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Missing Matter... Still Missing
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!"
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
http://www.busyweather.com/
For much more info, head to the CDMS homepage, which includes links to preprints of the mentioned Phys. Rev. Letters article (note, the paper hasn't been published yet), as well as other (published and unpublished) papers, as well as general info.
--Xandu
Anyone in high school knows that if a wimp hits anything, no one notices. If someone did notice, he wouldn't be a wimp.
"As God is my witness, I thought turkeys could fly." A. Carlson
If a Wimp is about a thousand times more massive than a proton - what does that make a proton? a Wuss? or a Nerd?
- Your stupidity got you into this mess, why can't it get you out? -Will Rogers
The real dark matter in the universe is the massive SCO intellectual property rights that no one else has yet seen.
"It's the height of ridiculousness to say for those 9 lines you get hundreds of millions."
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 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.
Zero.
Opps. I meant, seven.
All they have to do is reverse the polarity of the anti-proton injectors in the warp core, re-route the resulting subspace pulse through the plasma conduits, synchronise the comm-system to transmit the frequency of the subspace distortion field to the deflector dish and emit a sub-tachyon particle scan over a wide area. That'd surely reveal what they're looking for!
Drill baby drill - on Mars
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.
AccountKiller
I am not a physics/math expert, but assuming that dark matter does exist, it only proves that the equipment currently used has a sensitivity that is approaching zero, but not zero. But anyone who has seen a graph of an asymptope, it is not very promising especially if you push x approaching infinity. Even if you were to multiply x by 20, while you are out to infinity, by not knowning where exactly they are relative to the origin on the graph, a factor of 20 may not be all that significant... :-/
But at least they are still trying... and trying... and trying some more.
Cooling is done in tiers (over a distance of many meters). I would assume that the outermost is cooled to 76K with LN2 since that is dirt cheap. And then inside that LHe cools it down to a couple Kelvin or so, maybe less if they use superfluidic Helium. This much is pretty standard by now. As far as the last degree or so, I would guess they mess with the pressure a bit to get the temperature as low as possible.
I hate to say it, but CDMS II (this experiment) was SUPPOSED to not find WIMPs in this range. There was an experiment called DAMA which had found a modulation in their noise consistent with their being WIMP dark matter, and they claimed detection. The whole purpose of this press release is to say that DAMA's claimed detection is now *ruled out*.
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...
Wow, what a suprise, hyperbole in the Slashdot summary.
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.
NB: YMMV. IANAL. Take the above with a grain of salt.
> How could a wimp be so large and yet unnoticed?
You just described my entire high school career.
The detector is also chilled to within a tenth of a degree of absolute zero [...]
How do they do it?
Ever been to Minnesota? In the winter? You wouldn't have to ask.
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?
Reality has a conservative bias: it conserves mass, energy, momentum...
Nice with the conspiracy theory, AC. Too bad that you're wrong. The first tip-off that there's dark matter is the rotational speed of galaxies. Your decaying speed of light won't explain that.
Fascism trolls keeping me up every night. When I starts a preachin', he HITS ME WITH HIS REICH!
I'd just like to be the first to say that it's an honor, Mister President, to count you amongst the Slashdot readership.
Size, mass and interaction strength are unrelated. For example, imagine trying to detect clouds by throwing rocks at them. Clouds are big, but they only interact with rocks very weakly.
--Tom
Blasphemy is a human right. Blasphemophobia kills.
CDMS detectors detect heat (vibrational energy) which is deposited in their superconductors when any kind of particle flies in and hits them. The localized heat causes the hit region to go non-superconducting, and as a result they can measure a reduced current as would be expected from a normal conductor.
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
Maybe there is no dark matter. Science only describe predicted observations. Reality doesn't necessarily obey the laws of science. Belief in such is similar to belief in a deity. Maybe the universe is governed by the laws of science, but then again, maybe it is governed by such-n-such a deity.
So if a theory isn't cutting it, then create a new model of whatever observation that you are trying to describe. It seems silly to try to fit nature to the theory, and not the theory to nature.
One of the possible outcomes of string theory is multiple universes, each separated by a fairly small distance (of course this distance is in a higher dimension so we can't notice them). If these alternate universes do exist, it is thought that the gravity from particles in our universe affects the other nearby universes. Imagine our universe as a flat sheet and another universe is a parallel flat sheet close to ours. In this model, gravity would still be three dimensional - ie, it would be able to bridge the gap between universes and affect the other universe. Perhaps this is what we're noticing - the gravity of massive particles in another universe?
BTW, I am not a physist but I have read up on this stuff. The theory of gravity carrying over to other universes actually does make sense - it explains why gravity is so much weaker than the other forces, because much of gravity's effect isn't on this universe. There's experiments going on now to test and see whether this is actually the case but I don't know the outcome. Anyway, this is just my thought on perhaps why we can't detect the dark matter - because it's not physically in our universe.
The theory of dark matter is based on the assumption that the basic properties of the Universe have never changed over time. If the intrinsic properties of space itself HAVE changed significantly, then there is no need to postulate such a thing as dark matter. Scientists are very reluctant to accept new data that shakes their preconceived pet ideas to their foundations. It took over 200 years after Roemer first measured a finite light speed, for the majority of scientists to accept the fact that light did not get instantaneously from point A to point B, as was the belief for centuries. In the same way, the majority of scientists today refuse to even consider the idea that some very fundamental "constants" may have changed dramatically since the beginning of time. For example, the cause for the "Red Shift" of distant star light is traditionally attributed to the Doppler effect, and in light of that INTERPRETATION of the cause for an observed fact, (the shifted light) all sorts of cosmological observations are very difficult to explain. Humans (including scientists) like to assume that certain things stay the same for all time, but that is a fervently desired wish based on faith, not observed fact. It seems that in the physical universe, there is nothing as constant as change! AAW
All theory is gray
The numbers come from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) which measured fluctuations in the Cosmic Microwave background (afterglow of the Big Bang). There's a good review of their results in hep-ph/0308251 accessible from the LANL preprint server though it might be a bit technical for most.
The thing that makes the dark matter explanation compelling is that it makes so many different observations work. We don't have to fine tune things so much - it all fits together. Here are some examples.
1. Galaxy rotation curves - you can watch the orbits of stars in a galaxy to determine the distribution of matter in the galaxy. This shows that there is a lot more matter than can be accounted for by the stars and that it is distributed differently.
2. Gravitational lensing - you can see how light is bent by distant galaxies to map out their matter distributions. Again, there's a lot more matter than the stars can account for, distributed differently.
3. The cosmic microwave background - this one is complicated, but the idea is that you look at the "afterglow" of the big bang, released when the universe was as dense and hot as the surface of a star. We understand the physics of matter at these temperatures very well, and by studying the signatures of vibrations in this hot plasma, we can measure the properties of the early universe. We can see from this that the universe contains a lot of matter, and that the large majority of this matter is not composed of ordinary atoms (hard to explain, but fairly rock solid).
4. Light elements - Most of the universe's helium, deuterium, lithium and beryllium were created in the early universe, not in stars (the conditions aren't right). Again, the physics is very well-understood, nothing fancy. By studying the relative ratios of these elements, we can figure out the properties of the plasma in which they were formed (a bit hotter and you get less deuterium, the temperature falls too quick and you get less helium, stuff like that). Again, the universe has a lot of matter, and most of it isn't made of atoms.
5. Structure formation - if you work things out on supercomputers, you find that (if the universe containst only ordinary matter) the universe hasn't been around long enough to form the galaxies and galaxy superclusters that we see. Adding dark matter to the mix makes galaxies form faster - just enough faster!
And the beautiful thing is that all of these different arguments give essentially the same answer for the amount of dark matter and its basic behavior. You can tweak your theories to explain some of these observations, but no one has been able to explain them all - except with dark matter, the SIMPLEST explanation!!
Before you say something is "clearly inferior intellectual flotsam", learn what you're talking about...
Comment removed based on user account deletion
There seems to be a common sense here that all of the evidence for dark matter could be equivalently explained by changing the force law.
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]