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Cuneiformist
The Imperfectionist
USA
4955 Posts |
 Posted - 06/12/2012 : 16:24:43
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Hi all-- long time, no see.
I am here because I'd love some feedback on something I recently stumbled across regarding Dark Matter.
For those not in the know (and that largely includes me), Dark Matter is a proposed form of matter that exists in a way that doesn't interact normally with light like normal matter (so we can't detect it via conventional means) but which nevertheless interacts with other matter via gravity.
As I understand it, this was proposed because scholars who were looking at the rotations of galaxies noticed that such actions didn't behave according to how we understood gravity-- in particular, the furthest points of observes galaxies were rotating as fast as the central parts. Assuming out understanding of gravity is correct, the only other explanation was that the outer parts of galaxies contained massive amounts of this invisible dark matter.
I bring this up, because I stumbled across this SciAm blog.
The blog itself is not particularly interesting to me, though. Much more interesting is one of the [url='http://blogs.scientificamerican.com/critical-opalescence/2012/06/11/is-dark-matter-a-glimpse-of-a-deeper-level-of-reality/#comment-5
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Dave W.
Info Junkie
USA
26020 Posts |
Posted - 06/12/2012 : 16:43:22 [Permalink]
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It's not just the rotation of galaxies that suggests Dark Matter exists, it's a bunch of other observations as well. The only theory to date that better explains every related phenomenon is Dark Matter. Other theories may explain one or two phenomena well, but they fall flat for others. Apply Occam's Razor. 
Ethan Siegel wrote an awesome summary a couple months ago. |
- Dave W. (Private Msg, EMail)
Evidently, I rock!
Why not question something for a change?
Visit Dave's Psoriasis Info, too. |
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Dave W.
Info Junkie
USA
26020 Posts |
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On fire for Christ
SFN Regular
Norway
1273 Posts |
Posted - 06/13/2012 : 00:42:06 [Permalink]
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| I think the comment is basically talking about modified Newtonian dynamics. It's more of a fringe theory at the moment. |
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HalfMooner
Dingaling
Philippines
15831 Posts |
Posted - 06/13/2012 : 00:47:25 [Permalink]
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Yes, welcome back, Cune! Got that cuneiform avatar finished?
I've been baffled by barred spiral galaxies since I was a kid. I could not figure out how features like a straight bar across these galaxies could exist for any significant time, given that the gravity well of galaxies should cause everything within them to rotate faster towards the center than at the edge, quickly twisting the bar into unrecognizability. I suppose the distribution of dark matter at the edges of galaxies must explain that, eh?
The dark matter's distribution must make the gravity well an entirely different shape than that of objects such as the sun or planets. Instead of sharply down-curving well that has steeper and steeper sides as one approaches the center, perhaps galaxies have more of a bowl-shaped gravity well? |
“Biology is just physics that has begun to smell bad.” —HalfMooner
Here's a link to Moonscape News, and one to its Archive. |
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sailingsoul
SFN Addict
2830 Posts |
Posted - 06/13/2012 : 03:39:19 [Permalink]
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| Consequently, some astronomers and physicists suspect there may be no dark matter after all. If you notice the floorboards in your house are sagging, as if there is too much weight on them, you might conclude there is an 800-pound gorilla in the room with you. You see no gorilla, so it must be invisible. You hear no gorilla, so it must be silent. You smell no gorilla, so it must be odorless. After a while, the gorilla seems so improbably stealthy that you begin to think there must be some other explanation for the sagging floorboards—the house has settled, say. Likewise, perhaps the laws of gravity and motion which led astronomers to deduce dark matter are wrong. “I think dark matter will be a sign of another type of physics,” Verlinde said. |
There more to the topic than this gorilla analogy implies. That may be. 
Patricia Burchat sheds light on dark matter here. |
There are only two types of religious people, the deceivers and the deceived. SS |
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Tim Thompson
New Member
USA
36 Posts |
 Posted - 06/13/2012 : 18:43:11 [Permalink]
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The blog comment referenced by Cuneiformist refers only to the study of galaxy rotation curves, established primarily in Rubin & Ford, 1970 (not sure why he cites a 1980 paper), but in fact anticipated in Oort, 1932. The lack of Keplerian motion is most simply interpreted as the presence of matter as yet unseen and it's requisite gravitational influence. But dark matter, or "missing mass", was first introduced by Zwicky, 1933 (which I think may have gone largely unnoticed until the study was published in English, Zwicky, 1937), which deals with the motion of galaxies in galaxy clusters. But modern cosmic microwave background (CMB) cosmology strongly implies that the dark matter cannot be ordinary baryonic matter, and must be of a more exotic form, because its lack of ability to scatter off of electromagnetic photons alters the power spectrum of acoustic wave patterns in the observed CMB.
I have actually posted quite a bit about dark matter in another thread, so perhaps I may be allowed to rather extensively quote myself, which puts all of my strictly dark matter related posts from this forum in one place. First, on the history & science of dark matter ...
Originally posted by Tim Thompson
Originally posted by Sebastian (09/14/2011 07:02:46)
The CERN laboratory now routinely traps and examines hydrogen antimatter, but no sign of dark matter nor dark energy yet, despite the fact its existence was first postulated as early as 1934 to explain anomalies of the orbital velocities of galaxies.
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But this is poor reasoning altogether and deserves criticism. The time it takes to verify any hypothesis depends critically on the scientific context. Isaac Newton was well aware of the problems that form the basis of the general theory of relativity, and tried to solve them himself. However, the knowledge available in his time, both in theory & practice, was insufficient to the task. The time between the publication of Newton's Principia (1687) and Einstein's solution for general relativity (1915) is 228 years, and they were not the only 2 people aware of the issues. At what point should all scientists have felt justified in declaring that, since they had not already solved the problem, it must be unsolvable, and we should now all give up and quit?
When Zwicky first introduced the problem of "missing mass" (Zwicky, 1933; Zwicky, 1937; the latter translation into English was the first time many astronomers became aware of the issue) he had no reason to believe it was not ordinary matter; gas, dark clouds of dust, dim stars, planets & etc. And there is no single moment when suddenly & without warning astronomers exclaimed "it must be non-baryonic dark matter". It just slowly dawned on astronomers as they continued to look and did not see. In this case, the context was set by technology. If you look for something and do not see it, even though you know it lies well within the capabilities of your technology, then the failure to detect it is significant, and increases in significance as time & technology advance with continuing invisibility of that which should be seen. The advent of radio & infrared astronomy is particularly damaging to baryonic dark matter, since cooler stars & especially clouds of dust & gas will stand out clearly at theses wavelengths, where they would be truly invisible to Zwicky. Likewise, X-ray astronomy reveals hot gas & low-mass stars (the latter, being fully convective, are somewhat more prone to flare than solar-type stars and so become evident as X-ray flare sources). In the case of our own Milky Way, specific searches for low mass stars have clearly demonstrated that they cannot be the ultimate source of dark matter (e.g., Hubble Space Telescope, 1994).
Inference from observation is a crucial element of the scientific process and should not be discounted as "merely hypothetical". Not all hypotheses are created equal nor should they all be considered on equal footing. In this case, the hypotheses of dark matter & dark energy are both supported by significant observational data. If astrophysical dark matter is baryonic, then where & what is it and how does one explain that it is not visible to instruments which have the capability to see it? That is just as serious an issue as the fact that non-baryonic dark matter has not been directly detected, and in my mind a more serious issue in fact, which gives significant weight to the dark matter hypothesis.
But there is much skepticism in the scientific community, and while cold dark matter is the basis of the "consensus cosmology" it is by no means universal in the professional community. After all, the hypothesis of dark matter relies on the assumption that the law of gravity is well known. Modify gravity, and you can do away with dark matter altogether, and there is significant research published along those lines as well. Likewise, dark energy could be an error in evaluating the light curves of distant supernovae, or like dark matter, due to a modification of the laws of gravity or to general relativity. These too are active areas of investigation.
So perhaps the bottom line for this thread is that there is in fact a healthy skepticism about big bang cosmology that is in fact enshrined in the scientific literature itself. But the consensus is such because it is in fact the most scientifically reasonable conclusion.
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And the rest of my posts are a rather extensive list of references to current literature, some of the general review type, and some perhaps more detailed ...
Originally posted by Tim Thompson
Comments on Cosmology and Dark Matter
Long, long ago, in a galaxy far, far away, we were talking about dark matter. Let me return to that topic by once again quoting myself:
Originally posted by Tim Thompson
My conclusion: I don't see how anyone can look at the full weight of the evidence, both direct & indirect, and not conclude that that the presence of non-baryonic dark matter is in fact the best solution currently available to the scientific community as an explanations for the "missing mass" problem. I do not mean by this that the alternative solution, a modification of the law of gravity, should be ignore; quite the opposite, it should be pursued as best we can. But credit, as they say, should go where credit is due, and non-baryonic dark matter is in fact the superior solution on offer, by virtue of both quantity and quality of both observational evidence and theoretical considerations.
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There really isn't much more for me to say now. In that post I laid out the empirical evidence favoring the case for the existence of non-baryonic dark matter. I will add to that a reference which in fact appeared only today:
Evidence for dark matter modulation in CoGeNT
Chiara Arina, et al., preprint dated 14 November 2011.
Abstract:
We investigate the question of whether the recent modulation signal claimed by CoGeNT is best explained by the dark matter (DM) hypothesis from a Bayesian model comparison perspective. We consider five phenomenological explanations for the data: no modulation signal, modulation due to DM, modulation due to DM compatible with the total CoGeNT rate, and a signal coming from other physics with a free phase but annual period, or with a free phase and a free period. In each scenario, we assign to the free parameters physically motivated priors. We find that the no modulation model is excluded with odds in excess of 10^5 : 1 when all energy bins are included in the analysis. The DM models are strongly preferred over explanations due to other physics, even when astrophysical uncertainties are taken into account and the impact of priors assessed. However, the evidence for the DM model in which the modulation signal is compatible with the total rate is significantly weaker than for a DM model in which this prior is not implemented, a result driven mainly by the large modulation amplitude observed in the energy range (0.9 - 3.0) keV by CoGeNT. Classical hypothesis testing also rules out the null hypothesis of no modulation at the 4.5-sigma to 4.8-sigma level, depending on the details of the alternative. Lastly, we investigate whether anisotropic velocity distributions can help to mitigate the tension between the CoGeNT total and modulated rates, and find encouraging results.
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Originally posted by Tim Thompson
And on the specific topic of dark matter & dark energy, dominant of late, aside from the books above, all 3 of which address the topic appropriately, one might wish to consult the current research. Of course, research papers are the means of communication between professionals in sometimes narrowly focussed fields, and can be dense reading for outsiders. Still, there are some general review papers that fit this discussion well enough. So here is a selection from the literature; I present these papers because I know about them, no necessarily because I an familiar with them. But they all come from reliable sources and should be themselves therefore reliable.
On Dark Matter: This should be enough, along with the books, to at least get the point across that dark matter is not just some casual idea with little merit. Rather, it is an idea that essentially forces itself on astronomy for lack of any better explanation.
The missing matter problem: from the dark matter search to alternative hypotheses; Capozziello, ]et al., October 2011; preprint of astrophysics text book chapter.
Searches for Particle Dark Matter: An Introduction; Pat Scott; Textbook-level introductory review section of Stockholm University PhD thesis, posted by popular demand. Defended May 4, 2010. 84 pages
Astrophysical Constraints on Dark Matter; Charling Tao; Proceedings of the 3rd International conference on Directional Detection of Dark Matter (CYGNUS 2011), Aussois, France, 8-10 June 2011
Dark Matter; J. Einasto; Baltic Astronomy 20: 231-240 (2011)
Dark Matter: A Primer; Garrett & Duda; Advances in Astronomy 2011
Highlights and Conclusions of the Chalonge Meudon Workshop: Dark Matter in the Universe and Universal Properties of Galaxies: Theories and Observations; de Vega & Sanchez; July 2010
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Originally posted by Tim Thompson
To the documentary evidence I have already provided, which clearly falsifies the notion that the dark matter hypothesis is in any way "beyond scientific verification and falsification", add the following recent sources ...
Possibility of a Dark Matter Interpretation for the Excess in Isotropic Radio Emission Reported by ARCADE by Nicolao Fornengo, et al., Physical Review Letters 107, 271302, 30 December 2011
Abstract: The ARCADE 2 Collaboration has recently measured an isotropic radio emission which is significantly brighter than the expected contributions from known extra-galactic sources. The simplest explanation of such excess involves a “new” population of unresolved sources which become the most numerous at very low (observationally unreached) brightness. We investigate this scenario in terms of synchrotron radiation induced by weakly interacting massive particle (WIMP) annihilations or decays in extra-galactic halos. Intriguingly, for light-mass WIMPs with a thermal annihilation cross section, the level of expected radio emission matches the ARCADE observations.
There is also a related news article at Physics World: Radio wave excess could point to dark matter, dated 1 December 2011. This is evidence in the true scientific sense, in that it constitutes a valid inference from observation. The observation is radio emission from outside the galaxy, which cannot yet be explained by any other known sources of radio emission. By itself, of course, the study proves nothing. But like most scientific evidence, it is indicative, it points towards an implied conclusion, which is made explicit in the last sentence of the abstract: We know that physically reasonable, hypothesized dark matter particles, would be expected to generate a radio emission consistent with the radio emission we see. This does not prove that dark matter is responsible for the observed radio emission, which might well have some other source. But it is an intriguing coincidence that what we see so nicely matches what we would expect to see if dark matter were the actual source. It is just one element in an expanding library of evidence favoring the presence of dark matter particles. Each element in the library, by itself, does not amount to much, but taken together, as a whole body of evidence, they have significant implications that scientists cannot meaningfully ignore, as Sebastian seems to think they should.
And while we are on the dark matter trail, here is another recent entry to the lists:
The Empirical Case for 10 GeV Dark Matter by Dan Hooper, online preprint: arXiv:1201.1303v1 dated 5 Jan 2012
Abstract: In this article, I summarize and discuss the body of evidence which has accumulated in favor of dark matter in the form of approximately 10 GeV particles. This evidence includes the spectrum and angular distribution of gamma rays from the Galactic Center, the synchrotron emission from the Milky Way's radio filaments, the diffuse synchrotron emission from the Inner Galaxy (the "WMAP Haze") and low-energy signals from the direct detection experiments DAMA/LIBRA, CoGeNT and CRESST-II. This collection of observations can be explained by a relatively light dark matter particle with an annihilation cross section consistent with that predicted for a simple thermal relic (sigma v ~ 10^-26 cm^3/s) and with a distribution in the halo of the Milky Way consistent with that predicted from simulations. Astrophysical explanations for the gamma ray and synchrotron signals, in contrast, have not been successful in accommodating these observations. Similarly, the phase of the annual modulation observed by DAMA/LIBRA (and now supported by CoGeNT) is inconsistent with all known or postulated modulating backgrounds, but are in good agreement with expectations for dark matter scattering. This scenario is consistent with all existing indirect and collider constraints, as well as the constraints placed by CDMS. Consistency with xenon-based experiments can be achieved if the response of liquid xenon to very low-energy nuclear recoils is somewhat suppressed relative to previous evaluations, or if the dark matter possesses different couplings to protons and neutrons.
More evidence, this time a combination of the admittedly ambiguous evidence I already mentioned, from the direct detection experiments, plus observations of radio emission from inside, not outside, the Milky Way, as well as gamma rays. Again, by itself only suggestive, but combined with other evidence it becomes a key player. We see multiple observations, from both laboratory experiments and astronomical observations, all coincidentally consistent with what we would expect to see from dark matter particles.
On the modified gravity front, I said above, "none of them have been able to produce consistency with so wide a collection of observation as the more mundane notion of dark matter". Here is what one researcher from Case Western Reserve University presents:
Modifying Gravity: You Can't Always Get What You Want by Glenn Starkman, online preprint: arXiv:1201.1697 dated 9 Jan 2012
Abstract; The combination of GR and the Standard Model disagrees with numerous observations on scales from our Solar System up. In the concordance model of cosmology, these contradictions are removed or alleviated by the introduction of three completely independent new components of stress-energy -- the inflaton, dark matter, and dark energy. Each of these in its turn is meant to have (or to currently) dominate the dynamics of the universe. There is still no non-gravitational evidence for any of these dark sectors; nor for the required extensions of the standard model. An alternative is to imagine that GR itself must be modified. Certain coincidences of scale even suggest that one might expect not to have to make three independent. Because they must address the most different types of data, attempts to replace dark matter with modified gravity are the most controversial. A phenomenological model (or family of models), Modified Newtonian Dynamics, has, over the last few years seen several covariant realizations. We discuss a number of challenges that any model that seeks to replace dark matter with modified gravity must face: the loss of Birkhoff's Theorem, and the calculational simplifications it implies; the failure to explain clusters, whether static or interacting, and the consequent need to introduce dark matter of some form, whether hot dark matter neutrinos, or dark fields that arise in new sectors of the modified gravity theory; the intrusion of cosmological expansion into the modified force law, that arises precisely because of the coincidence in scale between the centripetal acceleration at which Newtonian gravity fails in galaxies, and the cosmic acceleration. We conclude with the observation that, although modified gravity may indeed manage to replace dark matter, it is likely to do so by becoming or incorporating, a dark matter theory itself.
So Starkman thinks the evident weaknesses of the modified gravity approach are significant enough that, even if we allow that gravity must be modified, we still can't avoid dark matter.
Science is a moving target. There will be more papers after this, more evidence presented as time goes by. Eventually the weight of scientific opinion will come down somewhere, and so far it looks like that somewhere will be the realm of WIMP dark matter, and for pretty good reasons. In light of the evidence, how can anyone sanely argue that the dark matter hypothesis is "beyond scientific verification and falsification"?
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There. Hopefully, this does not count as overdoing it. But I think it's not a bad idea to have all of these references in one place, for anyone interested in further pursuing the matter of dark matter. I have trimmed material on dark energy or other topics, but include links back to the original posts is you feel the need for context.
Reference was made to "Modified Newtonian Dynamics" (MOND), which is the brainchild of Israeli physicist Mordehai Milgrom (Milgrom, 1980a, Milgrom, 1980b, Milgrom, 1980c, Bekenstein & Milgrom, 1984). Milgrom seeks to modify Newton's law of gravity over very large distances, thus creating an effect that looks like extra mass but really is not. Although a subject of much ongoing study, MOND is not currently a strong contender; Ferreras, et al., 2008 & Ferreras, et al., 2012 show that MOND alone cannot explain gravitational lensing observations, and dark matter is still required, while Shi, 2011 finds MOND incompatible with the motions of the galaxies in our local group. While MOND appears to be able to do a good job on galaxy rotation curves (absent the gravitational lensing problem), it does a poor job with galaxy clusters, and most seriously, is not compatible with the CMB power spectrum (Dodelson, 2011). I think it is too harsh to call MOND a "fringe theory"; it is probably the most successful of the alternative theories, but it certainly is inferior to the prosaic choice that there is simply more mass than we can see.
Edited to add this:
I did say I was interested in putting all of my dark matter posts in one place (at least to the extent that I can remember them). That said, I should also point out the nearby short topic "Vast Structure Discovery....Nixes Dark Matter and my two posts therein, number 10 and number 13, the latter being perhaps more directly relevant. |
The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell |
| Edited by - Tim Thompson on 06/14/2012 15:59:32 |
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Machi4velli
SFN Regular
USA
854 Posts |
Posted - 06/13/2012 : 19:17:43 [Permalink]
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| Thanks Tim! |
"Truth does not change because it is, or is not, believed by a majority of the people."
-Giordano Bruno
"The greatest enemy of knowledge is not ignorance, but the illusion of knowledge."
-Stephen Hawking
"Seeking what is true is not seeking what is desirable"
-Albert Camus |
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Dark Matter misconception?
