Dark Matter is Just Messing With Us Now

By Sean Carroll | May 3, 2011 10:49 am

The state of play in dark matter searches just refuses to settle down. Just a few weeks ago, the XENON100 experiment released the best-yet limits on WIMP dark matter (a two-dimensional parameter space, “mass of the dark matter particle” and “cross section with ordinary matter”). These limits seemed to firmly exclude the hints of a signal that had been trickling in from other experiments. But… the story isn’t over yet.

Remember that XENON, like CDMS and other experiments, tries to find dark matter by making a very quiet experiment and picking out individual events where a dark matter particle bumps into a nucleus inside the detector. There is a complementary strategy, looking for annual modulations in the dark matter signal: rather than being very picky about what event is and is not a DM interaction, just take lots of events and look for tiny changes in the rate as the Earth moves around the Sun. Dark matter is like an atmosphere through which we are moving; when we’re moving into a headwind, the rate of interactions should be slightly higher than when our relative speed through the ambient dark matter is smaller. The DAMA experiment was designed to look for such a modulation, and it certainly sees one. The problem is that lots of things modulate on a one-year timescale; as Juan Collar explained in a guest post here, there were many questions about whether what DAMA is detecting is really dark matter.

Now one of Juan’s own experiments, CoGeNT, has seen (very tentative) hints of an annual modulation itself! CoGeNT had already teased us with a hint of a dark matter signal, which (like DAMA) seemed to imply lower masses (about 10 GeV, where 1 GeV is the mass of a proton) rather than the usual masses for weakly-interacting dark matter favored by theorists (hundreds of GeV). But the competitor experiment CDMS, and later of course XENON, seemed to put the kabosh on those claims. The CDMS result was especially hurtful to CoGeNT’s claims, as both experiments use germanium as their detector material. Theorists are very clever at inventing models in which dark matter interacts with one substance but not some other substance (see e.g.), but it’s harder to invent models where dark matter interacts with one substance in one experiment but not the same substance in some other experiment.

Yesterday Juan Collar gave a talk at the April Meeting of the APS, where he revealed something about CoGeNT’s latest findings. (I don’t think there’s a paper yet, but it’s supposed to come very soon, and they are promising to share their data with anyone who asks.) Now, unlike for their earlier results, they are explicitly looking for annual modulation. And … they see it. Maybe. Well, not really enough to take it seriously, but enough to be intrigued. Or, in science-speak: it’s a 2.8 sigma result. It doesn’t seem to have hit the news very hard, but there are writeups by Valerie Jamieson and David Harris. The CoGeNT folks have 442 days of data, with a rate of about three events per day.

Ordinarily, a tasteful physicist would claim that a 2.8 sigma result doesn’t even rise to the level of “intriguing”; you need three sigma to count as “evidence,” and five sigma for “discovery,” by the accepted standards of the field. The reason this is even blogworthy (a low bar indeed) is that it’s the first attempt to check DAMA by looking for an annual modulation signal, and the result matches the phase of DAMA’s oscillation, and is claimed to be consistent with its amplitude (the experiments use different materials, so it’s hard to do a direct comparison). Also, of course, because the team was looking to bury DAMA, not to praise it: “We tried like everyone else to shut down DAMA, but what happened was slightly different.” On the other hand, what you would need to explain this purported signal is at first glance still very much incompatible with XENON’s limits.

In the end: probably still nothing to get too excited about. But at least it will keep the pot boiling a while longer. Don’t fear; the experiments are getting better and better, and temporary confusions eventually evaporate. Or are swept away by the dark matter wind.

  • simplicio

    If they were trying to verify (or de-verifty, anyways) the signal from DAMA, and they knew the parameters of DAMA’s signal, wouldn’t they build the experiment so that the detection of such a signal would give a better confidence then 2.8 sigma, or if better confidence was impossible, then not bother to build it at all?

  • Todos Manos

    There is a vacuum background active toward massed fermions (matter) but not toward massless bosons (photons, arxiv:0912.5057, 0905.1929, 0706.2031). It would be better to discover what it is rather than to use it to curve-fit physically defective, politically facile prior theory.

    Two much larger detectors are required: More of the same to confirm, and a much bigger XENON to deny. The truth bridges the gap. BTW, are the phase angles of the annual sinusoidal signals identical?

  • Paul Clapham

    “Just messing with us”? No, I don’t think that’s quite right.

    The designers of the universe set the galaxies’ rotation, but they didn’t do it right. Then some people noticed that. Fortunately the designers had time to fix the bug, so they invented dark matter. But now it seems they didn’t do it quite right, and some people have noticed.

    You don’t think that’s a reasonable description of things? Then what about the neutrino fiasco? The designers didn’t get the mass of the neutrino right, and when people noticed, they had to invent that ridiculous neutrino which changes flavours as it flies. And from what I see lately they didn’t quite get that right either.

    And then there’s the proton which doesn’t decay like it was supposed to. That appears to be still waiting for a bug fix from the designers.

    I’m sure you could go on and produce more examples of sloppy design. (The “flatness” problem?)

    It appears that humans’ ability to find these bugs is speeding up. If the designers don’t fix them fast enough then things could really start to unravel. Especially if they put in fixes which are worse than the original bugs. However the designers could deal with this problem by causing the funding for the bug-finders to be reduced; perhaps this strategy is already underway.

    (Anyone know how to put smilies in comments?)

  • Sili

    Where’s the Southern Hemisphere verification?

  • jpd

    “Dark matter is like an atmosphere through which we are moving;…”

    did you mean aether?

  • Chris

    If they could build more sensitive detectors (bigger lump of Ge?), would they then be able to see a daily variation, and then confirm the effect by the phase difference of detectors on opposite sides of the planet..?

  • Dan Carney

    I’ve heard 4.8 and 2.8 sigma now, from two different places, neither with a citation; anyone know for sure what the number is?

  • David

    If there’s 5 times as much dark matter as visible matter that means the Milky Way has 5 times more mass in dark matter. That also means the solar system should have 5 times more mass than we can detect. Why don’t we see the gravitational effects of dark matter on the solar system? Is all the dark matter concentrated at the galaxies core so that it only affects the rotation of the galaxy but not small scale objects like the Solar System?

    The curve describing the orbital velocity of stars in galaxies shows that orbital velocities are constant as you move out from the core of the galaxy. This implies that the dark matter isn’t concentrated in the core but is evenly distributed throughout the whole galaxy. Again, if this is the case then why don’t we see the effect of dark matter on the orbital velocities of the planets?

  • Matthew Saunders

    Hmm…’annual modulation’…*chuckle* I can guess that this is triggering some scientist’s “NOT ASTROLOGY!!!” gag reflex :)

    (similar to the finding that it seems that some radioactive decay changes over time as well…)

    # 3: Hmm, Douglas Adam’s as soon as we understand universe, it will disappear and come back as something more inexplicable :)

    You asked how do you put smiles in comments? Just do it the regular ascii way, do it in text, and it will, after you save the message, appear as a graphical smiley. So colon shift-zero will produce a :) smiley and so forth.

  • Matt B.

    It’s “kibosh”, not “kabosh”.

    @Paul Clapham, to see what to type to make a particular smily, just hover your pointer over it for a couple seconds. :roll:

  • http://www.flickr.com/photos/sedary_raymaker/ Naked Bunny with a Whip

    did you mean aether?

    Sure, because the aether was a completely unexpected phenomenon that was only postulated after observations suggested it existed. *rolls eyes*

  • http://washparkprophet.blogspot.com ohwilleke

    “That also means the solar system should have 5 times more mass than we can detect.”

    CDM models generally assume a dark matter halo that is distributed within a galaxy with a distribution different than that of the ordinary matter in the galaxy. Thus, a given proportion of dark matter does not imply a given proportion of dark matter in a particular part of a galaxy.

    Also, the 5-1 ratio (which is probably off, as it was based on an underestimate of the amount of ordinary matter in ellipical galaxies and is probably closer to 1-1), isn’t necessarily the same for all types of galaxies (and indeed probably isn’t the same). The ratio at the galaxy level may very well be different in ellipical and spiral galaxies of particular sizes.

    One of the notable annual cycle events that ultimately turned out to be nothing was an apparent change in beta decay rate of some radioactive substance that some initial studies had seemed to support at levels far greater than GR time dialation effects could explain.

  • Matthew Saunders

    # 12: “One of the notable annual cycle events that ultimately turned out to be nothing was an apparent change in beta decay rate of some radioactive substance that some initial studies had seemed to support at levels far greater than GR time dialation effects could explain.”

    Nifty, got a link? :)

  • Annika

    To me, this says DAMA and CoGeNT are seeing annually-varying backgrounds. When experiments that can distinguish between electronic and nuclear recoils see no signal, but background-dominated experiments see something, then it smells to me like a background issue.

  • jpd

    not sure of your point

    both of these things are true.

    aether was a completely unexpected phenomenon that was only postulated after observations suggested it existed.
    dark matter is a completely unexpected phenomenon that was only postulated after observations suggested it existed.

    *rolls eyes*

  • Ciaran

    “If there’s 5 times as much dark matter as visible matter that means the Milky Way has 5 times more mass in dark matter. That also means the solar system should have 5 times more mass than we can detect. ”
    I don’t know much about Dark Matter physics but this is an error in logic. It’s similar to saying that if 70% of the earth’s surface is ocean, 70% of my back garden must be ocean.

  • Matthew Saunders

    # 16: Yes, it could mean too that 70% of Earth is dark matter, 70% of all elections are dark matter (well, we already knew that, right?) and 70% of our bodies are dark matter 😉

  • vn

    # 13: arXiv: 0808.3283 and 1004.1761 (season variations of
    beta-decays) (zhogin.narod.ru — am I black-listed? fie)

  • z

    Perhaps they’re seeing a GeV-scale sterile neutrino signal? A low mass would seem to imply something that should have been seen at LEP?

    Sean is right, this is very confusing!

  • chris

    yeah, that’s what Michelson-Morley dod, too. after not seeing a straight signal they were looking for anual modulations. and then starlight.

  • PTM

    Could dark matter be not a separate entity but rather a “tail” of normal matter?

    Right now the mass of an elementary particle is thought to be completely localized within a small distance from the particle. But imagine instead that the distribution of mass is not of a Dirac delta sort, but rather there is a very high peak where all the mass that we ascribe to particle is located and then there is a very low tail which extends for light years and which is responsible for dark matter effects. This tail would not affect physics on our scales, it would only become important on astronomical scales.

  • James

    @8: Even assuming a perfectly even distribution of dark matter (in the ratio of 5:1 to normal matter), what gravitational effects are you expecting to see within the solar system?

  • Tom

    So where are the CoGeNT slides and/or the arXIv link for the paper?

  • george briggs

    people are wasting the governments m0ney.A.ll the dark matter in our mature galaxy is long gone. Our galaxy is no longer growimg because it has no fermibosonic matter left to eat. Dark matter is the bosonic half of fermibosonic matter. We must have anther big bang to restore the fermi bosonic entity

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  • Cutter McCool

    @jpd. Your first comment is exactly what I thought as well when I read that sentence. Seems entirely possible that future scientists will regard the current belief in dark matter in the same way that current scientists regard the old belief in the aether.

  • Matthew Saunders

    #18 vn: thankee for the link :) I’ve got a friend who is from “Moscow/Ukraine” (he’s told me about the games one must play there), a scientist, who is currently enjoying being underemployed in his new country — it’s amazing how my country screws their new citizens.

  • David

    @16 If the Earth’s oceans were evenly distributed over the Earth’s surface then, yes, your garden would be 70% water. The flat rotation curves of galaxy’s implies that the dark matter is evenly distributed rather than concentrated at the core of the galaxy.

    @22 I would expect to see much higher orbital velocities for the planets and other objects within the solar system than we currently see. After all, there would be 5 additional solar masses in the solar system.

  • Marc

    @28 David—if the velocity is constant, that means that the density falls off like 1/r^2. So it is still concentrated at the center. Check Newton’s Law. And do you really think that the thousands of physicists who have worked on dark matter have all missed the velocities of the planets? We aren’t all that stupid. The density of the solar system is much bigger than the density of our galaxy, and dark matter gives 1-5 times the density of our galaxy. Utterly irrelevant in the solar system, and this is very easy to calculate.

  • http://washparkprophet.blogspot.com ohwilleke

    “The density of the solar system is much bigger than the density of our galaxy, and dark matter gives 1-5 times the density of our galaxy. Utterly irrelevant in the solar system[.]”

    I’ve never seen this stated quite so clearly and compactly. (And the fact that it rarely is suggests that the outraged tone in response to #28 wasn’t really necessary, even if it feels good to vent sometimes.)

    There is a lot a educated layman literature explaining why we infer dark matter and how much of it is inferred (rotation curves, lensing, etc.) There is also a fair amount of literature at the generalist physicist level explaining the way that we infer things about dark matter from the cosmic background radiation profile.

    But, there is a real dearth of information in the literature accessible to non-specialists about the nuts and bolts of leading dark matter therories (e.g. regarding the distribution of dark matter implied in theories) at an educated layman/freshman physics student level, e.g., what assumptions about dark matter distributions, dark matter interactions, constants used to fit the model, go into the theory that is used to fit the data from astronomy.

    This is a shame because these theories use very little more than Newtonian physics and gravity — there isn’t much in GR that modifies the conclusions at the galactic/galactic cluster or finer scales (although GR obviously is relevant when one tests theories against cosmology models) , and one doesn’t have to be very specific about dark matter particle masses or properties to get a quite detailed exposition of the theory.

    Similarly, while quantum mechanics points us towards possible dark matter candidates and to theories that could explain their origins in Big Bang processes, the experiments like COGENT that we are doing to try to find a dark matter candidates aren’t anything that Bohr wouldn’t have understood. The mathematical and physics knowledge pre-requisites to having a fairly firm technical grasp of dark matter theories are quite a bit less demanding than those necessary for a really firm grasp of what is going on at LHC for example.

  • Blake

    I don’t understand how a hypothetical dark matter particle of a mass in the range of just a few GeV COULDN’T have already been seen at the detectors in our particle colliders ages ago. I mean, we’ve been able to see new particles of a few GeV mass like the J/psi since the mid-’70s! Shouldn’t we have been seeing weird missing mass or some such in collider events long ago?

  • http://washparkprophet.blogspot.com ohwilleke

    “Shouldn’t we have been seeing weird missing mass or some such in collider events long ago?”

    Not if W bosons don’t decay into them. Atom smashers see the decay products of high energy unstable fermions and leptons mediated via W bosons. But, if W bosons don’t decay into dark matter particles, then the decays of high energy particles in atom smashers wouldn’t produce them. If the ordinary matter sector and dark matter sector only interact through gravity and not through any Standard Model forces (i.e. strong nuclear force, weak nuclear force, electro-magnetism), then we wouldn’t expect W bosons to produce them.

  • spaceman


    Another ~3 sigma result! There seem to be a lot of these popping up as of late. But the simple fact remains: our cherished theories are NOT panning out! No higgs, no direct detection of gravitational waves, no dark matter, etc. How much longer will people who are so so smart continue to cling to these out-dated, unproven, and increasingly untenable notions in spite of the ample evidence against them? Maybe I am being too negative, but really, I read an article in the NYTimes magazine about how fruitless and futile the search for dark matter is becoming (Even Vera Rubin herself, when interviewed for the article, expressed doubts about the stuff after all of these years!). The stuff probably does not exist so how about finally embracing the search for alternative theories e.g. diverting a modest portion of the money, IQ points, and effort into testing OTHER ideas?? Should we not by this late date take this state of utter disarray and excitement over ~3 sigma results as a sign that the dark matter hypothesis is probably wrong?

    Below is my case for claiming that in fact we do not know much about dark matter at all other than to say that there is a discrepancy in our observations and the reigning theory of gravity.

    A few years ago I read an online power point presentation about the nature of dark matter. What amazed me then and still does is the sheer number of different dark matter theories and associated candidate particles: warm dark matter, cold dark matter, mixed-dark matter, axions, neutralinos, sterile neutrinos, gravitinos, etc. What many of the candidates seem to have in common is as follows: (i) motivation by physics outside of cosmology (e.g usually as potential solutions to a particle physics problem), (ii) proponents can cherry pick astrophysical observations that support “his or her” candidate, and (iii) the candidates’ respective parameter spaces are increasingly being narrowed by a variety of observations (or are they– look at the state of disagreement about what has been ruled out by the various direct detection experimental teams).

    The take home message is that the field is still wide open and scientist A’s guess is as good as scientist B’s. Also, this state of confusion is before you throw in the modified gravity theories which are diverse bunch as well.

    The situation is akin to one in which a crime has been committed and the case detectives each have their own theories as to who did it. Each detective can point to evidence that supports his “guy” just as each physicist can point to evidence that supports his particle. This aforementioned group of detectives at least agree that a human being is behind the crime. Now, there is another (minority) of case detectives who believe that a human being is not behind the crime just as there exists a minority of physicists who believe that a particulate matter is not behind the “missing mass problem.”

    The only thing we know is that there is a “missing mass problem” which is like the case detectives saying that at least we know that a crime has been committed. Duh!!

    Therefore, I would say that not only can we not claim to know much about DE, we can also not claim to know much about dark matter!

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  • anonymous007

    “The only thing we know is that there is a “missing mass problem” which is like the case detectives saying that at least we know that a crime has been committed. Duh!!”

    We do not have a “missing mass” problem. That is a bundled assumption. We have an apparent excess of gravitation in galactic rotation speed measurements. I might suggest that – the “missing mass” and ‘missing Higgs’ to give mass – means that the wellspring of gravitation/mass/inertia is not yet fully understood…although the theory of how it interacts is well understood.

    please note that I am less than an amateur, however.

  • Doug

    Juan gave a talk on this yesterday at the STScI dark matter meeting. A video of it can be found at https://webcast.stsci.edu/webcast/detail.xhtml?talkid=2426&parent=1

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  • Lloyd B. Johnson

    @ anonymous007. I would agree! Gravitation/Mass/Inertia mechanism is not understood at all. I think a Substantivalist explanation is required.

  • george briggs

    @my # 24:The dark matter is long gone from our old galaxy statement is only true if the fermibosonic ancestor was relatively static. If the precursor was a very low mass ordinary matter particle, it would be zooming around at near light speed and would be in our galaxy today where we could mistake it for the real thing. The black hole at the center of our galaxy tears the weightlees fermibosonic matter into its constituent dark matter (bosonic) and ordinary matter (fermionic) parts.

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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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