Two Blows for Dark Matter Model

Two recent experiments limit physicists’ favorite candidate for the elusive and invisible matter lurking in the universe.

It’s been a bad couple of weeks for wimps.

Weakly interacting massive particles (WIMPs) are the top candidates for dark matter, the invisible stuff that makes up about 84% of the universe’s matter. But two recent experiments designed to sniff out the elusive particles have come up empty-handed, calling previously promising results into doubt.

WIMP-y Hints of Dark Matter

The Large Underground Xenon experiment is a six-story titanium tank filled with a ⅓ ton of liquid xenon. If WIMPs interact with xenon nuclei, they'll set off tiny flashes of light that can be detected by arrays of photomultiplier tubes at the top and bottom of the detector (pictured here). But so far, LUX hasn't seen any WIMP-like events.
Monica Young
By definition, dark matter doesn’t interact with light, so not only is it dark, it’s transparent too. And while we know it interacts with gravity, that interaction leaves only indirect evidence of its existence, such as its effect on galaxy rotation.

But WIMP theory says dark matter particles should also interact via the weak force, a fundamental force that governs nature on a subatomic level. So a WIMP particle will very rarely smash into a heavy nucleus, leaving a detectable signal. The chance for a direct hit is very, very low, but underground detectors stacked full of cold silicon, germanium, or xenon have the best odds for success.

Several detectors have claimed to see hints of WIMP interactions (but not actual detections, which would merit a flight to Stockholm). Some of these, such as the CDMS-II, CRESST, and CoGeNT detectors, have seen a possible signal from a WIMP particle with a mass near 10 GeV — a signal important enough to announce, but not statistically significant enough to be named a real detection. Another detector, DAMA/LIBRA, has seen a much stronger signal, but not everybody is convinced it comes from WIMPs.

But not every experiment has seen positive signs of dark matter. The XENON-100 experiment had already cast some doubts on previous results because it didn’t see any WIMP interactions. But this result wasn’t anything physicists couldn’t theoretically massage away. For example, some suggested that WIMPs might interact differently with the silicon nuclei of CDMS-II’s detector than with the xenon in XENON-100.

A Null Result

Now a new experiment called LUX (short for Large Underground Xenon) may have barred that way out. Richard Gaitskell (Brown University) and Dan McKinsey (Yale University) announced today the results from LUX’s first 85 days of operation. The new detector is 20 times more sensitive than XENON-100. If, say, the CDMS-II detections were real, the researchers say LUX’s six-foot-tall titanium tank of liquid xenon ought to have interacted with some 1,550 WIMPs.

Instead, it saw zero.

Direct-detection experiments seek to find WIMP particles, identifying their mass (x-axis) and the probability that they'll interact with ordinary matter (y-axis). All experiments are labeled except for the magenta line, which is from the ZEPLIN-III experiment. Blobs mark regions where an experiment has seen signs of WIMP interactions, while lines mark upper limits set by experiments with no WIMP detections. The LUX experiment is one of the latter. The blue line marks the upper limit set by LUX data — all parameter space above this line is ruled out. (Click image for larger version.)
Courtesy of Jeremy Chapman
“It does not appear possible to reconcile [CDMS-II and LUX],” Gaitskell says. Claiming that WIMPs interact differently with some elements than others won’t work this time. “LUX’s results are in direct conflict with DAMA, CoGeNT, and CRESST.”

(The graph at right says it all — the take-away is that the blue line, which marks the upper limit set by the LUX experiment, lies below all the detections seen in previous experiments, marked by colorful blobs.)

Dan Hooper, a dark matter expert at Fermi National Accelerator Laboratory who was not involved in the LUX study, agrees: "It would be fair to say that dark matter interpretations of the CoGeNT and CDMS data seem pretty unlikely to me in light of the new results from LUX."

"I would be lying if I said I was not more than a little disappointed," he adds.

If LUX’s results rule out all previous detections, then what were the other detectors seeing? For the barely there results reported by CDMS-II, CoGeNT, and CRESST, Gaitskell says it’s difficult to trust data at the margins. “I think unfortunately what we’ve seen is a series of partial results and experiments where they’re reporting right at the limit of their capabilities.”

But DAMA’s results are by no means marginal — on the contrary, they’re almost impossible to explain as a statistical fluke. “There is no doubt that DAMA is measuring something,” Gaitskell explains. But the equipment isn’t precise enough to point to a cause. “The challenge there is to determine whether it is of an astrophysical nature, or whether it comes from some more mundane, anthropological, or just earthbound explanation.”

Though LUX appears to contradict the hints of direct detection seen so far, Gaitskell cautions, “one should never say never. Theorists are very inventive.”

Even if all previous WIMP hints are wrong, there are still many more models to test, up to 1,000 times below the interactions LUX is sensitive to. “We will be searching for WIMPs for a while longer,” Gaitskell says, undeterred. “They remain the favored quarry.”

Missing Gamma Rays

Direct detections of a slippery particle that avoids ordinary matter like the plague is difficult at best. So some scientists prefer a more indirect approach. The standard WIMP model says the particles are their own anti-particles, so if WIMPs meet their siblings in deep space, they should unleash a flurry of gamma rays at an energy corresponding to the mass of the individual particles.

Intern A (Johannes Hirn)
NASA’s Fermi Gamma-Ray Space Telescope has been scouring the sky to look for this annihilation signature, focusing especially on dwarf galaxies near the Milky Way. Astronomers think these galaxies have much more dark matter than ordinary matter, which should make them decently bright gamma-ray sources. But the Fermi-LAT collaboration recently submitted a study to the journal Physical Review D that reports no significant gamma-ray emission from 25 nearby dwarfs.

Unlike LUX, the Fermi observations don’t contradict the hints seen from CDMS-II and other experiments, at least for now. That’s because it’s not clear how the probability of WIMP particles smashing into each other relates to the probability of WIMP particles smashing into ordinary matter. But that will change as Fermi continues to collect gamma-ray observations — if dark matter really is made of WIMPs, the collaboration expects to see a signal soon.

Cosmology, News

About Monica Young

Astronomer turned science writer, Monica Young is web editor of S&T, where she creates, manages, and maintains website content as well as contributes to the magazine.

29 thoughts on “Two Blows for Dark Matter Model

  1. Robert L. Oldershaw

    There is not a shred of empirical evidence that suggests that the dark matter is in the form of any mythical WIMP, axion, or other hypothetical particle.

    A far better candidate for the dark matter is low-mass black holes with masses in the 0.1 to 0.6 solar mass range. The MACHO microlensing experiment apparently detected billions of objects in this mass range that were inconsistent with conventional stars. Primordial black holes were considered the best interpretation of the empirical results.

    Alas, primordial black holes are not fashionable today, while fairytale WIMPs are. So it goes in the pseudo-science era.

    Robert L. Oldershaw
    Discrete Scale Relativity/Fractal Cosmology

  2. Erik Forbes

    "There is not a shred of empirical evidence" – experiments are conducted to find evidence that confirms or refutes theory. Complaining about the lack of evidence before the experiments are complete is rather premature, don’t you think? =)

  3. Dennis K. Stepanek

    It would certainly be a low blow to the body of conventional cosmological wisdom if low-mass black holes turn out to be a significant component of dark matter. Yep, and that’s why they should be known as LoBloes.

  4. Paul Vondra

    "Dark matter" increasingly reminds me of the Ptolemaic system of epicycles and deferents to explain observations that were not in accord with contemporary cosmology. As the discordance persisted, the epicycles and deferents got increasingly complicated and unwieldy. Finally along came Copernicus with a flash of insight that ultimately resulted in a new paradigm. What will ultimately solve the gravitational discordance will not, in my opinion. be new searches for (non-existent)dark matter. It will be a new Copernicus, with a flash of refreshing new (testable)insight on the nature of gravity.

  5. T McG

    I have a MS in Astrophysics and this issue was the one that bothered me all the way through it. I asked professor after professor and it just supported the adage: The best way to argue against something is to argue for it badly.

    As far as I have ever seen "Dark Energy" and "Dark Matter" has been "created" by astronomers because the universe is so close to being exactly balanced, it must be.

    If it isn’t, well Dark Energy is unnecessary, Dark Matter is largely understood.

  6. Robert L. Oldershaw

    Does Erik Forbes have any idea how many experiments looking for WIMPs have come up negative?

    They have been doing experiments for decades and nature keeps saying "NO!".

    Maybe theoretical physicists should consider that they have been badly mistaken for assuming that the dark matter must be some form of weird hypothetical particle.

    Is asking for a new open-mindedness too much to ask for?

    Is particle dark matter the only game in town?

  7. George

    I have wondered if it is our understanding of gravity that is incomplete. We thought we understood mass, space and time, but Einstein’s relativity demonstrated our understanding was only valid within certain boundaries. At relativistic speeds our former equations were invalid. Perhaps at the large gravitational scales of rotating galaxies , etc., that support the dark matter hypotheses, our current description of gravity is incomplete.

  8. Erik Forbes


    Actually no, I wasn’t aware of the prior experiments until I started looking into it after writing my previous post. Perhaps I was a little too hasty – my apologies! =)

    I’ve been reading your work on Fractal Cosmology by the way. I have a very basic layman’s understanding of theoretical physics, but it’s been my intuition that your view of the universe is very likely correct given the abundant examples of fractal geometry in nature at a wide variety of space and time scales. I don’t understand the experimental models well enough to comment on the atomic / stellar / galactic scale analogues you’ve researched, unfortunately, but I’ve been thinking about the fractal nature of reality for years, ever since I came across fractals as a teenager. Fascinating stuff! =)

  9. Robert L. Oldershaw

    Thanks for your comments Eric.

    I think we are way overdue for a paradigm shift in cosmology and I think a discrete fractal paradigm is a very natural candidate.

    I am perplexed by the academic resistance to this new way of understanding nature’s hierarchy. It can explain several key enigmas in theoretical physics, like the cause and resolution of the vacuum energy density crisis, the way to unify QM and GR, the meaning of the fine structure constant and Planck’s constant,…

    Fractal Cosmology

  10. Charlie Culbertson

    Robert, the graphic in this article and others does in fact show "shreds" of evidence. The Super CDMS experiment detected a "WIMP-like signal at the Sigma 3 level" (99.8% chance, with the term "discovery" reserved for 5 Sigma or better). Regarding "Fractal Cosmology", it’s well known that fractals occur throughout nature. But a fractal description of a phenomenon is just that – a description, not a Theory. So, isn’t a Fractal description of the Universe just that – a description, and not a Theory? And Wouldn’t WIMPS be consistent with the well established Standard Model?

  11. HRPuffenstuff

    Some of you seem to be a little confused…these experiments are trying to discover what dark matter is not whether it exists. The existence of dark matter is well established with multiple lines of evidence (rotation and velocity of galaxies just to name two) going back to the 1930s.

  12. Peter Wilson

    The evidence and arguments for both dark matter and dark energy are compelling, if not overwhelming. Yet, as with the Higgs boson, until a laboratory detection is made, niggling doubts remain. The most troublesome is that dark matter is necessary at “small” scales to hold things together, while dark energy is needed at “large” scales to push things apart. The dividing line between large and small is the mysterious Ri introduced before. At distances of less than Ri (the distance between galaxy clusters, about 10^23 meters), dark matter must be added to the equations; at distances greater than Ri, dark energy is required. To reiterate, Ri is not included in the Standard Model; it‘s my contention that it should be. And speaking of equations, GR is not used in the former, because it is too cumbersome. In other words, models of galaxy rotation that require dark matter are Newtonian models, not relativistic. It is “hoped” that Newtonian gravity is a good enough approximation, and it probably is. It seems impossible that the non-linearity introduced by GR could have a 10x impact in inferred mass. But no one knows for sure, because no one has successfully modeled galaxy rotation or galaxy clusters using GR.

  13. Robert L. Oldershaw

    The "Standard Model" did not predict the dark matter and cannot retrodict its fundamental properties.

    All we have seen of "evidence" for WIMPS are "hints" that are subsequently found to be false positives.

    A fractal understanding of nature is as fundamental theoretically as any existing cosmological paradigm, and as much a theory as any other theory of physics. It has passed 38 retrodictions and makes more than 15 definitive predictions.

    Saying a fractal model is only "a description" is pejorative bias. The fractal paradigm that I espouse can make many definitive predictions, and has already passed some of those definitive scientific predictions, like the existence of pulsar-planets. That is the true test of the correctness and fundamentality of an idea.

    You might want to make an effort to learn about Discrete Scale Relativity.


  14. Robert L. Oldershaw

    Hi Charlie. Here is my second attempt to answer your questions. Maybe this post will pass muster.

    The "Standard Model" did not predict the dark matter.

    The "Standard Model" cannot retrodict the fundamental properties of the dark matter, especially the dark object masses and other identifying physical properties.

    So far, any "shred" of positive evidence for WIMPs has subsequently been shown to be a false positive.

    Saying fractal models can only provide "descriptions" is false, and ignores the well-known application of fractal modeling in many fields.

    Discrete Scale Relativity (the new discrete fractal paradigm) makes at least 35 fundamental retrodictions, and it makes at least 15 definitive predictions (some of which have already been verified). This evidence is freely available at several places on the web.

    All of the above are verifiable scientific facts.

  15. Gordon

    Since dark matter has never been observed as far back as we can see I would assume it has ‘never’ radiated. This should mean the particles are still at the temperature they formed. This could be billions of degrees. This would make for a large gravity effect from a somewhat light particle. These detectors would never detect them.

  16. alzie

    As Mr. Oldershaw has stated,
    exotic matter isnt needed to explain dark matter.

    Its always bothered me that
    theres not a hint of dark matter in our own solar system if
    dark matter out weighs regular matter 5:1

    I also like the theory that there are
    100K ejected free roaming planets for every star.
    Thats a huge amount of extra dark mass!

    I hope to see fingered out this in my life time.

  17. Robert L. Oldershaw

    The number of planetary-mass "nomad" objects inferred from microlensing observations ranges from 100 billion to about 1,000 trillion objects.

    That is indeed an extremely large amount of astrophysical stuff out there that we did not know about before 2011.

    It is reasonable to expect that there might be other huge populations of astrophysical objects out there that remain to be discovered.

    Also, the mass range for individual "nomad" objects is known approximately, but not their physical states. They could be planets, black holes, or something completely different.

    WIMPs, axions, sparticles, mirror matter, sterile neutrinos … ?
    In a pig’s eye

  18. Adam Smith

    As a graduate student actively researching this phenomena I feel like I’m riding a rollercoaster ! There are highs and lows, twists and turns. Only 12 months ago, if asked, I would of described the WIMP particle as our best guess and even now I’m not yet ready to ditch it completely. However, as a scientist I have to accept what the data is telling me. As the quest for answers continues I do find myself looking (again) at MOND theories and asking if we are indeed looking for one answer to more than one question. Most of the matter in the Universe is missing, this is an undeniable fact. We think we know where it is but we still have no idea what it is. As Richard Feynman once said ".. better to live not knowing, than to have answers that may be wrong" I’ll keep looking, thinking, and most importantly smiling !

  19. Adam Smith

    As a graduate student actively researching this phenomena I feel like I’m riding a rollercoaster ! There are highs and lows, twists and turns. Only 12 months ago, if asked, I would of described the WIMP particle as our best guess and even now I’m not yet ready to ditch it completely. However, as a scientist I have to accept what the data is telling me. As the quest for answers continues I do find myself looking (again) at MOND theories and asking if we are indeed looking for one answer to more than one question. Most of the matter in the Universe is missing, this is an undeniable fact. We think we know where it is but we still have no idea what it is. As Richard Feynman once said ".. better to live not knowing, than to have answers that may be wrong" I’ll keep looking, thinking, and most importantly smiling !

  20. Bruce Mayfield

    This article and discussion are fascinating, with everyone contributing good comments. The points made about unseen ordinary matter such as great numbers of stellar massed black holes and rogue planets possibly contributing to the missing mass problem are interesting. Considering that there is still so much more to be learned about the complete contents of something as close as our own solar system, how can we be so sure that astronomers have accurately tallied up the true contribution of ordinary matter throughout space? Is empty space really as empty as it seems? On the other hand we do see areas, such as in the bullet cluster where super clusters have collided, in which gravitational lensing evidence shows that ordinary matter can be decoupled from dark matter. To me it seems like dark “matter” is behaving more like a field than a true particulate substance, a field that produces regional gravitational attraction. But the attraction doesn’t appear to be mutual. Dark “matter” is just not that into us.

  21. Peter Wilson

    Gordon: The high temp of dark matter creates another problem. How does DM clump if it does not radiate? Many models of galaxy formation put the DM into clumps first; light matter then “falls” onto the clumps by radiating away energy. But dark matter cannot cool down by radiating, so how does it clump first? Adam Smith: As implied in my earlier post, there really is no point in looking at MOND or any other alternative gravitational theory until GR has been ruled out. Since GR can be thought of as “modified” Newtonian dynamics anyway, might as well start there.

  22. Allan Holmgren

    Instead of fixating on particles perhaps we should be investigating fields. Basically what we are detecting is mass which is causing the effects that we are attributing to dark matter. In flat spacetime we have a scaler field of vacuum energy distributed in all three special dimensions. When this spacetime encounters a massive object like a galaxy the flatness of these special dimensions is distorted. Perhaps the value of the vacuum energy field increases as a consequence of the local distortion of spacetime. Relativity’s equilvalence could be the higher local level of vacuum energy which we are detecting as mass.

  23. K L Rajpal

    Abstract: More energetic than the cosmic gamma photons, there should exist dark photons and Planck photons. Also, similar to the cosmic microwave background radiation (CMB), there should exist a ‘cosmic dark photons background’ (CDB). Dark photons may be the particles of the elusive dark matter.

  24. K L Rajpal

    Dark Photons and Dark Matter. Abstract: More energetic than the cosmic gamma photons, there should exist dark photons and Planck photons. Also, similar to the cosmic microwave background radiation (CMB), there should exist a ‘cosmic dark photons background’ (CDB). Dark photons may be the particles of the elusive dark matter.

  25. Martian Bachelor

    How many more blows can WIMPs take and still hang in there? Their tenacity — what?, ~25 years now — is at odds with their catchy acro-name.

    It’s almost an admission of huge failure or incompetence for all of astrophysics, after all this time, effort, and money spent, to proclaim ignorance of what 84% of their universe consists of.

    I know this is always presented as if it’s some profound mystery, rather than befuddled confusion on their part, but it really should be a source of embarrassment because they’re going in the wrong direction!

    Anyway, the sooner WIMPs join the ranks of phlogiston, the better.

  26. Lyle Tompsen

    Well, I think we are jumping the gun here a bit. I agree “WIMP like” signals are a 3 sigma polite way of saying "I don’t know", but although it is elegant, I am not quite sold on “discrete fractal” paradigm either. But, I would say all this evidence is –highly- indicative that something is wrong with our beliefs (and I use that word intentionally) around gravity. That is the major tie to everything. At what point do you look at a patched up pot and call it a sieve?