

Has dark matter finally been detected? - indy
http://www.guardian.co.uk/science/2009/dec/17/dark-matter-detected

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roundsquare
_One of the great mysteries is why time only goes in one direction, and one
candidate to explain that is a dark matter particle._

How?

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anigbrowl
Surprisingly accurate for a newspaper. Project: <http://cdms.berkeley.edu/>
Nice to see that upgrades are planned to the detection equipment.

Besides confirming theoretical predictions and presumably improving our
understanding of cosmology, are there any potential applications for this
knowledge at an earthly scale?

~~~
amichail
_Besides confirming theoretical predictions and presumably improving our
understanding of cosmology, are there any potential applications for this
knowledge at an earthly scale?_

One idea is to use dark matter as fuel for interstellar travel:

[http://www.newscientist.com/article/mg20427361.000-dark-
powe...](http://www.newscientist.com/article/mg20427361.000-dark-power-grand-
designs-for-interstellar-travel.html)

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richardw
IANAP.

"To detect dark matter, scientists have to wait for the extremely rare
occasion when a dark matter particle knocks into an atomic nucleus in the
detector and makes it vibrate"

Why? If dark matter is 90+% of the mass of 'everything', then surely many more
anomalies would occur without us having to be hit by a chunk of dark matter.
It would simply have to be anywhere near us (or a nearby body we could observe
carefully) to observe weird gravitational effects. Nearby ones, not 'out there
in other galaxies' ones.

Assuming it's not near us, why would that be? Our star isn't much affected by
it. We can't see nearby stars being pulled willy-nilly. How far out must we
search before we could reasonably assume it doesn't exist and there might be
another explanation?

[edit] And if it's a weird non-localised 'blanket' that can't hit you in the
foot, then it'd be great to have some guesses as to the structure so we can
test for it. Does it exist inside all galaxies? Is it in the middle, or does
it rotate like normal 'matter'? If it exists in the perimeter, is much
heavier, and spins like visible matter, do we have to invent a super-ultra-
dark matter to explain why common-or-garden dark matter doesn't fly off into
space? If it's in the middle and clumpy, could it collapse into a dark-matter
black hole, especially since there's no radiation to force particles apart? Is
it only one particle type, or a family like visible matter? Since it has mass,
is there another form it can convert to as visible mass does? Since it doesn't
radiate (being dark) what happens to the energy when (if) it converts?

If it's clumpy, stop worrying about visible asteroids. It's the invisible ones
that can really hurt you :)

~~~
Locke1689
_Why? If dark matter is 90+% of the mass of 'everything', then surely many
more anomalies would occur without us having to be hit by a chunk of dark
matter._

Well, if the dark matter is ~90% of the mass of everything, that doesn't
necessarily mean that 90% of the mass of every thing is dark matter. As far as
I understand it, we see the effects of the large amount of extra matter but we
don't really know where it is or how it's distributed. That could mean that
there isn't any dark matter clustered in large enough quantities close by for
us to detect.

 _Assuming it's not near us, why would that be? Our star isn't much affected
by it. We can't see nearby stars being pulled willy-nilly._

Well, a really spread out distribution across the universe would explain that.

 _And if it's a weird non-localised 'blanket' that can't hit you in the foot,
then it'd be great to have some guesses as to the structure so we can test for
it._

Well we have a prediction for what the particle would look like (/act like).
That's what the detector is there for. Well, not just dark matter. Think
neutrinos and Cherenkov radiation.

As far as the other questions go, I'm not sure enough to answer and I'm pretty
sure that some of those questions are open.

~~~
richardw
I asked mostly for the conversation value, I certainly am not sure of anything
myself!

 _Well, a really spread out distribution across the universe would explain
that._

It would, but then there's the 'why' it's spread out. Maybe that's answered in
the particle predictions but something is keeping it apart, but doesn't affect
visible matter in the same way. It's obviously spread out just enough to not
be clumpy, but not so much that it can't pull a galaxy together. If it were
everywhere it wouldn't have the effect we think we see.

~~~
hugh_
Ordinary matter forms dense clumps like planets and stars because it's sticky.
Atoms may be attracted to one another by gravity, but in order to get them to
actually stick together into dust grains, planets, stars et cetera, you need
them to collide inelastically, and that requires the electromagnetic force.

Two dark-matter particles in the void will be attracted to each other
(slightly) by gravity, but when they meet they'll just sail straight on
through one another without colliding. This prevents the formation of small-
scale (ie sub-galactic scale) dark matter clumps.

------
rms
<http://resonaances.blogspot.com/2009/12/cdms-live.html>

_"18:20 Summarizing, no discovery. Just a hint of a signal but with a very low
statistical significance. Was fun anyway."_

As the newspaper article said, a 75% chance of finding new particles.

~~~
hugh_
Also from Cosmic Variance:

[http://blogs.discovermagazine.com/cosmicvariance/2009/12/17/...](http://blogs.discovermagazine.com/cosmicvariance/2009/12/17/dark-
matter-detected-or-not-live-blogging-the-seminar/)

2:50: Bottom line: “The results cannot be interpreted as significant evidence
for WIMP interactions, but we cannot reject the possibility that either event
is signal.” –Risa

To which I reply "Hang on, so you made a big deal out of this for what? Wake
me when you have something which _can_ be interpreted as significant evidence
for something"

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simon_
IANAP, but wouldn't confirmation of the existence of supersymmetric WIMPs not
necessarily prove that they account for a large portion of dark matter?

EDIT: Not to imply that it wouldn't be a _huge_ deal anyway.

~~~
hugh_
IAAP,BNTRTOP but I assume you could count how many you detect to get a
reasonable idea of how common they are in this part of the universe, at least.
Then you could adjust it for the fact that WIMPs tend to prefer to hang out in
gravity wells, like around here, and get a pretty good idea of the overall
distribution. I haven't done the maths, but if you're detecting _any_ of 'em
then they've got to be reasonably common, since they should be relatively
uniformly distributed throughout the galaxy.

What I _didn't_ understand until just now was this: if WIMPs interact by
gravity why don't they fall into stars and planets, or even form stars and
planets of their own? The answer is that they _do_ fall into stars and
planets, but just keep going through the middle (since they are not subject to
any ordinary interactions) and out the other side. And they can't form dark-
matter planets of their own since they don't interact strongly with each other
either, so they won't stick together. Neato!

~~~
rms
BNTRTOP?

~~~
JacobAldridge
But Not The Really Top Order Physicist?

Boy, Newton Totally Responded To Our Planet?

Basically, Nobody Trusts RMS To Out-think PG?

~~~
hugh_
The others who said "but not the right type of physicist" correctly
ascertained my meaning, though yours are all quite accurate as well.

~~~
carbocation
I was imagining, "But not the relevant type of physicist." Excellent new
acronym.

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thras
No it hasn't. This is 2 events in an expected background of 0.8. According to
reports it's a 1.5 sigma event. Noise.

This can be dismissed until they get to 3-5 sigma.

~~~
gjm11
It shouldn't be _dismissed_. Suppose two candidate hypotheses are (a) no dark
matter and (b) enough dark matter to produce an expected signal at +1sigma in
this experiment. Then a 1.5sigma event is about [EDIT: following ratio was
wrong, sorry] 3:1 Bayesian evidence for (b) over (a). Of course that's nowhere
near enough to justify saying (b) is right and (a) is wrong, and of course the
simple normal-distribution model I've assumed is likely too simple; but I
think a 1.5sigma event should certainly lead you to adjust your probability
estimates a little.

