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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 Letters article (note, the paper hasn't been published yet), as well as other (published and unpublished) papers, as well as general info.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
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...
> How could a wimp be so large and yet unnoticed?
You just described my entire high school career.
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.
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.
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!
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
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 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...