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Basic question about dark matter interaction



 
 
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  #11  
Old July 23rd 15, 06:36 AM posted to sci.astro.research
Richard D. Saam
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Posts: 240
Default 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  
Old July 23rd 15, 06:37 AM posted to sci.astro.research
Martin Brown
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Default 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  
Old July 23rd 15, 06:37 AM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Default 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  
Old July 24th 15, 07:59 AM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Posts: 273
Default 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  
Old July 25th 15, 09:15 AM posted to sci.astro.research
brad
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Posts: 102
Default 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  
Old July 28th 15, 05:34 PM posted to sci.astro.research
Jos Bergervoet
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Posts: 126
Default 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  
Old July 28th 15, 05:34 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Posts: 273
Default 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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