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#11
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Basic question about dark matter interaction
On 7/21/15 7:17 PM, Steve Willner wrote:
What actually happens is that density contrast grows over time, the negative potential energy being offset by increased (on average) kinetic energy. Numerous dark matter simulations show this very clearly. There are a few movies of this process on youtube, but I've seen much better ones. (Some movies illustrate the matter distribution at the present epoch "z=0", but what you want is one illustrating the evolution from high redshift to now.) One (now older) simulation is described at http://www.mpa-garching.mpg.de/galfo...um/index.shtml At the bottom of the page, there are pictures illustrating the density distributions at various epochs. I'm disappointed that there doesn't seem to be a movie. There is an entirely different field of study related to wastewater treatment with its own terminology that deals with density distributions in a fluid medium in hydraulic shear state: Google:activated sludge filamentous bacteria images https://en.wikipedia.org/wiki/Activated_sludge_model These filamentous forms seem to have the same visual characteristics as DM filaments. Richard D Saam |
#12
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Basic question about dark matter interaction
On 22/07/2015 01:17, Steve Willner wrote:
In article , Martin Brown writes: Are there any decent theoretical bounds on the characteristics of candidate CDM WIMPs/axions that allow the experimenters to know that interactions with xenon or other crystal based scintillators will occur? I gather there are predictions for particular particle types and vaguely remember an article, perhaps in _Physics Today_, from some years ago. A very quick web search found https://www.astro.umd.edu/~ssm/darkm...periments.html I bet a careful search would turn up much more. Thanks. That link is very interesting. I had thought that MOND was now also ruled out of the game now but that page seems to say not. Are any of the detectors located in the tropics where there might be a slight daily modulation from the gravitational focussing effect of the WIMP particles as they pass through the Earth? Wouldn't the major component of motion be the Sun's orbit around the Milky Way center? Yes. And any passing CDM will still be fast enough that the Earths gravity isn't going to alter their trajectory by much. The Boulby DRIFT detector exploits this to distinguish real signal by direction. Since gravity is the only thing they really see does any of the Earth, Jupiter or Sun offer enough gravitational pull x nuclear collision cross section to ever capture some of them in bound orbits? The cross sections are presumably similar to (or less than) those of neutrinos, so I wouldn't think so. Maybe it could happen if the elastic collision cross section is much higher than the inelastic cross section. If it's that high, though, I'd expect (without having done the calculation) major astrophysical consequences, which we don't see. I found some claims that they might and that as a result WIMP anti-WIMP interactions might lead to annihilation reactions with observable high energy neutrino and gamma emission as a consequence. I presume with such a low interaction cross section the residual mix of WIMP and anti-WIMP is much closer to being 50:50 than for real matter. What is the fate of a gravitationally bound WIMP in the suns core? How did it get captured? Assuming it did, it probably just orbits around the Sun's center, coming into thermal equilibrium with the local gas if the elastic interaction cross section is high. Eventually, assuming a non-zero inelastic cross section, the particle would undergo some nuclear reaction with products depending on just what the particle is. Thanks. It is hard to see the wood for the trees sometimes. I can find all sort of claims and counter claims about CDM in a web search. It was very interesting to see the lab where Zepplin lived. A cleanroom built inside a working potash mine (in the more stable salt layer) is an interesting thing to visit. Outside everything is coated in dirty crystalline salt which glistens in the helmet lamp light. Keeping that muck from getting inside and wrecking experiments is non trivial. They are presently qualifying materials for the next generation of CDM detectors in a screened environment where background radioactivity is about a million times lower than at the surface. All we saw go through it in realtime was alpha particles not even a neutron The detectors are inside "castles" of nested lead and copper block shielding from locally generated radiation eg. from visiting humans! Also interesting geology and tests for exobiology in the same vicinity since they have brine seeps with extremophile archeobacteria in too. An exobiology lab is colocated with the dark matter facility and in addition they have some beautiful mineral samples from the mine. -- Regards, Martin Brown |
#13
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Basic question about dark matter interaction
In article , Steve Willner
writes: Are any of the detectors located in the tropics where there might be a slight daily modulation from the gravitational focussing effect of the WIMP particles as they pass through the Earth? Wouldn't the major component of motion be the Sun's orbit around the Milky Way center? Of the motion, yes, but the timescale is too long to notice any modulation. |
#14
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Basic question about dark matter interaction
In article , Martin Brown
writes: Thanks. That link is very interesting. I had thought that MOND was now also ruled out of the game now but that page seems to say not. MOND was developed to explain flat rotation curves of galaxies without the need for dark matter. Of course it still works for this. There is only one adjustable parameter (which seems to have a "natural" value), and over time many other phenomena could be explained with the same value of the adjustable parameter. This has not changed either. This original MOND was not really a theory, just an empirical fit. Certain theories based on MOND have been ruled out. There is now much evidence for dark matter apart from flat rotation curves of spiral galaxies. This cannot be explained by MOND, at least not by the empirical MOND or a small extrapolation of it. However, it could of course be the case that the MOND effect is real but that there is also dark matter. There is no reason this could not be the case. Psychologically, it might not be desirable to supporters of MOND who don't like dark matter, but the universe is not obliged to conform to our tastes. It might not be desirable to supporters of dark matter, but the simplest explanation is not always the best. For example, it turned out that neutrinos have mass, and are thus non-baryonic dark matter, but the masses are too small to allow neutrinos to make up all non-baryonic dark matter, so we already have a case where the simplest explanation didn't turn out to be right. Of course, one should prefer the simplest explanation unless a more complicated one works substantially better (that a more complicated one could work somewhat better is not sufficient). The question is whether, in the case of spiral galaxies, MOND or DM is the better explanation. Note that MOND doesn't give just approximately flat rotation curves, which DM simulations can also give, but predict many details. It is not obvious that these simply "fall out" of DM, and at least some claims that they do is not convincing. Personally, I am a bit sceptical of MOND, at least until some underlying theory predicts something like it, but on the other hand my impression is that the supporters of MOND in general know more about astronomy and astrophysics than many of the detractors, and many "refutations" of MOND are quite superficial and show that the detractors haven't investigated MOND in detail. Note that Oxford University professor James Binney, who literally wrote the book on galactic dynamics, has been a MOND supporter for a while now. MOND is not some fringe or crackpot theory, but an alternative hypothesis which has not yet been ruled out. |
#15
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Basic question about dark matter interaction
On Sunday, July 19, 2015 at 11:46:15 AM UTC-4, Phillip Helbig (undress to reply) wrote:
Suggest another candidate WHICH IS NOT ALREADY RULED OUT BY OBSERVATIONS. mimetic dark matter http://arxiv.org/abs/1308.5410 sincerely, Brad Johnson (no! not that one, or that one, or that one |
#16
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Basic question about dark matter interaction
On 7/24/2015 8:59 AM, Phillip Helbig (undress to reply) wrote:
.. ... For example, it turned out that neutrinos have mass, and are thus non-baryonic dark matter, but the masses are too small to allow neutrinos to make up all non-baryonic dark matter, so we already have a case where the simplest explanation didn't turn out to be right. Of course, one should prefer the simplest explanation unless a more complicated one works substantially better But it would be strange if DM consisted of only one type of particle! Luminous matter is a mix of many particle types and there is 4 times more dark than luminous matter, so shouldn't it (DM) be entitled to consist of a complete new particle zoo? -- Jos |
#17
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Basic question about dark matter interaction
In article , brad
writes: Suggest another candidate WHICH IS NOT ALREADY RULED OUT BY OBSERVATIONS. mimetic dark matter http://arxiv.org/abs/1308.5410 Actually, this is more a theory of modified gravity than a dark-matter candidate. Like MOND, the idea is that a modification of the laws of gravity can give effects similar to dark matter. |
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