector-scalar gravity.  TeVeS, unlike MOND, is consistent with relativity; but it has been argued that TeVeS cannot simultaneously account for both galactic dynamics and gravitational lensing.  Both TeVeS and dark-matter theory are also unable to completely explain the Bullet Cluster observations without postulating additional factors; TeVeS requires the presence of dark matter as well to fully explain the observations, while dark matter theorists have found it necessary to invoke not only both dark matter and dark energy but a possible fifth basic force in order to fully explain them.  A related theory, STVG (scalar-tensor-vector gravity), takes a slightly different approach from TeVeS, and is able to successfully explain galaxy rotation curves, galaxy cluster mass profiles, gravitational lensing, the Bullet Cluster observations, and the accelerating expansion of the Universe without requiring the presence of either dark matter or dark energy.  Another very recent theory (to which I’m unable to find a reference right now), based on M-theory, posits that gravitons are weakly bound to the brane and can drift off of it and diffuse away into the bulk, resulting in a net gravitational force that follows the Newtonian inverse-square law at “normal” distances, is slightly stronger at huge distances as required by MOND and TeVeS to produce the effects which CDM theory explains via dark matter, and on truly vast scales becomes weaker than inverse-square again, in order to explain universal expansion without dark energy.

We don’t know yet which, if any, of these theories may be correct, and new theories are being proposed all the time.  However, with this finding, particularly if it can be confirmed, the holes in cold dark matter theory are beginning to become larger and more evident.

Comments

[smandal.livejournal.com]

To be specific, in the news article they claim that tidal formation contradicts CDM, in the actual paper they don't.

Here's an article (http://www.jpl.nasa.gov/news/news.cfm?release=2009-024) from a (IMHO) more reliable news source, talking about dwarf galaxies formed by tidal stripping that don't have dark matter:

In a recent study, Thilker and his colleagues found the ultraviolet signature of young stars emanating from several clumps of gas within the Leo Ring. "We speculate that these young stellar complexes are dwarf galaxies, although, as previously shown by radio astronomers, the gaseous clumps forming ,b>these galaxies lack dark matter," he said. "Almost all other galaxies we know are dominated by dark matter, which acted as a seed for the collection of their luminous components--stars, gas and dust. What we see occurring in the Leo Ring is a new mode for the formation of dwarf galaxies in material remaining from the much earlier assembly of this galaxy group.

[unixronin.livejournal.com]

Um, I thought that was the point: that they don't have associated dark matter, but that a dark-matter explanation of their orbital parameters requires that they do.

[smandal.livejournal.com]

The disc-like distribution and the fact that they all orbit in the same direction suggest tidal formation. This is what the two (http://arxiv.org/abs/0901.1658) papers (http://arxiv.org/abs/0901.1658) discuss, and nothing else.

The press release (http://www.eurekalert.org/pub_releases/2009-05/uob-sps050509.php) argues that tidal formed dwarf satellites cannot have CDM, but the dynamical observations of the dwarfs show that they spin too fast not to have dark matter.

I prefer to believe something that others researchers will read and scrutinize, not a press release. The article I link above shows that dwarfs can indeed form without dark matter, so let's wait for a confirmation of this group's observations before getting too excited ...

Many experiments reports anomalous results only to be contradicted by later, more capable experiments. (Most recently, the Fermi satellite did not confirm the same excess of electrons and positrons reported by PAMELA. Many in the community were excited by the prospect that the excess could be caused by dark matter decays ...)

[unixronin.livejournal.com]

The article I link above shows that dwarfs can indeed form without dark matter, so let's wait for a confirmation of this group's observations before getting too excited ...

Oh, sure. It'll become a lot more interesting if confirmed. On the other hand, if no-one else can reproduce it, or if someone else points out an error in the math ...

I guess to me, the main thing that makes this particular result interesting — if correct, of course — is that it might possibly be the first finding that has a chance of giving us a definitive answer on CDM. It doesn't have the ability to prove CDM or any other theory, but if it were to confirmed that this observation cannot be explained via CDM without incurring paradox, then it would be a strong indication we need to look elsewhere. (It'd be nice if it had the potential to prove CDM as well, but you take what you can get.)

That's what really caught my eye about it: the chance to narrow the uncertainty. All the guessing and "If X", "If Y" is interesting speculation, but there's a frustrating lack of confirmation in either direction. We're making progress; Milgrom's MOND is pretty much dead, and TeVeS seems to have run into irreconcilable problems. But those themselves do not constitute proof that CDM is correct.

I guess a part of the influence on my position on CDM is that I'm bothered by the apparent attitude of some of the more outspoken CDM theorists that "They're wrong, therefore we're right." It doesn't work that way. But I shouldn't allow my reaction to that to color my perception of the entire field.

[smandal.livejournal.com]

Indeed, if there is a confirmed observation with which CDM is inconsistent, that's problematic for CDM.

Even though I work on CDM, I'd be pleased as punch if the entire parameters spaces of DM candidates were eliminated by null observations -- it means the story must be more wonderful than, say, the lightest supersymmetric particle. I also have a hope that the LHC will see something far more interesting than the Higgs ...

[unixronin.livejournal.com]

I certainly hope we get some significant new results from the LHC. It's going to be potentially a big blow to future science funding if, after all the money spent on the LHC, it doesn't make some major findings. It's hard enough to get basic science properly funded now.