Most Sensitive Detector Yet Fails To Find Any Signs of Dark Matter

ananyo writes "A U.S. team that claims to have built the world's most sensitive dark matter detector has completed its first data run without seeing any sign of the stuff. In a webcast presentation today at the Sanford Underground Laboratory in Lead, South Dakota, physicists working on the Large Underground Xenon (LUX) experiment said they had seen nothing statistically compelling in 110 days of data-taking. 'We find absolutely no events consistent with any kind of dark matter,' says LUX co-spokesman Rick Gaitskell, a physicist at Brown University in Providence, Rhode Island. Physicists know from astronomical observations that 85% of the Universe's matter is dark, making itself known only through its gravitational pull on conventional matter. Some think it may also engage in weak but detectable collisions with ordinary matter, and several direct detection experiments have reported tantalizing hints of these candidate dark matter particles, known as WIMPs (Weakly Interacting Massive Particles). Gaitskell says that it is now overwhelmingly likely that earlier sightings were statistical fluctuations. Despite the no-shows at XENON-100 and LUX, Laura Baudis, a physicist on XENON-100 at the University of Zurich in Switzerland, says physicists are not ready to give up on the idea of detecting WIMPs. They may simply have a lower mass, or may be more weakly interacting than originally hoped. 'We have some way to go,' she says."

Physicists know

[Spy Handler]

from astronomical observations that 85% of the Universe's matter is dark"

They don't *know*, they're deducing this from reconciling observed data with general relativity but it's far from certain.

However relativity is not infallible, maybe it's true only in a special case -- like how Newtonian mechanics works great but only in a special case (bigger size than quantum scale, less velocity than ~1/10 c, etc)

Maybe at very large size and mass such as galaxies, general relativity doesn't hold and there's a better theory for explaining motion and gravity. If so we wouldn't have to invent nonexistent dark matter to account for the faster-than-expected galactic rotation and other things.

Re:Have they considiered...

[invid]

It's there. We've detected it from its gravity. They were just hoping that it wasn't completely dark. It's starting to look like it is. The trouble with it being completely dark is that would make it difficult to prove any theories about it. What they're doing is searching for their keys under the streetlight when they've probably fallen down the sewer.

Re:Maybe

[Anon-Admin]

What we have is a phenomenon that is not explained by the calculated mass of the universe. As a filler we have titled it "Dark Matter" and "Dark Energy" and given it a mathematical correction to the calculations.

The mass issue is fixed if we realize that the size of the universe is larger than the visible horizon. Meaning it is bigger than we can see. With that we can assume that we can only see 13% of the whole universe and that the reset of it is too far away to see. Now, run those numbers through the formula to calculate the expansion rate of the universe and you get some great results!

The energy issue disappears when you realize that the closer an object is to a gravity well the slower time moves. Thus there is a large time differential between the edge of a given galaxy and intergalactic space. This time differential accounts for the perceived added gravity.

Better yet, paint it hot pink and put an SEP field around it. It is a better solution.

Re:Maybe

[occasional_dabbler]

One of the most enlightening books I ever read was Peter Woit's crticicism of string theory [amazon.co.uk] The problem with modern physics is that it now takes so long to learn what has gone before that you are past your productivity peak by the time you have the tools needed to be able to contribute. Put very simply - mankind is close to the limit of what we can work out. We need either a genius way further out on the curve than Einstein or Hawkins (who doesn't want to just become an investment banker...) or we need an extrordinarily lucky break. We won't be getting better data than the LHC has provided for another century,

Re:Maybe

[Immerman]

I believe a solution to the rotation curve problem has actually been proposed by analyzing galactic motion using General Relativity-based gravity equations rather than the much simpler Newtonian ones. Using the more accurate equations renders an expected rotation profile far more consistent consistent with observations to within a tiny percentage.

Of course that doesn't explain some of the other phenomena that supports Dark Matter, but it could mean we're looking for it in the wrong conceptual places.

Re:Maybe

[boristhespider]

The difference here is that whereas normally the "indirect" signals we receive are photons directly from a particle, or indeed a measurable and reproducible influence on known quantities in a laboratory setting (which includes the tracks of known particles through accelerators), dark matter is not easily amenable to such tests. We only see it (interpreting "it" loosely -- the way I use the words, 'dark matter' should be interpreted as 'the fact that galaxies, clusters and the universe as a whole act as though there is more matter than we observe', which is probably infuriatingly vague :( ) through its gravitational effects, and by the sheer weakness of gravity and the impractical idea of creating, well, galaxies in a laboratory setting it is never going to be directly detectable that way.

The Higgs boson, on the other hand, was seen in reproducible experiments. I do agree that we can quibble on whether it was a direct detection, or whether it was indirect, given that its existence was ultimately deduced from the pattern of particles around it - but there are big differences. For one thing, a (relatively) quick analysis of the shrapnel from a collision that produced a Higgs will point to a particle of a particular mass and nature. That can then be reproduced (albeit at a low likelihood, given the nature of the experiment), and has been. We only even saw announcements from CERN when two independent experiments both reported an excess at the same mass. (In particle physics these certainly used to be called "resonances" -- when you find that collisions with a particular energy change nature dramatically, you can be pretty certain there's a particle there. For all I know, they're still called resonances, but my particle physics is second-hand through textbooks and therefore about 25 or 30 years out of date.)

It basically comes down to a detection on local scales, under conditions we can control, through a force other than gravity. We can't examine anything through gravity - it's uselessly weak, and impossible to control. That's a "direct detection", and can be through interactions with photons, or the influence of the new particle on the particles we observe coming out of its interactions and annihilations, or anything along those lines that can be seen, influenced, reproduced, observed. We can't do that with the evidence for dark matter. All we have is that galaxies rotate faster than they should (and they do, unequivocably), and that clusters should not really be bound (but they are, equally unequivocably), and that we cannot account for this with our current theories of gravity. The easiest solution is at least one particulate dark matter, certainly -- but if that exists it *is* amenable to production in a lab, even if to actually observe it we would have to wade through ten times more data than the LHC pours out, or a billion times more. But that isn't the only solution, because the only evidence we have is through gravity, and there is absolutely no reason at all (and it would be a mild form of intellectual blindeness) to prematurely declare that "dark matter" is definitely particulate and not, say, a sign that gravity does not behave on kpc scales the way it does on AU scales, let alone on Mpc and Gpc.