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Missing Matter... Still Missing
Posted by
timothy
on Wed May 05, 2004 04:08 PM
from the hey-it's-zero dept.
from the hey-it's-zero dept.
squidfrog writes "Nature.com, PhysicsWeb, and the BBC all report on the latest results from the Cryogenic Dark Matter Search. 'The most powerful search yet for the Universe's missing matter has come up empty handed, contradicting an earlier study that claimed to have seen new particles.' 'A favoured theory is that the dark matter consists of Wimps (weakly interacting massive particles) about a thousand times more massive than a proton, one of the particles found in an atom's nucleus... on the rare occasions a Wimp strikes an ordinary atom, the effect should be noticeable.' 'Writing in the Physical Review Letters, the team says that while a detection has yet to occur, there is now a better idea of how much dark matter must exist.' They 'hope to improve the sensitivity of the experiment by another factor of 20 over the next few years.' What's 20 times 0? And don't tell me zero!"
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Missing Matter... Still Missing
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I "detect" a grant money detector at work... (Score:5, Insightful)
However since it started running in November last year, the detector has not seen a single WIMP.
Then they decide to make a more sensitive detector so that they can "not" detect at an even higher level?
Physicists with the CDMSII experiment say they will now add another 24 crystals to the detector, increasing its sensitivity tenfold.
Okay, maybe I am being a bit silly, but, I still don't see how they can know the detector is working. I don't even know how the WIMP can make the thing "ring" once it, itself, is subject to the 1/10 degree above absolute Zero conditions. And then, somehow, with no data, they can extrapolate more accurately how much dark matter is in the universe. Well, they would say the lack of WIMPS is data but I'm not buying it. Enough /. folks have worked in research to know better than to buy into those kinds of statistical games (you can prove almost anything with non-parametric statistics).
Happy Trails!
Erick
Re:I "detect" a grant money detector at work... (Score:5, Informative)
(http://matt.truch.net/ | Last Journal: Wednesday August 29 2001, @07:19PM)
Re:Why do dark matter found (Score:5, Informative)
(http://www.stileproject.com/ | Last Journal: Friday June 22, @03:09PM)
Re:Why do dark matter found (Score:4, Informative)
There is a thery that there is little or no dark matter, and the difference is accounted for by the assumption that the inverse square law for gravity fails at large distances -- based on a theoretical model of graviton particle exchanges that would not follow inverse square -- This just happens to match the observed data pretty well without need for dark matter.
A second alternative is combines light speed decay along with big change in assumed age of universe, so that spiral galaxies look the way they do because they are quite young compared to the standard model.
I'll bet there are other non-darm matter models that are explain observed data as well as the dark matter model too.
Re:Why do dark matter found (Score:5, Interesting)
However, that isn't true. One unique test of dark matter is that it is dynamical; it can move. And there are a bunch of tests that have started to be made that show evidence for dynamical dark matter:
- in order to explain rotation curves without dark matter, models like MOND require force laws that would make the derived "shape" of the dark matter halo spherical at large radius. You can test this by looking at the shapes of clusters using X-ray emitting gas (eg. Buote et al. 2002, ApJ, 577, 183; Lee & Suto 2003, ApJ, 585, 151; Lee & Suto 2004, ApJ, 601, 599) or the Sunyaev-Zeldovich effect (LS03,LS04). You can also look at the shapes of dark matter halos around galaxies using weak gravitational lensing (Hoekstra et al. 2004, ApJ, 606, 67). So far all of the tests indicate that dark matter halos are not spherical, but flattened exactly as predicted by cold dark matter.
- the bars in barred spiral galaxies should slow down and disperse quickly in a spherical static halo potential, like you'd get from modifying the force law, but they can be maintained for long periods of time if they can exchange angular momentum with the dark matter (Athanassoula 2002, ApJ, 569, L83; Valenzuela & Klypin 2003, MNRAS, 234, 459).
- there's a weak gravitational lensing observation of a group that is falling into a cluster, where the mass of the infalling group is offset from the light - the gas is moving slower because it's interacting with the cluster gas, while the dark matter has kept moving (Clowe et al. 2004, ApJ, 604, 596).
[TMB]
Basic assumptions may be wrong (Score:4, Insightful)
Re:I "detect" a grant money detector at work... (Score:5, Informative)
I'm sure this is part of thier validation that the detector is working.
I "detect" someone jumping to conclusions (Score:5, Insightful)
That the sensor has never detected something doesn't tell you that it's working or not working - or am I am missing something here?
Yah, you're missing the scientific paper. This is a one page write-up written by a journalist. The one page write up doesn't describe how they know the detector works, but I'm sure they have _some_ means of testing that it does. Blame the article, but at this point you can't really accuse anyone of doing shoddy science for grant money.
Re:I "detect" someone jumping to conclusions (Score:5, Funny)
Sure they do... the system has a green light on. If the red light were on it would be on standby and no light may mean there is no power, or the light is broken. But as long s the green light is on they know it's working.
Surely everyone knows that. Now please increase my grant.
Re:I "detect" someone jumping to conclusions (Score:4, Funny)
>
>Surely everyone knows that. Now please increase my grant
You forgot the third possibility. Suppose the power indicator LED is orange: it's hard to tell if we're in a superposition of states or merely oscillating very rapidly.
Or I just want a high-speed digital camera for Christmas.
Working detector? (Score:5, Informative)
All sorts of particles are constantly flying in and creating signals in their detectors. This is how they know that it is working. The trick is to veto the known signals by surrounding their superconductors with other detectors which can detect ordinary matter, but not dark matter. Therefore if the other detectors tell you that some ordinary matter entered the superconductor, then you would reject that signal.
In the context of a dark matter flux (flow) measurement, greater sensitivity means a greater ability to detect low fluxes. So far they've measured 0 dark matter particles in a few years of running. This means that the flux is less than 1 particle per detector area per few years (also per detector efficiency).
Suppose the numerical value of their measurement is that the flux is less than 100/m^2/year (just to use round numbers). Then, if the true flux given to us by nature is 1/m^2/year, then they would have to run for another ~100 years in order to detect 1 dark matter event. On the other hand, if they make their detector 100 times larger, then they can detect the 1 dark matter event with only 1 more year of running. This is what they mean by increased sensitivity by building a larger detector. Meanwhile, in the time taken to see the 1 dark matter event, they probably reject several trillion false events which are caused by ordinary matter particles.
A. Physicist
Check the actual webpage... (Score:5, Informative)
(http://matt.truch.net/ | Last Journal: Wednesday August 29 2001, @07:19PM)
Re:Chilled out (Score:5, Informative)
Re:Chilled out (Score:5, Funny)
How do they do it?
Ever been to Minnesota? In the winter? You wouldn't have to ask.
Re:Chilled out (Score:4, Informative)
How do they do it?
I assume you mean how do they cool it that low rather than how they found an abonadoned mine in Minnesota.
First, I imagene you have a series of refrigerators. If you've seen the movie Akira you have an idea what I'm talkign about. You put various types of refrigerators inside of eachother to limit the heat coming in from outside.
Take Helium (He) and put under pressure till it is in liquid form. If you let it boil, it will cool down to about 4K at atmospheric pressure. if you lower the atmospheric pressure by pumping out all the atmosphere, it will cool lower. This will take you to about 1K.
To get lower you can use a mixture of He3 and He4 (Helium atoms with different atomic weights) and cool it to make a dilution refrigerator. The lighter He3 will spearate from the He4. The He4 works to absorb the He3. You pump off He3 out of the He4 at the othe end of the tube and it cooles the remaining He3 as it is disolved into the He4. This should take you to the temperatures needed for this experiment. Simply put your experiment inside of the cold He3.
You can get even lower with various magnetic traps that allow fast atoms to "evoprorate" out of the traps but this tends to be for a small amount of atoms.
Re:Chilled out (Score:4, Informative)
They use a "dilution refrigerator" to get that cold. Dilution refrigeration uses a mixture of He3/He4 (mash) and cycles between two phases of the mixture (a He3 rich phase and a He3 dilute phase). The He3 and He4 are both liquids at this point.
Here's a basic overview of cryogenics. Liquid Nitrogen (LN2) liquifies at 77 K in 1 atmosphere. N2 is abundant, and LN2 is priced cheaper than milk. LHe4 liquifies at 4.2 K, and costs (here in the USA) about $4 per liter. I think it's much more expensive elsewhere in the world, but helium is mined w/ natural gas companies, so is more plentiful here than elsewhere. LHe3 is a rare isotope of Helium and vastly more expensive. It liquifies (I think) around 3K, and costs several hundred dollars for a few gaseous liters (here in USA).
So one can easily get to 4.2 K by dipping something in LHe4. One can employ evaporative cooling, and 'pump' on the LHe4 dewar, and get down to temperatures of about 1.5K. Perhaps slightly lower for bigger pumps. This cooling is quite easy and cheap to do, but often doesn't get low enough in temperature. If one has LHe3, that can be pumped on to get down to about 200 mK. But this is difficult because LHe3 is so expensive, and closed-cycle pumps are needed so as not to waste the cryogen.
Dilution fridges can get to lower temperatures. We just got one of these fridges in our lab, and using that I've cooled some samples down to about 20 mK. Dilution fridges have fundamental limits around 6 mK or so, but physical limits usually kick in earlier than that due to equilibrium between cooling 'power' and heating (mostly due to radiation and vibration). The basic thermodynamics are actually quite similar to your standard fridge, and you can think of it as He3 'evaporating' out of the mash, absorbing energy as they do so. And later the He3 is condensed back into the mash.
Fridge operation basically has a mixing chamber, which is the 'cold' point of the system. One hopes to create the phase boundary between the two phases here. The mixture absorbs heat from the sample, and the dilute phase travels up to the still, where it's pumped on by some big-ass pumping lines. The liquid is effectively warmed up, gets circulated around and re-condensed by a cold block at about 1.5 K. [This block is called the 1-K pot and is only pumped LHe4]. There's a flow impedance put in (to calibrate the pumping power with the circulation to get the phase separation at the right place). Then it's back into the mixing chamber. Meanwhile there are many heat exchangers along the way, exchanging heat from the incoming rich phase and outgoing dilute phase. The cooling power of the fridge is greatly increased depending on these heat exchangers. The effective sample size in our fridge is a cylinder about 1 inch diameter and 10 inches long. The dewar itself is about 7 feet tall and 3 feet diameter, and there's a rack of electronics and four pumps to go with it. So it's a big unit for a relatively small cooling volume.
Dewers are designed using stainless steel and other components to minimize thermal conductance to room temperature as much as possible. Radiative heating, however, is a problem. The dewar is evacuated between the 'cold' part and the outside, to minimize conductance. Radiation goes as T^4, and this power law is greatly exploited in dewar design. If one surrounds the 'cold' part of the dewar with a LN2 shroud, the cold part sees radiation at 77K instead of 300K. This factor of ~1/4 translates to a drop in radiative heating power of about 1/250.
Beyond this dewars use superinsulation, whereby aluminized mylar is wrapped around many times (with spacers), so each successive layer sees a colder temperature. So the 20mK part of the dewar might only be surrounded by an effective layer of a few K. These methods cut radiative heating down by factors of millions or more.
But it's obvious... (Score:5, Funny)
Wimp?! (Score:5, Funny)
(http://www.cookingwiththewarchef.com/ | Last Journal: Friday February 04 2005, @03:24PM)
The Real Dark Matter (Score:4, Funny)
Gravity is wrong (Score:4, Interesting)
(http://www.ckwop.me.uk/)
I think the answer to the dark matter problem and the quantum theory of gravity is one in the same. Our description of gravity is wrong. It has recently been discovered [physicsweb.org] that dark matter is 'missing' from three elliptic galaxies. One would think that on the scale of something as big as a galaxy and with WIMPs being so massive that you ought to detect some quite major effect..
Add that to the fact that the universe's acceleration is getting quicker rather than slowing down and I think we have a strong case for our description of gravity being incorrect.
Simon.
Re:Gravity is wrong (Score:5, Informative)
How this contradicts general relativity is, quite frankly, a mystery to me. For the past 10-15 years various teams around the world have been working on large-scale simulations of the field equations in attempts to provide accurate numerical simulations of effects predicted by general relativity. An area which receives considerable attention is that of galaxy formation due to aggregation of matter. Practically all such simulations proceed in the same manner, namely they add a matter source to the full four-dimensional field equations, decompose these into 3+1 form, and then evolve the initial spacelike hypersurface numerically in such a way that both the Hamiltonian and momentum constraints remain satisfied.
Granted, such simulations are prone to error, but that's more to do with the fact that Einstein's equations rely on a dynamical metric rather than any problem with general relativity per se.
Dark Energy
Again, see my original post. Here's a recipe for dealing with dark energy in general relativity: take a compact manifold without boundary. Modify the scalar curvature of said manifold in such a way that R -> R - L, where L is some strictly positive constant. Work out the field equations and look at what you get. For those too impatient to do this by hand, the result is that you get a universe whose expansion is governed by the cosmological constant L. Tune the parameter L to fit observations and hence prove that general relativity, once again, describes large scale gravitational effects. Next!
Dark Matter
Again, begin with a compact manifold without boundary. Transform the four-dimensional field equations into 3+1 form using the usual technique. Develop a Jacobi-type action functional. Conformally transform the spatial metric such that g_ij -> (phi)^4 g_ij for some strictly positive scalar function (phi). Require that (phi) implements volume-invariance so that we introduce a volume term into the action (for the curious, the reason for this stems from Yamabe's theorem on invariants of compact manifolds). Work out the dynamical equations for g_ij and K^ij and the constraints. Proceed as usual.
There seems to be a misunderstanding on the part of almost everyone who's taken part in this thread as to the definition of a physical theory. If you had taken the time to read the part of my original post where I said
GR is perhaps the most well-tested physical theory yet developed and, as such, you can't say that it's "wrong". It plainly isn't once you remain within its field of reference.
then perhaps we wouldn't have had the unfortunate sight of people bleating about GR being "wrong". As I said, general relativity *is not wrong* within the realm of its applicability. If people want to go off and bleat about exotic particles then they should talk about supersymmetry and graded Lie algebras rather than GR.
The impressive thing is that while many alternative theories have been proposed, they probably all reduce to general relativity in some particular limit. It doesn't matter whether you want to consider metric, affine, or metric-affine theories, they all most likely reduce to the familiar formulation of GR.
The bottom line is this: if you're looking for something which describes gravitational processes, use general relativity. If you're looking for something which explains why gravity is the way it is, go use something else, preferably something that includes a spin 2 graviton.
Re:Gravity is wrong (Score:4, Informative)
The important phrase in this is "the presumed distribution of mass." The presumption you speak of rests on notions derived from quantum field theory about the behaviour of matter. Certainly, general relativity indicates a discrepancy in this case. However, if you move to supersymmetry and introduce the possibility of WIMPs, then calculations based on that work out quite well.
Expensive shot: Gravity in the presence of delocalization. One can, in principle, convert huge amounts of mass-energy (gigagrams) into photons, and diffract those photons over large distances (hundreds of meters). GR predicts that those photons will create a substantial gravity field, one that is "trivially" measurable with a spatial resolution of meters. GR does not predict the values of those measurements. Ergo, GR does not describe the universe. It is easy to devise other quantized systems for which GR predicts a particular average result, but makes no predictions about the distribution of results.
Nobody, including me, has ever laid claim to general relativity being an acceptable description of the universe. General relativity, as I mentioned in the original post, works fantastically well in its field of reference. Quantum processes, as you're perfectly aware, are outside that field of reference and hence cannot be used to disprove the validity of general relativity.
Well, if it is... (Score:5, Informative)
(http://www.phys.ufl.edu/~siegel | Last Journal: Tuesday May 11 2004, @06:04PM)
As for the description of gravity being incorrect, I hate to tell you this, but general relativity solves *so* many problems that cannot be solved otherwise that it's preposterous at this point to consider anything else. Gravitational lensing, bending of light by masses, binary pulsar decay, Mercury's perihelion precession... etc. etc... NO other theory of gravity explains any of this, unless it starts with General Relativity and expands on it.
As for your proof that there is no dark matter because it's there in small quantities in three (out of ~250,000) galaxies, give me a break. Normal matter clumps and interacts with itself, so it's quite reasonable to expect we will get some cases where we have more normal matter than dark matter.
On average, though, Dark Matter is well known (see my comment history for examples) to exist in about 6-7 times the abundance of normal matter.
Sorry if this is a rant, but talk about throwing the baby out with the bath water...
Maybe - (Score:3, Funny)
What if software bugs emit gravitons? Wouldn't that explain the apparent extra mass in the universe?
The Answer (Score:4, Funny)
(Last Journal: Saturday May 10 2003, @07:35PM)
Zero.
Opps. I meant, seven.
Re:The Answer (Score:5, Interesting)
IBM may agree with you! Try this code on AIX:
#include"stdio.h"
int x,y,z;
int main() {
x=1;
y=0;
z=x/y;
printf("%d", z);
}
On most unix implementations you get floating point exception since the divide operator takes floating point operands. On AIX, when the denominator is cast to a float, it's a zero approximation rather than the official floating point zero. The result is that instead of a core dump, you get... 15.
Re:Forgive my ignorance (Score:5, Funny)
(http://slashdot.org/)
You just described my entire high school career.
Re:Forgive my ignorance (Score:4, Interesting)
(http://www.stox.org/)
Re:Forgive my ignorance (Score:5, Informative)
--Tom
Dark Matter (Score:3, Interesting)
(http://postbohemia.blogspot.com/ | Last Journal: Thursday January 08 2004, @09:33AM)
Missing Matter (Score:4, Insightful)
(http://slashdot.org/)
Obvious, but ... (Score:4, Funny)
(http://www.jasani.org/ | Last Journal: Monday September 09 2002, @04:33PM)
Did anyone check under the cushions on the couch?
The obvious solution (Score:5, Funny)
(Last Journal: Tuesday April 24 2007, @07:35PM)
Re:The obvious solution (Score:4, Funny)
What's 20 times 0? (Score:4, Funny)
Not completely zero (Score:5, Insightful)
But at least they are still trying... and trying... and trying some more.
No events != 0 sensitivity (Score:5, Insightful)
(Last Journal: Saturday May 01 2004, @12:15AM)
The fact that the detector hasn't found the thing it was designed to detect doesn't mean that it has a zero sensitivity or that the hypothesis is bogus(you can't readily prove a negative except by proving a contradictory positive), just that, in the finite time it's been running, it hasn't been sensitive ENOUGH to detect anything. 20 x 0.00000000000000000(you get the picture)001 is still an improvement, and may be enough to make progress.
Unusual science (Score:5, Interesting)
(http://www.livejournal.com/users/control_group)
This research, though, seems to be taking the same route: rather than questioning the model, they continue a so-far fruitless search for the "missing matter." If the model demands something the existence of which we are completely unable to verify, shouldn't we be questioning the model? Doesn't the very fact that there's all this "missing" matter indicate that perhaps our understanding is flawed?
Or am I just displaying rampant ignorance of the current state of physics and cosmology by asking this?
Re:Unusual science (Score:4, Interesting)
It's not proven or anything, and it competes with inflation theory. But it looks like it might be experimentally verifiable, as opposed to inflation.
The answer lies on the other side of the aether (Score:3, Interesting)