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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.
Re: bah. stupid 4300-character comment limit again. (Part 1)
1. CDM is circumstantial
That's right -- all theories are :) We have observations that can't be explained by current theories, so we cook up a new one and then test their novel predictions. The problem with theories like MOND is that they are not even consistent with the data; TeVeS has to have a lot of moving parts to be consistent, so in addition to being less predictive it has technical problems that spin the theory out of control.
2. CDM is not testable
This is not correct. CDM may interact weakly, i.e. through electroweak interaction, and so we could detect them in underground labs, in colliders or via their astrophysical decay/annihilation products. Even if CDM only interactions gravitationally, it may still be detectable by other strategies such as lensing.
3. CDM makes the universe too massive, so we invoke dark energy
This is not right. The universe is not too massive to keep expanding -- the cosmic microwave background (CMB) tells us that the energy density in the universe is very close to the critical value, where the Hubble parameter is constant. The question that remains is what are the relative fractions of normal matter/light, CDM and dark energy.
We have three unknowns and one equation, so we need two more relations to get the parameters of the theory, and yet one more to falsify it. Supernova observations gives us the ratio total matter-to-dark energy, CMB also gives baryon-to-CDM ratio, and finally galactic observations of total luminosity as well as their dynamics gives us the total matter density.
4. Bullet Cluster dynamics are not explained by dark matter alone
Is that so? Please send me a reference arguing this!
It is very unlikey that dark energy would be involved since the cluster is ~1 megaparsec in size, and the effects of dark energy are negligible below ~100 megaparsec.
Re: bah. stupid 4300-character comment limit again. (Part 1)
SVTG... er ... STVG (too many acronyms!) looks as though it may have promise, but needs more evaluation yet. The weakly-bound-graviton theory is self-consistent and truly elegant (and I suspect it could have applicability to other aspects of M-throry as well), but it's very new, there hasn't been a lot of time to examine it yet, and it's going to be a bitch to test.2 — I didn't say CDM wasn't testable, I just said that so far it has not been successfully tested and either verified or refuted, but I dislike the tendency to regard it as all but proven despite that. As mentioned,¹ if — say — the LHC detects the Higgs boson, and they measure the mass and say, "Well, dang, the math works out for CDM with these numbers", hey, I'm ready to shrug and say "I guess you guys have got something after all." But in the meantime, I consider it hypocritical of CDM theorists to dismiss non-Newtonian gravitational theories as a class as "purely descriptive" when, at this point in our knowledge, the same charge can with equal fairness be leveled at CDM.
3 — I'll freely admit I don't fully grasp the arguments behind dark energy and its role. It still bothers me that CDM has to invoke multiple new entities, required to behave in certain tightly specified ways, but for which we are given no explanation of what they actually are — or why they should exist — other than that this specific mathematical framework relies upon the assumption that they do. It's the "If we had some ham, we could have ham and eggs, if we had any eggs" problem.
4 — I know I've read papers (my memory says out of Germany) pointing out aspects of the Bullet observations which [the authors assert] cannot be fully explained by CDM without positing additional factors not currently contained within CDM, even if they considered the contribution of dark energy. This is one of several things I went looking for links to, and couldn't find. (I also tried — but failed — to find the paper that invoked the possibility of a fifth basic force in the Universe, and the weakly-bound-graviton theory.)
[1] Hmmm .... actually, in hindsight, I'm not certain I did explicitly mention that. But I intended to. Honest...
Re: bah. stupid 4300-character comment limit again. (Part 1)
Re: bah. stupid 4300-character comment limit again. (Part 1)
3. Any new theoretical construct is a placeholder for our ignorance. Take for example the neutrino: Fermi basically pulled it out of his ass in order to conserve momentum in neutron decay. Purely circumstantial. But it seems to have worked out ...
Just to be clear, the expansion history depends only on the ratio of total matter-to-dark energy, whereas CMB, galactic dynamics and structure formation depend on CDM but not dark energy. So it's not right to say that CDM and dark energy need each other ...
Re: bah. stupid 4300-character comment limit again. (Part 1)
Basically, the location of the first peak tells you the geometric curvature of the universe; the ratio of the first to the second tells you the baryon abundance; and the ratio of the first and third tells you the dark matter abundance for that given baryon abundance.
(Technically, the third and higher peaks tell you the dark matter-to-radiation ratio for the given baryon abundance, but the total radiation is very tightly constrained by Big Bang nucleosynthesis (http://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis).)