Most Sensitive Detector Yet Fails To Find Any Signs of Dark Matter 293
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."
Direct dark matter detection is confusing (Score:5, Informative)
Several different experiments have tried to measure dark matter directly in the lab, and the experimental situation is pretty confusing. This plot [ggpht.com] shows the confidence intervals and exclusion limits for various experiments (but it does not include LUX yet). The shaded regions are confidence intervals, that basically say "we've seen dark matter, and its properties lie somewhere in this region. But the dotted lines say "we haven't seen it, and if it exists, it can't lie above these lines".
What is strange, then, is that all of the detections are in regions that have been excluded by other experiements. LUX just makes the situation even more strained by pulling those upper bounds even lower. Still, those bounds and intervals depend on assumptions about the properties of dark matter, and it may be possible to reconcile [blogspot.co.uk] the results.
It will be interesting to see what happens to those tentative detections when they get more data. My bet is that in the end some systematic effect will be found to be responsible for the apparent signal. Or (much less likely) that they were just flukes. But who knows?
Re:Have they considiered... (Score:5, Informative)
Guess they should have given up on the Higgs boson search 10 years ago, too? A negative results is not a "failure", it just constrains things a little more.
The most compelling evidence for dark matter is http://en.wikipedia.org/wiki/Bullet_Cluster [wikipedia.org]
Obviously we should always be open to alternate hypotheses, but at the moment dark matter is still the most straightforward explanation.
Re:Maybe (Score:4, Informative)
We can certainly detect dark matter. The CMBR studies have show it fairly directly (we've "observed" dark matter as much as we "observe" things with an electron microscope or radio telescope). The ratio of "normal" matter to "dark" matter in the early universe has been measured to 2 significant digits (perhaps more since last I looked into it).
The unknown part is what dark matter is made of. We know it's there, we just don't know what it is.
Re:Maybe (Score:4, Informative)
This experiment tries to find some other interactions, but none so far were detected.
Re:Dark matter fighting dark energy (Score:5, Informative)
Or to put it another way:
1. Scientists come up with theories to explain a phenomenon
2. Test to confirm
3. New observation breaks the theory
4. Theory refined to account for new measurements
5. Goto 2
That doesn't look like bad science at all.
The dark matter thing is stuck at step 2 as it may be either (a) the theory is wrong or (b) dark matter is really really hard to test for.
Science is a process, not a big book of answers. If you want a big book of answers there are any number of religions willing to accommodate you. Just be aware that the answers you get may be (1) vague, (2) contradictory and (3) of limited predictive use.
Re:Maybe (Score:5, Informative)
At what point did it become ok in the scientific community to keep on with a theory that evidence contradicts?
Where has it been contradicted here? The failure to observe WIMPs by this experiment doesn't mean that they don't exist -- just that they don't have certain properties that would make them detectable by this instrument.
It's like the search for the Higgs boson. There were theories that allowed for the Higgs to exist at lower energy levels than it was eventually found at. We tested them with the LEP and with Tevatron, in the 1990s. As we ruled out those lower (and some higher) energy levels, we got closer and closer to the truth. The Higgs boson exists are a mass somewhere around 125 GeV/c^2.
All this experiment has done is narrow the parameters a bit so far. Did you make a similar cry in 2011, when Tevatron shut down that we shouldn't have been wasting money on the LHC because the Higgs was contradicted? If so, then shame on you then. If not, then shame on you now.
The day I realized that the previous three chapters I had read were not science, but rather theories that were based on other theories based on yet other theories that only existed because the first theory was shown to be wrong at some point, was a real downer.
How is that not science? Science is all about filling in the gaps and trying to find explanations for what we don't know -- including the things we didn't previously know we didn't know. It's not some divine revelation that you either get right the first time or you disregard it as heresy and falsehood. It's a global learning process.
Re:Have they considiered... (Score:3, Informative)
Re:Maybe (Score:5, Informative)
Protip... that still doesn't explain the rotation curve problem observed in spiral galaxies.
Re:Maybe (Score:5, Informative)
"What we have is a phenomenon that is not explained by the calculated mass of the universe."
Vague statement. What we have are two phenomena, one which is not explained by the observed mass in galaxies or in clusters, and one not explained by the present (and currently only serious) model of the universe. Feel free to propose alternative models for the universe... but make sure that they fit the current observations *at least* as well as that model and fails to break the Solar System. That is hard to do.
"As a filler we have titled it "Dark Matter" and "Dark Energy" and given it a mathematical correction to the calculations."
True, with the correction above.
"The mass issue is fixed if we realize that the size of the universe is larger than the visible horizon."
No it isn't. That will do precisely nothing for the rotation curves of galaxies and will also basically do nothing for the cosmological problem either. Vague hand-waving and appeals to Mach's principle don't hold without a concrete model. Provide that model and people may be convinced, but at the minute what you're suggesting is startlingly acausal and, as a result, unacceptable.
"Meaning it is bigger than we can see."
Very true. No-one thinks that the entire universe is the observed universe.
"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!"
Nope, you get precisely the same results that we currently get, because while it may startle you, that's what we currently do -- effectively. Thanks to causality, matter outside of our horizon cannot have an effect on us. Basically, something which is far enough away from us that light cannot have made the distance cannot possibly have influenced us. That, or you have to propose a new theory of gravity -- good luck with that one. It's a common game in cosmology, and one which precious few people since Einstein have had any luck at.
"The energy issue disappears when you realize that the closer an object is to a gravity well the slower time moves."
No it doesn't. Do you think that we're using non-relativistic models of cosmology? Relativity is at the heart of your statement that gravity wells dilate time, and relativity is at the heart of cosmological models.
"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."
Now this is a much more interesting statement. Dig out Wiltshire's attempts to use time dilations between galactic clusters and voids to explain the dark energy problem, firmly in the context of general relativity. The fundamentals are not well-studied, but it is promising. However, it goes the opposite direction from your surmise -- it tends towards providing a dark energy rather than a dark matter. It does drive home the point though that it is vital to actually try and calculate something based on an idea, properly rooted in a concrete theory. The answers might be rather different from what you expected...
Re:Have they considiered... (Score:4, Informative)
Actually, it separated the hot gas in the galaxies from the stars and dark matter in the galaxies. Stars are so small compared to the distances between them that when galaxies collide, the stars just pass right through each other. The same applies to the dark matter (because it doesn't interact electromagnetically (or it would be visible), it does not experience any significant friction force). But the diffuse, hot gas collides and gets left behind in the collision. So you end up with dark matter and stars on each side of the collision point, and a huge amount of hot gas stuck in the middle. That gas is much heavier than the stars, so without dark matter, the gravitational field should be concentrated around the gas. But instead we see it (through gravitational lensing) to be concentrated around the stars (which is where we would expect the dark matter to be as explained above).
Re:Maybe (Score:5, Informative)
I'm a professional cosmologist, and I have to take issue with your first statement. The instruments did not, and categorically have not, detected the presence of something that is matter. If they had, that would be a direct detection of dark matter, and a Nobel prize would already be sitting on their desk. What they have detected are indirect signals of dark matter. It is very hard to reproduce the observations - particularly the cosmological observations - without adding at least one component of dark matter. So the observations are typically interpreted in terms of dark matter.
But this is very much not, strictly speaking, necessary. What we have is something that has an effect which, when viewed through a Robertson-Walker model, looks for all the world like a species of massive, weakly-interacting particle (or two or three such species - no-one ever said there has to be only one). On smaller scales, we have what for all the world appears to be a large amount of mass that can't be seen.
Any of this could be down to a modification of gravity. We know the nature of gravity roughly up to the position of the Voyager craft -- call it 300AU to be generous. We are extrapolating that a thousand times to get to galactic scales, a million times to get to cluster scales, and a thousand million times to get to cosmological scales, all without evidence. Of course, without a better theory to replace relativity, it's the best we can do, so we do it - but don't try and claim that instruments have detected that it is matter (they haven't), nor that we are wedded to particulate dark matter (with caveats, we aren't; the caveats are firstly that neutrinos have a mass and are therefore a rather warm dark matter, and secondly that it seems rather unlikely that there isn't at least one species of weakly interacting matter which would act as CDM, but maybe not in sufficient abundance to answer our woes).
Re:Maybe (Score:5, Informative)
It's always enlightening to see how it looks to people who have had occasional glimpses from the outside but never bothered looking any further.
No-one is so wedded, philosophically, to the idea of CDM as is. Everyone knows its an approximation. The arguments over what it *is*. Mirage, particle, multiple particles, modifications to gravity, unanticipated effects of relativity on large scales, unanticipated effects of *averaging* observations across large scales, or a combination of the lot of them. And I can guarantee that practically no-one has been arrogant enough to stand up in a room and declare that we know what dark matter is.
I saw one person - who shall remain nameless - say something along these lines. He said to a room full of distinguished cosmologists (and me, I'm not distinguished at all), and I paraphrase since this was a few years back, "We can be absolutely certain that supersymmetry exists". That quite took my breath away. Firstly: no we can't be. Secondly: lol. Thirdly: winning that prize obviously turned you into an even bigger prick than you already were. I can't remember if anyone made these points to him because his talk was so stultifyingly boring, and so overlong, that I was comatose long before the end. Anyway, the corollary of his flabbergastingly inaccurate statement is that he also believes firmly that there is a single species of particulate dark matter, since this is more or less a prediction of general supersymmetric theories.
He's wrong, anyway. There may very well be supersymmetry, but we can in no way be certain that it exists.
Same goes for "dark matter", whatever you want to call it. The only thing you can't do is deny that the problem is there, and that the simplest explanation, which basically works all the way from galactic scales up to cosmological scales, is that it is composed of massive, weakly-interacting particles.
Re:Maybe (Score:5, Informative)
Yeah I tried to go through some of that stuff years back, and it was distinctly unconvincing, sketchily-laid out, and in a far weaker state than the author(s) would wish you to believe. Ultimately, if they feel they have a truly viable theory they have to apply it, in as much detail as the current LCDM model has been applied. That means they have to start off in the early universe (or the distant past, if you prefer; we don't *have* to assume a Big Bang), then justify in some way the existence of both the cosmic microwave background, and the exact spectrum of perturbations on it; then in the same, self-consistent coherent model, they have to account for structure formation and the presence of a wave imprinted on the largest scales of galactic structure which just happens to have a wavelength that perfectly matches that on the CMB... if the universe evolved as predicted by a Lambda CDM model; they have to include a form of nucleosynthesis to explain the ratio of elements we see in the oldest stars; they have to explain why old stars tend to be metal poor and young stars are metal rich; they have to explain the collapse of shards in clusters to form galaxies; and so on and so on.
Do that, and people might just start paying attention... but they have to do it at a level of rigour that is equivalent to that employed in professional cosmology. If they can't, they don't have a theory, they have words, and words are extremely cheap. It has to be couched in a mathematical language, and that's because it has to have a surmise and make a testable prediction. It has to be directly testable. I am very definitely not a fan of Lambda CDM, and a hunt back through my posts on /. that relate to cosmology would probably make that quite clear, but I've spent many years looking at it and its perturbations anyway. In my view, Lambda CDM has one absolute killer of a prediction: the wavelength which it predicted, from that on the CMB, was imprinted on the large-scale structure, and which was later found, exactly where it said. That wavelength, and the amplitude of the wave, is exquisitely sensitive to any change in the evolution of the perturbations, which is itself exquisitely sensitive to a change in the background spacetime. Lambda CDM got it right; any successor model -- and I hope to God there is one, because Lambda CDM is not satisfactory -- also has to.
The last that I knew, the Electric Universe stuff doesn't do any of this. (I would emphasise again that to gain acceptance it is not enough to posit a model -- and it's not even enough to present some back-of-the-envelope calculations. Frankly, the absolute minimum is a full analysis of possible backgrounds -- containing at least photons, neutrinos and standard model matter -- before you can even think of putting a paper out. That would then need to be followed up with an analysis of the perturbations, which we are all after all made from. Effectively, a version of the CAMB code, or one of its competitors, is necessary. Without it, you don't really have a viable model, just yet another model that can recreate something with observables matching the background Lambda CDM, and those come ten a penny. And so on. This is not an easy job, which is why we have no answers yet -- but it sure as shit isn't because the people working in the field are purblind idiots devoid of imagination or soul. Well, certainly not all of them ;) )
Re:Maybe (Score:4, Informative)
So basically you're using the same logic people use to justify the existence of God? How very..... scientific of you
No...
We have observed 'Y'. We think that 'X' might be what is causing 'Y'. We setup an experiment to test for 'X' The experiment did not detect 'X'. The observed 'Y' still exists, but we now know it is not caused by 'X'.
Or an example:
Every morning, my newspaper is delivered. I think that it is being delivered by car. I have a special 'newspaper delivery car' detector. I setup the detector, and check the results the next morning. The detector did not detect any 'newspaper delivery cars'. The newspaper was still delivered, but I now know it was not delivered by car.
Y = Newspaper delivery
X = Delivery by car
Experiment = Check for delivery cars
Result = proof that delivery was not by car
Yet we know 'Y', the newspaper delivery, occurred/exists even though we have eliminated one of the ways in which it could be occurring.
Re:Maybe (Score:4, Informative)
> The mass issue is fixed if we realize that the size of the universe is larger than the visible horizon.
Sigh, completely wrong.
1. Dark Matter (or some kind of "unseen gravity source") has to be present *within each galaxy* to stop galaxies flying apart because of their spin, which calculations based on their visible matter says they should do.
2. Dark Matter (or some kind of "unseen gravity source") has to be present between us and certain distant objects, because of the visible effect of "gravitational lensing" (ie. visible distortion of light) being caused by something we can't see.
There may be other examples of why DM is a thing, but those are the main two that pop to mind.
TL;DR it's a LOT more than just "adding up" the required matter in the universe.