DARK AND STORMY: A variety of competing dark matter detectors are producing results at odds with one another. So far dark matter has been convincingly detected only by its gravitational effects. Here the inferred location of dark matter in galaxy cluster Abell 1689 is marked by a purple glow.
Image: NASA, ESA, E. Jullo (Jet Propulsion Laboratory), P. Natarajan (Yale University), and J.-P. Kneib (Laboratoire d'Astrophysique de Marseille, CNRS, France)
BALTIMORE—The generic line on dark matter is that nobody really knows what it is because nobody has seen it. The former claim remains basically unassailable—there are many forms dark matter could take. But one research group would dispute the latter assertion. Over the past several years, the Italian DAMA (for DArk MAtter) collaboration has been making the claim that their subterranean detector has registered the signature of dark matter as Earth passes through a sea of the stuff. But despite an ever-strengthening observational case for their claim, the DAMA collaboration's finding remains a source of broad skepticism within the scientific community.
The situation was highlighted this week at a symposium on dark matter here at the Space Telescope Science Institute (STScI). A May 2 talk by a DAMA scientist was immediately followed by a presentation about a competing dark matter detector—one whose results are more widely accepted and whose data contradict DAMA's finding. A few days later a third scientist representing a third detector weighed in, and confused the situation even more.
Dark matter provides the universe with a great deal of its total mass, so it is critical to cosmic evolution, but it is both invisible and barely interactive with normal matter, making it an incredible challenge to detect directly. So far its existence has been inferred from its gravitational effects in shaping galaxies and other large-scale structures.
Pierluigi Belli of the University of Rome Tor Vergata and the DAMA collaboration explained that his team's detector has measured an annual fluctuation in particulate hits that seems to fit the bill for dark matter. DAMA is a detector meant to measure changes in the ambient particle environment—including, presumably, dark matter—as Earth passes through its orbit. "The velocity of Earth in the galactic frame is different at different times of the year," Belli explained. As the sun moves steadily through its orbit around the galaxy, Earth orbits the sun in turn, and the planet's velocity either adds to or subtracts from the sun's velocity. If dark matter rings the galaxy as theory predicts, Earth should be oscillating back and forth through a sea of particles. And DAMA should be able to register a yearlong ebb-and-flow cycle in the number of dark-matter particles passing through the detector.
The DAMA experiment consists of 250 kilograms of crystalline sodium iodide concealed deep underground in Italy's Gran Sasso National Laboratory. Burying the detector underground shields the instrument from more mundane particles, whereas dark matter should pass through rock and into the lab relatively easily. The hope is that some of those particles, having cruised cleanly through Earth, will then bump into one of the atoms in the DAMA detector, announcing its presence by depositing a tiny bit of energy in the crystal.
In more than a dozen years of operation, DAMA has registered a seasonal fluctuation in particle hits that agrees with what a dark matter sea should look like. As predicted, the fluctuation cycle peaks around the start of June and lasts almost exactly a year. "The results are well compatible with many dark matter scenarios," Belli said. Specifically, DAMA could be seeing a very lightweight form of the preferred candidate for dark matter, known as the weakly interacting massive particle, or WIMP. "We have positive evidence for the presence of dark matter particles at a very high confidence level," Belli said.
The general criteria for announcing evidence suggestive of a new particle or physical effect is three standard deviations, or 3 sigma; the benchmark for claiming a new discovery is 5 sigma. The DAMA seasonal flux is now a roughly 9-sigma effect. But doubts of the dark-matter interpretation still loom large.
"I think everyone would agree at this point that they see a signal," says STScI astronomer Mario Livio. "The question is: What is it?" Other researchers at the meeting echoed the same sentiment—after all, a 9-sigma result demands some explanation, even if it is not the dark-matter explanation the experimenters have offered. "It's an intriguing hint," Stanford University physicist Peter Graham said in an April 4 panel discussion at the symposium. "They clearly have a signal, and they've been seeing it for a long time."
Albert de Roeck, a physicist working at the Large Hadron Collider outside Geneva, Switzerland, suspects that DAMA scientists may have gotten off on the wrong foot with their peers. DAMA first announced evidence for a seasonal modulation indicative of dark matter more than 10 years ago, when the signal was much weaker than it is now. Not everyone was convinced. "It sort of seemed like they wanted it to be there," de Roeck says. Now, even with much stronger evidence, the field remains dubious.
Physicists have complained that the DAMA group has not been open enough with its data. During Belli's talk, physicist David Cline of the University of California, Los Angeles, noted that other researchers have not been given access to the raw, unprocessed data from DAMA to see how the collaboration arrived at their 9-sigma result. "It would be nice to get a look at that," Cline said. "Usually in science people do open their doors to let other people look at their data."
Belli said that that was simply not the procedure. "These data have been analyzed in the simple standard method," he said. "So we don't think that it's important to give the particular data."
The neighboring Xenon100 experiment has not helped DAMA's case. Xenon100, which is located in the same underground lab, works in a similar way to DAMA. Its detector, with a 62-kilogram liquid xenon target, has been awaiting collisions from dark matter since 2009. It boasts a very low level of background noise that should make it exquisitely sensitive to detecting ambient dark matter particles.
In a talk immediately following Belli's, Elena Aprile of Columbia University showed data from an April Xenon100 study that appeared to rule out the kind of lightweight dark matter particle favored by DAMA. What is more, the experiment did not pick up any kind of seasonally varying signal. In 100 days of taking data Xenon100 was unable to gather any evidence for the presence of dark matter, despite its sensitivity. "I didn't want to say it in words," Aprile said when asked to explain the discrepancy between her experiment and DAMA, "but…of course, already since some time, these data and others are in friction with that. They're not compatible."
Later, during the panel discussion, Belli was pressed to offer an explanation for what the DAMA signal might represent if it is someday proved not to be dark matter, as most observers suspect. "It is quite difficult to demonstrate that it's not dark matter," Belli said, adding that he and his colleagues have tried to exclude every other effect they can think of. "We cannot explain it in another way."
The last speaker of the symposium gave DAMA a glimmer of hope. Juan Collar of the University of Chicago, presenting May 5 on behalf of the CoGeNT (Coherent Germanium Neutrino Technology) experimental collaboration, announced that his group also registered a faint seasonal modulation in particulate hits. The CoGeNT dark matter detector had its experimental run cut short at 15 months due to a March fire in the Soudan Mine in Minnesota where it resides. But in analyzing that limited data Collar and his colleagues found a possible signature of seasonal modulation akin to what DAMA has seen.
CoGeNT's results are in broad agreement with a DAMA-like dark matter ebb and flow in terms of the timing, duration and amplitude of the seasonal variation. With a few basic assumptions, the CoGeNT signal becomes a 2.8-sigma result—not enough to make a strong statement, but enough to raise a few eyebrows. "We're not going to claim that we're seeing WIMPs," Collar cautioned.
He charged that the Xenon100 collaboration has overstepped in using its own nondetection to rule out the DAMA claim. "Xenon is not a good medium for light WIMPs," Collar said. "Light WIMPs are wicked.... With present technologies there's very little we can say [about them] with certainty." He had harsh words for a smaller, earlier iteration of the Xenon100 detector known as Xenon10. In one of the slides accompanying his talk, Collar referred to some results from Xenon10 as "pure, weapons-grade balonium."
CoGeNT may turn out to be the ally DAMA has long lacked—Belli, who tucked himself quietly in a back corner of the auditorium for most of the symposium, moved up to the third row for the CoGeNT talk. But Collar maintains he is not taking sides. "Maybe DAMA's wrong, maybe they're right, but we have to remain neutral," Collar said. "I find myself caught between the believers and heathens."
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20 Comments
Add CommentIs there anyone out there with academic street cred willing to pose an obvious alternative? Namely, that 'dark matter' is dark because it isn't matter. Since matter and it's motion have, once again, been confirmed to mold space-time, is it not reasonable to hypothesize that so-called dark matter is a second order affect of groupings of matter, whose relativistic affects superimpose to warp space-time in a distinct manner at galactic scales? Is there any obvious reason why this option shouldn't share the stage with the "believers"?
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Reply | Report Abuse | Link to thisIn my opinion astrophysicists have already observed the galactic dark matter objects - in fact, billions of them.
The MACHO microlensing results, and the 6x radio background excess seen in the ARCADE-2 experiment, are consistent with a huge population of "primordial" stellar-mass black holes.
Perhaps the evidence for dark matter has been in hand and staring us in the face for over a decade, but it has been ignored because it is the "wrong" answer, i.e., not the one the particle physicists desperately want.
Knee-jerk nay-sayers should informed that "primordial" black holes are fundamental objects that do not conflict with nucleosynthesis constraints.
Robert L. Oldershaw
http:/www3.amherst.edu/~rloldershaw
Discrete Scale Relativity; Fractal Cosmology
They are obviously measuring our velocity within the Ether.
Reply | Report Abuse | Link to this3 sigma is 99.73002039% (of a population), or
Reply | Report Abuse | Link to this2699.80 missed opportunities per million.
5 sigma is 99.99994267% (of a population), or
0.573 missed opportunities per million.
9 sigma (rounded to 30 decimal places is 100.000000000000000000000000000000%, or 0.000000000000000000000000000000 missed opportunities per million.
Could matter be 'dark' because it's time is slightly skewed from our own? Multiple universes should be able to birth elements and effects that we cannot measure or perceive.
Reply | Report Abuse | Link to thisAnybody who claims to know "for sure" what dark matter is, is delusional, self-aggrandizing, and severely lacking in humility. The jury is still out, and I suspect it will be out for a long time -- longer than anyone currently alive will get to appreciate. I will continue to be happy to entertain all the theories, but dark matter and dark energy seem fundamentally to demand some new science, new paradigms, and new instruments.
Reply | Report Abuse | Link to thisSo, you've posited a hypothesis with some corroborating evidence. That is useful. You're "knee jerk" rhetoric is not. In fact, relativistic lensing is also consistent with second order relativistic behavior. At first glance, I don't think 6x radio background is counter to my submission in this still open quest, but I'm intrigued and will look into it.
Reply | Report Abuse | Link to thisIf evidence is found for actual matter that is dark, then great; but until then, I recommend being open to all parsimonious hypotheses that account for the available evidence.
doesn't dark matter "disappear" if the gravitational constant isn't?
Reply | Report Abuse | Link to thisBoy oh boy David are you ever right! I hate the CRAP he (I assume it's a he) and his ilk pollute SA's site with. SA needs a spam filter like "The Economist" uses.
Reply | Report Abuse | Link to thisForgive a crazy conjecture, but if matter collapses space, why can't anti-matter source / expand space. As such, anti-matter would be responsible for what is called dark energy and dark matter.
Reply | Report Abuse | Link to thissee:
Reply | Report Abuse | Link to thishttp://www.benthamscience.com/open/toaaj/openaccess2.htm
Macho's and stellar mass black holes are just one part of the answer.
The other part of the answer lays in understanding that such black holes and the supermassive black hole at the centre of the galaxy exert an extra amount of gravity; 6.25 X the normal amount to be exact
I would like to comment vis-a-vis another discussion in SciAm pages last year [In praise of scientific error
Reply | Report Abuse | Link to thisBy George Musser | Dec 20, 2010 01:00 PM |]. Jim Dwyer = jtdwyer commented there that dark matter on the scale of spiral galaxies as inferred from spiral disk rotation curves is actually misguided inference. This has also been my view for many years now, starting end of 2005. I have written about this over the years [arXiv:0806.1131, arXiv:0805.4163, arXiv:0809.1972, arXiv:astro-ph/0703430]. Jim (jtdwyer) recently sent me a couple of corroborative papers [http://arxiv.org/abs/1007.3778, http://iopscience.iop.org/0004-637X/679/1/373/]. The bottom line from many independent individuals & groups around the world is: DISK GALAXY ROTATION CURVES ARE FULLY ADEQUATELY MODELLED USING NEWTONIAN DYNAMICS AND GRAVITY WITHOUT ANY NEED DARK MATTER. Jim has commented on this topic in several more (sometimes inapproriate) SciAm discussions as well.
b4 I start I think they got mad at me for comments intended towards the clown that keeps posting ads on the site. In the words of Gilly on Saturday Night Live 'Sorry'...
Reply | Report Abuse | Link to thisRegarding Jtdwyer he is on a mission to reject dark matter, dark energy and most of the 96% of the universe we cannot explain. His comments are interesting and thought out but I feel like he goes to Fox News only. I have read Green, Sussking, Hawking and Smolin (plus anything I can find on Smolin and Feynman). Smolin is on to something when I read his work on loop quantum gravity and he and Feynman together present some interesting concepts.
I like Mandelbrot's take on fractals and chaos and as I read QED I keep thinking that as we put more energy in the more interesting things come out but they are similiar to existing objects just at a higher energy level (Heavier). Maybe we are looking through the mirror the wrong way. Maybe the more massive objects are layers of a multidimensional universe where our view is directly related to our energy level.
By that I mean we are a mere 270 degrees Celsius from absolute zero. We are very close to the low end of energy and I am sure that affects our view of the whole picture.
Again sorry about the add crap but that guy needs to stop so how about pretty please?
I meant to include Freeman Dyson. He is one of the most important underrated physicist of our time and he is still with us. Thanks to Freeman the world can actually use quantum math. Plus he reads a lot like Einstein in his love of mankind.
Reply | Report Abuse | Link to thisThey have removed all of my post. I was playing with that guy that kept putting adds on the site all the time. Maybe it was the magazine putting up the adds to make some money. Well if this disappears I ll be back and I guess use a made up name like everyone else. Thanks for the thanks.
Reply | Report Abuse | Link to thisIs it possible that since the only thing that can be "detected" is the gravitational effect, that the source of the gravitational effect is actually located in one of the rolled up dimensions posited by string or M-Theory? It seems to me that just because a dimension may appear rolled up from our perspective does not mean that it can not have some effect of the other dimensions. Who knows, all the dimensions could be fully expanded at the same time but most only appear to be rolled up from our perspective.
Reply | Report Abuse | Link to thisI keep wondering why other dimensions are rolled up and not considered outside of our dimension. Where does it say that extra dimensions are microscopic vs macroscopic?
Reply | Report Abuse | Link to thisThanks for your comments. I am on a mission to expose the analytical errors that produced the perceived requirement for, especially, galactic dark matter. I am doing so strictly as a professional courtesy to the scientific community and the public. As I always strive to make clear, I'm a retired information systems analyst - it is that expertise that I can apply to these issues. I have no financial or professional interest in these matters.
Reply | Report Abuse | Link to thisFYI, I don't pay any attention to Fox News, only sciam.com and news.sciencemag.org. I read "Scientific American" and "Science News" for a couple of decades and I have been an AAAS member and subscribed to "Science" in the past. I have watched all the silly Science Channel and other video programs in the past couple of years. Most of my detailed information comes from related research archives.
Regarding books written by famous scientists, keep in mind that they may contain quite a bit of highly qualified personal opinion and conjecture, perhaps aligned with professional agendas, along with some fluff to interest a general readership. Unlike scientific journal articles, book publication does not require any adherence to formal peer review procedures.
The existence of dark matter is not in doubt.
Reply | Report Abuse | Link to thisAstronomers have know for decades that some kind
of invisible material is present around galaxies, because the velocities of stars do not fall off
with distance from the centers of the galaxies
as they should if mass were present in the visible
matter (gas, dust, and stars) only.
The question posed here is what is it, light "WIMPS,"
or something else.
You might want to read a few more Scientific American articles before you declare that the existence of dark matter has been proven. Please see:
Reply | Report Abuse | Link to this"Reliance on Indirect Evidence Fuels Dark Matter Doubts"
http://www.scientificamerican.com/article.cfm?id=dark-matter-doubts
"Tweak Gravity: What If There Is No Dark Matter?"
http://www.scientificamerican.com/article.cfm?id=dark-matter-modified-gravity
"What's The Matter?: Cold Dark Matter and the Milky Way's Missing Satellites"
http://www.scientificamerican.com/podcast/episode.cfm?id=A2B71EFB-ABFA-C6D7-0A728C56892215F8