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."
First dark matter post (Score:5, Funny)
Pulling in other posts below it :D
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You don't seem to perceive the gravity of the topic.
Neither are they, despite going to Lead to find heavy stuff.
Re:First dark matter post (Score:4, Funny)
Did they remember to turn it on? (Score:2, Funny)
Just sayin'...
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Maybe (Score:5, Insightful)
Maybe it's just not there.
Dark matter always reminds of the 18th century hypothesis of the aether.
http://en.m.wikipedia.org/wiki/Luminiferous_aether
Same principle. Same made up matter that no one can see or detect but somehow fills the entire universe.
Re:Maybe (Score:5, Insightful)
Yes but I could pick another example, the nutrino and say it sounds like that too:
"In 1930 Wolfgang Pauli proposed a solution to the missing energy in nuclear beta decays, namely that it was carried by a neutral particle " ( http://www.ps.uci.edu/physics/news/nuexpt.html [uci.edu] )
It makes perfect sense. You have theories that test to a high confidence in every way you can test them, then you find an anomaly in specific instances. Whats the response? Take those theories and attempt to narrow down the properties of what would cause the anomaly.
It obviously doesn't always produce a hypothesis that pans out as correct, but, can you really say that Aether theory was so bad? It was wrong, yes, but, it lead to the creation of experiments that answered new questions and ultimately, shaped the theories that came after it.
and...at the time... that is, after light was shown to be wave-like AND before we knew that there was no motion relative to its "medium", postulating Aether made a lot of sense.
Re:Maybe (Score:5, Insightful)
True, even failed theories advance science in some way or other.
However at some point you have to let them go.
The summary where it clearly states:
Physicists know from astronomical observations that 85% of the Universe's matter is dark,
I suggest they KNOW no such thing, and merely postulate dark matter to get their equations to balance. But how many such equation balancing inventions are laying in the dustbin of Physicists' revised theories over the years?
Unless or until the Physicists can find fault with the detectors, all of which have failed to find a trace of something allegedly composing 85% of the universe , it would seem that the whole "dark matter is known to exist" statement needs to taken down a notch. Detectors designed to their own specs fail to produce a single trace. It doesn't matter that there are very precise measurements of exactly how much the equations are out of balance.
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So you know that this something doesn't exist because we have failed to detect it when it was named as it is because we have yet been able to detect it?
Re:Maybe (Score:4, Informative)
This experiment tries to find some other interactions, but none so far were detected.
Dark energy? (Score:3)
Neither electric fields, magnetic fields nor gravity fields consist of particles. So perhaps we ought to call it dark energy. It might form a "gravity-dark energy" pair analogous to electro-magnetism.
Much as magnetic fields interact with electric fields, perhaps dark energy interacts with gravity, giving us indications of it, but not being measurable using particle detectors (as the one described in the article).
Thoughts, anyone?
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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.
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All we know is that there is something creating a gravity-like effect on large scales. We don't even really know how gravity works, so I don't think we can know dark matter 'exists' per se, as a type of matter, until we fully understand how gravity works at macro and quantum scales, the number of dimensions of the universe, the shape of the universe, etc... For all we know, the effect which we attribute to dark matter could just be a consequence of some other fundamental property of the universe that we kno
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Plausible. But before we go down that path, I say we try the more conventional means of explaining dark matter ... Occam's Razor, and all that.
Re:Maybe (Score:5, Insightful)
We can certainly detect dark matter.
No, we can't.
We only know to what extent our speculation and our math fails to completely work to our satisfaction.
Se we invent a black-box term to get the math to work out. We are quite precise in our invention.
We design instruments to detect this stuff that the math predicts is there. Instruments fail, time after
time.
You always need to consider the fact that it might be something else in the math that is wrong.
Otherwise, you might just as well attribute it to unicorns.
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The thing is: instruments did detect the presence of something that was
* Matter
* Not interacting with electrons or photons
* At the ratio to normal matter (quite accurately) predicted by a dark matter theory for galaxy rotation
Many theories were invented "to make the math work out" for galaxy rotation, and one of them made a quite accurate prediction of what we eventually measured about the early universe. Now we're trying to make additional measurements, because while we've measured dark matter at a large
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).
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But all we ever detect is indirect signals about something, no? And anything detected by any instrument could always be some new, previously unknown effect that just happens to look like what we expect from theory - but that's not very helpful to say. I'd say we detected dark matter in the same way we detected the Higgs boson - a theory made some specific predictions about what we'd see under the circumstance, and we saw something nicely matching the prediction.
The fact that the same theory quantitatively
Re:Maybe (Score:5, Interesting)
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.
Re:Maybe (Score:4, Insightful)
Yes, in a sense. I have a strong suspicion that if we were able to do a proper statistical mechanical analysis of the situation we'd see some odd emergent behaviour -- a galaxy is, after all, a rarified gas of about 10^9 interacting, confined bodies. We'd get different behaviour in a cluster, and different on cosmological scales.
Of course, I may be wrong and what we'd get out would be effectively pressureless dust, which is what we currently put in. Thep roblem is that at the minute we can't do a proper statistical mechanical analysis. We don't even have a full theory to work with, though there's progress here, too.
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Well, science is about what the data points to, not just what sort of fun stories we can tell. The data points to some kind of matter that doesn't interact with electrons or photons, and that doesn't have some alternative way to clump due to friction (and I guess we know more about what it's not from TFA).
There were many explanations floated for galactic rotation rates, but one specific dark matter theory predicted the CMBR results with great accuracy, so the scientific method says we go with that until so
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For galactic rotation rates - the most convincing alternative I've seen is a re-analysis using General Relativity rather than the much simpler equations of Newtonian gravity, which apparently gives a predicted rotation profile very close to what we observe.
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There were many such theories, both differing kinds of dark matter and different things that might be "wrong with gravity", and no way to choose between them without new observations. Then we got new observations, and the WIMP version of dark matter predicted those new observations (from the cosmic microwave background radiation) quite well, while the others were falsified.
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I'd be interested in seeing the study you're talking about - the general relativistic models of galaxy rotation that I've seen have been pretty unconvincing, and at most provide around a quarter of the effect.
What is impressive - and is very definitely phenomenology and not fundamental physics - is the success of MOND. It's been startlingly successful at predicting rotation curves from the observed distribution of luminous matter -- far more so than a standard cold dark matter paradigm -- and it does so wit
Re: Maybe (Score:2)
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The theory may be incorrect, but at the same time, we're able to try and discover the boundaries of where our direct knowledge ends and our theories begin. As we define those bounds better and better, we make it easier to recognize other discoveries that might fill that hole and provide an actual explanation for what we're seeing.
Long story short, wrong or not, the theory is a useful one, even if it's only serving as a stand-in for the actual phenomena that explains those observations.
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Possible, although unlikely. The -CDM model does an astonishingly good job of modeling the observed universe. But, that doesn't mean it is right.
In the case of aether, people didn't stop investigating it until a) experiments that should have observed no matter what saw no evidence of it and b) another theory that agreed with this new data came along.
People who trot out the tired old "dark matter is just like aether!" line typically do so while patting themselves on the back for their cleverness, while
Re:Maybe (Score:4, Interesting)
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.
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Looks like you're a shoo-in for the Nobel prize in physics.
Re:Maybe (Score:5, Informative)
Protip... that still doesn't explain the rotation curve problem observed in spiral galaxies.
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Re:Maybe (Score:4, Interesting)
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.
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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...
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Sloppiness :( Sorry, I'll try and do that a lot more often in the future.
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)
> 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.
Re:Maybe (Score:5, Funny)
From Security Video:
Jedi WIMP: These aren't the WIMPs you were looking for!
Physicist: "These aren't the WIMPs we are looking for!"
Jedi WIMP:They can go about their business.
Physicist: "They can go about their business."
Jedi WIMP: Move along.
Physicist: "Move along... move along."
Physicist: "Damn, we still haven't found anything!"
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: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.
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First and foremost the scientists that decided it needed to be created to begin with had to first decide that their ability to calculate the masses of objects, (that are too far away to reliably gauge any relevant data except through speculation or guess work), somehow had to be irrefutable. Tell me first, how the data that was collected is irrefutable. You can't. Except in a land of non-science there isn't a need to create something to prove something else unless that something else is irrefutable.
There are a few good ways to figure out the mass of a star. First and foremost, the relationship been mass, gravity, and rotation around a common point is very well understood. From this, we can get a good measure of the masses of binary stars as they rotate around their center of gravity. After getting this for a large number of binary stars, we start to see a strong correlation between the luminosity and the mass of primary ignition stars. This lets us figure out the masses of other stars too.
From tha
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They're not random guesses people have pulled out of their asses and declared to be infallible truths. The framework was originally built out of example stars in our immediate neighborhood, such as Sirius, which we can examine in fairly good detail. We know that Sirius is x-many kilometers wide, has a specific luminosity, radiates in a set range of frequencies, and interacts in a certain manner with the other two stars in its system. From the frequencies we know that it's mostly hydrogen, from the gravit
Re:Maybe (Score:5, Interesting)
Re:Maybe (Score:4, Insightful)
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.
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Here's the issue: either there is something out there we can't see (hence "dark matter") which is taking on more and more fantastical properties the more we learn about it, or our understanding of the universe's mechanics on a grand scale is wrong (and wrong in such as way that they line up pretty well at our small-scale understanding). Or for that matter, both could be true to some degree.
The scale is beyond the range of direct experimentation, so what can you do? In the former case, you can try to find so
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Wimps? Wimps everywhere ! (Score:2)
"... 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"
So former wimps are having a hard time finding WIMPs themselves? That's an interesting turn of events !!
the WIMPS are on your desks, sillies (Score:2)
unless you are using DOS
Have they considiered... (Score:2, Insightful)
...that maybe they're not seeing it because it's just not there?
Just a suggestion.
Dark matter fighting dark energy (Score:3)
So undetected dark matter pulling stuff together more than expected and undetected dark energy pulling stuff apart more than expected.
Hmmm. Isn't it possible that the theory is just wrong about how gravity and spacetime works at really large scales?
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Nice to hear some skepticism here on Slashdot. It certainly seems like scientists desperately want dark matter and dark energy to exist because their numbers are never adding up. It looks like bad science when they keep fiddling with the numbers to patch up their deficient theories.
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.
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Yes, and the big problem is that a better, more testable #1 hasn't come along. We're stuck with dark matter at #2 until the variances seen can have another plausible explanation.
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No for dark matter, because the strong evidence is from the early universe. Yes for "dark energy", because the term (like "cosmological constant") is just a placeholder for "there's something we don't know yet about how gravity and spacetime works at really large scales". Also, there's something we barely understand about it at very small scales - postulating "faster than light expansion of the early universe" explains a lot of data, but not much progress on a mechanism for it, or whether it's the same as
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Possible, yes, it just seems less likely than the existence of WIMPs.
The trouble is the Bullet Cluster lensing pretty much requires non-visible matter, even with the theories that assume relativity is wrong at large scales. It seems you can reconcile TeVeS with the Bullet Cluster using lots of neutrinos instead of WIMPs, but then when you plug that sort of neutrino abundance in TeVeS, you apparently get other inconsistencies elsewhere.
(Now, apparently STVG manages to handle the Bullet Cluster and galactic
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At this point, the two are mostly equivalent. For example, Einstein's original "cosmological constant" in the general theory of relativity *is* a form of dark energy.
Re:Have they considiered... (Score:4, Insightful)
Hur hur, yeah, stupid scientists with their "degrees" and their "experiments."
What a bunch of losers.
Re:Have they considiered... (Score:5, Interesting)
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We THINK we detected some anomaly in gravity. Even that isn't certain.
When you look into that, the Galaxy Rotation Curve [wikipedia.org], (the source of much of the dark matter speculation), is itself pretty much of a huge kludge of assumptions and guesses.
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You say "it's there, we've detected it" -- isn't that not strictly true? I mean, aren't their theories (MoND) that could potentially explain things w/o Dark Matter. Not saying they're likely, just that as I understand it they aren't completely ruled out....or am I wrong?
You're technically correct. We've detected something or some phenomenon, and there are theories that can explain that something without dark matter, but like you said, they're not likely. Perhaps I was a little too dismissive of the fringe theories. Without direct evidence of what it is, all we really have is Occam's razor.
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Perhaps I was a little too dismissive of the fringe theories.
So are there theories that propose that there is something, that is not matter, but still creates the gravitational affect? Now that would be interesting.
I'm maybe not up to date, but I don't think that we've really managed to detect gravitational waves yet either. So maybe this gravity critter is just a little more complex than we have previously thought?
Only more experiments will give us more insight.
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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).
oh, it's there, (Score:5, Funny)
teeny-weeny black spiders.
hundreds of Quattuordecillions of teeny-weeny black spiders per cubic centimeter, crawling between the very fabric of creation.
crawling in your ear, in your eye.
SPIDERS.
It is very hard to avoid dark matter (Score:5, Insightful)
The main lines of evidence for dark matter:
* Galactic rotation curves [wikipedia.org]
* Velocity distribution in clusters of galaxies [wikipedia.org]
* Gravitational lensing [wikipedia.org] in general
* The Bullet Cluster [wikipedia.org] in particular
* The pattern of positions of galaxies in the universe [wikipedia.org]
* The pattern of Baryon-acoustic oscillations [wikipedia.org] in the cosmic microwave background [wikipedia.org] and in the galaxy distribution
* The primordial distribution of light elements in the universe [wikipedia.org]
We know of some kinds of dark matter already: There is a huge amount of neutrinos left over from the big bang, and since these interact very weakly with other stuff, they definitely qualify as dark. Other known kinds of dark matter are black holes, and compact, cold objects made out of baryons (normal matter). So dark matter exists.
The problem is that there isn't enough of the normal kinds of dark matter. To match the pattern in the cosmic microwave background and the amount of hydrogen, helium and lithium in the universe, one needs by far most of the dark matter to be non-baryonic (i.e. not normal matter, but something like neutrinos, but heavier). This kind of dark matter is something we have to postulate exists in order to match observations. But when we do assume it exists, the theory matches observations extremely well. As an example, look at the CMB power spectrum as mesured by Planck [ggpht.com]. The error bars are so small that you mostly can't see them, and the points lie smack on top of the theory curve. But only if dark matter is included.
And it just so happens that the amount of dark matter that makes theory match the points in that graph also makes the element abundances, galaxy distribution, lensing observations and galaxy cluster velocities work too. Such a coincidence is pretty telling, I think.
But yes, people have tried to avoid dark matter by modifying gravity instead (though nowadays, the most common motivation for modifying graivty is to avoid dark energy). MOND [wikipedia.org] is an example of that. MOND is like normal Newtonian gravity as long as the gravitational acceleration is large (like in the solar system), but instead of falling to arbitrarily low values as distances increase, the gravitational acceleration has an effective minimal value that it approaches as you move away. And such a constant value is just what you need to get the flat rotation curves we observe in galaxies. Which is the problem MOND was invented to solve.
MOND is an elegant solution for galaxies, but it loses all its elegance and predictive power when you try to apply it to the other areas where dark matter shows up. And in some cases it is plainly ruled out as an explanation. MOND, like Newtonian gravity, is a central force, which means that the force points towards the mass that generated it. But in the Bullet cluster [wikipedia.org], the gravitational force points towards areas with little visible matter, away from areas with much visible matter. This is impossible to fit into MOND. So the Bullet cluster basically killed MOND.
Some of MOND lives on in TeVES [wikipedia.org], which is an attempt at a relativistic version of MOND. Sadly TeVES has none of the simplicity and elegance of MOND, and while it can explai
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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.
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So, this dark matter, what is it? Because it seems to me that it hasn't been defined properly, it's just a massive kludge that scientists did when their observations didn't make scientific sense according to our current best theories of physics.
Dark matter is a theory without basis, it says oops, our measurements don't make sense. What is it, axions? - a type of particle that hasn't even been proved to exist. Dark matter is a theory shakily based on other unproven theories, proposed because the initial theo
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Actually the money is on SUSY (supersymmetric) particles, which from a mathematical point of view really "should" exist. Meaning it would be odd that the universe exhibits all these other symmetries, but not that supersymmetry. Basically it's one of those things that if you understand the math it totally makes sense there should be this whole other class of particles, otherwise it looks like a kludge if you don't. Obviously just because those two things fit neatly with each other, hey there should be these
No! Why didn't you mention this earlier? (Score:3)
Of course they hadn't considered it earlier! What fools they've been shown to be!
(Hint: If you're a random commenter on Slashdot, then, yeah, the experts in the field have probably considered your idea before you suggested it.)
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Can you prove that?
According to our math based on our hypothesis, it does.
The universe doesn't have to agree with our puny hypothesis, despite how well it explains most of what we observe.
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I'm pretty sure we both just said the same thing.
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Ok, I'll spell it:
GP: "maybe it's just not there"
parent "but it "has to" otherwise shit breaks apart"
Me: "According to our math based on our hypothesis, it does."
Since the universe isn't breaking apart, and we can usually trust math before millions are spent on detectors, either we have a somewhat good hypothesis and there is dark matter preventing shit from breaking apart, but we're trying to detect it wrong, or we have a very bad hypothesis, and there isn't dark matter.
i'm questioning the statement that i
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the equations for our current theory "break" but one must remember that it is a theory. while it seems like a very solid theory, we do not know for a certainty if it's correct or not.
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Theory is exactly the right word.
Perhaps you were mistaking the word Theory for something else?
obvious solution (Score:2)
clearly it's too dark to see so they should just use a flashlight.
Physicists know (Score:5, Interesting)
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.
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EVERY SINGLE TIME someone says they have broken relativity, even with "proof", it is later shown to be wrong.
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They probably said the same about Newtonian physics. There's a difference between jumping on every bandwagon driven by a guy who's "unlocked the true secrets of the universe" and refusing the call a scientific theory infallible.
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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.
Maybe. Physics does have people working that line (TeVeS with massive neutrinos to explain the Bullet Cluster, Moffat's STVG). But WIMPs still are considered the most likely candidate.
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That would be nice but actually physicists have only been using Newtonian mechanics to model galactic rotation. Mostly because doing it with General Relativity is too hard, and simulating it on a supercomputer is probably only been possible for the last couple of years, and even then it would take millions of CPU hours.
The biggest death nail for dark matter is if it makes up 85% of the matter of the Universe how come the solar system is utterly devoid of it? Remember General Relativity can explain the motio
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?
There is no dark matter in the universe, really (Score:2)
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I though it was the moon that was all dark [youtube.com].
Check the Phlogiston Compensators (Score:2)
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that was an awful lot of words to say "i should shut up"
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>nor have standing for my words to carry weight
I see what you did there.
MACHOs (Score:2)
I still think it is lots of baryonic matter in black holes or whatever, aka MACHOs [wikipedia.org](Massive Compact Halo Object).
Use a bigger flashlight! (Score:2)
Anytime I lose something in the dark I just get a bigger flashlight. All we need to do is launch a giant version of those 30 LED flashlights you can get at Harbor Freight for like $2. It has to be in pink though just to make it pretty.
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Was she scared of gravity as a child or something? Too much falling down perhaps?
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