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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!"
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 Letters article (note, the paper hasn't been published yet), as well as other (published and unpublished) papers, as well as general info.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
No, they don't extrapolate how much dark matter is in the universe. They say, if dark matter is of the 'WIMP' variety, we know that the mass and cross section (aka how easily they interact with other particles, namely the germainium nuclei in their detectors) of of these WIMPS is not in a certain range.
--Xandu
I'm sure this is part of thier validation that the detector is working.
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...
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!
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
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.
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