Of Particular Significance

A Murky Matter, Several Times Over

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/29/2013

Dark Matter, Dark Matter, Everywhere! It’s in your shoes, it’s in your coffee, it’s in the stars and even in your favorite cheese… at least, it’s widely believed to be wandering all about, mostly unnoticed.  Still it’s not quite as inscrutable as its reputation would lead you to believe.  It’s responsible for a galactic glow, an abundance of anti-matter, and now — three quiet little taps in an underground mine.

Or is it?

Apparent effects of dark matter have been “discovered” so many times in the last decade that you may by now feel a bit jaded, or at least dispassionate.  Certainly I do.  Some day, some year, one of these many hints may turn out to be the real thing. But of the current hints? We’ve got at least six, and they can’t all be real, because they’re not consistent with one other. It’s certain that several of them are false alarms; and once you open that door a crack, you have to consider flinging it wide open, asking: why can’t “several” be “all six”?  All of the dark-matter search experiments are difficult, as they involve pushing the technological envelope. And as anyone with experience in science knows, most of the exciting-sounding results emerging from forefront experiments don’t survive the test of time. Never underestimate the challenge of science at the frontier of knowledge!

Still, as of two weeks ago, we have a new dark matter hint to talk about. So here’s a summary of the various hints, including the new one, exploring their implications and their consistency.

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43 Responses

  1. “300 comments …” I think is a slight exaggeration. In view of the current evidence, what are the best hopes for SUSY WIMPs?

    1. I never cared one way or another regarding SUSY WIMPs myself; but there are lots of other ways to get dark matter. All you need is a single new symmetry, and a single new neutral particle, one that is stable because of that symmetry and has the right interactions with itself and with ordinary matter.

  2. “All of the dark-matter search experiments are difficult …” MOND might entirely eliminate the dark-matter hypothesis.
    According to McGaugh and Milgrom, “MOND appears to be in good agreement with the observed velocity dispersions of the dwarf spheroidals of M31.” http://arxiv.org/pdf/1301.0822v2.pdf “Andromeda Dwarfs in Light of MOND”, Feb. 2013
    According to Kroupa, Pawlowski, and Milgrom, “Understanding the deeper physical meaning of MOND remains a challenging aim. It involves the realistic likelihood that a major new insight into gravitation will emerge, which would have significant implications for our understanding of space, time and matter.”
    http://arxiv.org/pdf/1301.3907v1.pdf “The failures of the standard model of cosmology require a new paradigm”, Jan. 2013
    I say that Milgrom is the Kepler of contemporary cosmology. Thirty years of ignoring Milgrom is enough.

    1. 300 comments saying exactly the same thing is enough, Mr. Brown. I’m bored with this set of statements, with which you’ve flooded my website. We all know that MOND is a possibility, one that most experts disagree with, but which hasn’t died yet.

  3. True or false? ….

    1. Dark matter is slow low in mass that is lives (hides) in the vast “empty” space between the other massive particles.

    2. Dark matter has mass but may or may not have charge and spin, mmm, sounds a lot like a Higgs boson, but we already found that one.

    … Or maybe a graviton?

    1. 1. The hypothesized low cross-section of interaction of dark matter is more relevant than its mass. Dark matter is, by hypothesis, much heavier than neutrinos (otherwise Planck would observe more effective neutrino species and there would be no galactic structure), but the direct detection of neutrinos is something done routinely because they have a sufficiently great cross-section of interaction.
      2. Unlike the Higgs boson which has a lifetime of a fraction of a second, dark matter (or at least the predominant form of it) is stable or meta-stable.
      3. Hypothetical gravitons have zero rest mass and hence always move at the speed of light. The dark matter hypothesis requires a particle that is not relativistic.

      1. My line of questioning points to the graviton and/or a heavier Higgs boson or both … ?

        Dark matter not only prevents galaxies from flying apart from each other but could also be the dominate factor of keeping them intact. Interaction between “dark” and “visible” matter is there it is just to small (unit or quanta) for us to measure. But we can deduce the bigger picture. So what better particle could do this coupling other than the graviton?

        Gravitons are small and fast but the total is 5x more (and suspect the ratio is not a random number either). Keep in mind that has nothing to do with mass, but rather energy and momentum.

        In other words, the question could be posed like … are graviton or more accurate, the gravitation field responsible for quantum confinement? Are infinite arrays of gravitons responsible for creating the walls that keep massive particle “trapped” and stable?

  4. @Matt Strassler
    You did not consider the XDM (Exciting Dark Matter) candidate proposed by Nima Arkani-Hameed and others which can explain complex dark matter with interactions.
    BTW why do people want to consider “all” dark matter as inert and expect very simple structure and interactions (or lack of it among themselves).
    Since real matter has a complex life, symmetry should expect us similar interactions in a different sector.

    I think one should start by considering “all possible” configurations for dark matter and then see what all can fit cosmological and experimental observations

    1. “BTW why do people want to consider “all” dark matter as inert and expect very simple structure and interactions (or lack of it among themselves).”

      Not many degrees of freedom are necessary to model almost all observed dark matter phenomena. LamdaCDM does so at the large scale structure CMB level with just one. MOND models can describe all galactic scale dark matter phenomena effects quite precisely with just one (which means that even if the source is DM rather than gravity modification that the mechanism must be a simple one). Two or three parameter DM models that span the entire spectrum of phenomena aren’t uncommon. Models with few degrees of freedom can usually be explained with simple structures and interactions (or with situations where simple structures and interactions predominate even if the total reality is more complex – e.g. a proton-neutron-electron model is quite adequate to explain a huge share of non-DM phenomena outside extreme conditions despite omitting all sorts of exotic hadrons).

  5. I believe that the bulk of “dark matter” is the “fabric of space.” That is, that the bits that make up the “fabric” of the Universe are locked in the structure of the Universe but the “fabric” is distorted due to the mass of the these bits interacting with the mass of normal matter.

  6. Hello Matt,

    Hello Matt, Let me state my question less speculatively than before because I would really like to know the answer. The fact that WIMPS are said to interact through the weak force and the gravitational force (and not ONLY gravitationally) has this to do with the fact that it is impossible to generate the mass of the WIMP through the current version of the Higgs mechanism if it doesn’t also interact weakly?

  7. Wouldn’t there have to be a decoupling event in the big bang for dark matter the same way there was one for light? What effect would most matter suddenly becoming invisible have on the big bang?

  8. Maybe there is more than one kind of dark matter. In fact we know that there is. Neutrinos are essentially dark.

    1. That’s correct, there’s no reason dark matter can’t have multiple components (and in some sense neutrinos and anti-neutrinos make up a very small part of the dark matter.) We do know the majority of the dark matter can’t be made from particles as lightweight as neutrinos. But the majority might be made from several different types of objects.

      But the underground experiments I’ve described here don’t require that there be only one type of dark matter. As long as one type interacts strongly enough with matter, they may see it.

      The satellite experiments are more sensitive to this question, because they are seeing signals that are already somewhat larger than naively expected even if there’s only one (non-neutrino) type of dark matter.

  9. Are we living on the leading edges of the Big Bang shock wave? Is dark matter and dark energy the bulk of the “stuff” that is lagging behind?

    Was there a space-time before the Big Bang released it’s energy? ,,, and if so, how big is this universe anyway? … infinite?

    I truly believe if we ever decipher the concept of infinity, we will understand everything. Never mind the theory of nothingness and/or everything, the theory of infinity we be the key that opens the last door.

  10. My first guess would be:

    Fermion Bosons
    gimp gravitational
    neutrino gravitational, weak
    electron gravitational, weak, electromagnetic
    quark gravitational, weak, electromagn., strong

    but I think that in this case it is difficult to give the “gimp” its mass through the Higgs mechanism?

    1. (More than two spaces were substituted by one space when I submitted my contribution and now it look awful. Who decided that my multiple spaces were not necessary?

      1. You can’t align columns with spaces when using a proportional font anyway, so don’t even try.

        It should be done with tables, but it’s up to the designer of the comment system to allow the users to create tables. Most don’t.

        1. Of course you’re right but if I then find it acceptable to use spaces it should be my choice!

    2. Marcel,

      I don’t pretend to know the answer to your question on the Higgs generating the gimp mass, though I have learned through Matt that the Higgs is responsible for less mass than is generally advertised. I do however like the schema you are setting up. I like the hierarchy it incorporates. Have you considered the possibility that each of the fermions (quark, electron, neutrino, gimp) comes in 3 conserved values or families?

      I do not like our current neutrino oscillation theory where for example an electron-neutrino simply changes (presto-change-o) into a muon-neutrino. I would much prefer that an electron-neutrino becomes a muon-neutrino accompanied by a muon-antigimp and an electron-gimp. That would conserve family type, and the gimps would go undetected, so it would ‘appear’ to us that an electron-neutrino simply changes into a muon-neutrino.

      1. Hello S. Dino,

        Sorry I didn’t notice your response for some time!

        You are of course right, GIMP stands for “Gravitationally Interacting Massive Particle”, a name I made up myself but Google tells me it has already been used by other people in the past.

        That the Higgs is responsible for less mass than is generally advertised is only true for composite particles, for the particles we now often denote as fundamental particles (especially electrons and quarks) all (rest) mass is generated via the Higgs mechanism.

        The schema I proposed as something that is at least worth considering. The family replication is a complication I have ignored for the moment but it is reasonable to assume that it should exist also for the gimp, if the gimp exists. As far as I know there is no issue with “oscillation theory” for neutrinos or quarks. It’s a standard consequence of quantum mechanics and the existence or non-existence of gimps would be irrelevant for that.

        Probably there are good reasons why the existence of the gimp is unlikely but it would be nice if some specialist could explain us why.

    3. There is a time to feign knowledge and a time to admit ignorance. I know exactly which occasion has now arrived. So, what the hell is a gimp?

      1. Well, I can’t be positive (Marcel please correct me if I’m way off base here) but just as a ‘WIMP’ stands for a Weakly Interacting Massive Particle, a ‘GIMP’ would be a Gravitationally Interacting Massive Particle – a particle that interacts through no other force except gravity. We know of no such particle, but I think Marcel (again Marcel please correct me if I’m wrong) is suggesting that the gimp could be a good candidate for dark matter. Our current instruments could not detect such a particle…


        Particle Interaction(s)

        Gimp Gravity

        Neutrino Gravity, Weak

        Electron Gravity, Weak, Electromagnetic

        Quark Gravity, Weak, Eletromagnetic, Strong

        1. The next naive thing would be to complement this with (something like)

          Particle Interaction(s) symmetry group

          Gimp Gravity SU(0) (= do nothing)

          Neutrino Gravity, Weak +SU(2)

          Electron Gravity, Weak, Electromagnetic +SU(1) (:=U(1))

          Quark Gravity, Weak, Electromagnetic, Strong +SU(3)

          and we see that we have a problem putting this in the right order 0,1,2,3 but then 1 and 2 are broken anyway 🙂

  11. Matt, there’s something of a presumption that dark matter consists of particles. Remember the raisins-in-the-cake-analogy. Space expands between the galaxies but not within. Given dark energy and conservation of energy, every galaxy will be embedded in a region of higher spatial energy density, with a mass equivalence. IMHO Einstein pointed the way with “the energy of a gravitational field shall act gravitatively in the same way as any other kind of energy”. It doesn’t consist of WIMPs. Google on inhomogeneous vacuum.

    1. There is certainly a presumption implicit in my article, and one should certainly question it, check it, and decide about one’s point of view. But of course scientists have been questioning it for decades, and many alternative interpretations of what’s going on have already been discarded, because they disagreed with data.

      As for your specific point of view, it does not lead to a good description of galaxy formation, or of galaxy collisions such as the Bullet cluster, or of the Planck and other cosmic microwave background data.

      1. I think the data can fit, Matt. In his Leyden Address Einstein described a gravitational field as inhomogeneous space. So the non-uniform raisins-in-the-cake expansion results in inhomogeneous space which results in a gravity modification, and MOND fits the flat galactic rotation curves very well. It doesn’t fit the clusters where there seems to be much more dark matter, but you can liken them to a bunch of raisins that prevent a big lump of cake expanding. I think an underlying issue here is that Lambda-CDM is built on the FLRW metric which “starts with the assumption of homogeneity and isotropy of space”. It just doesn’t square with a gravitational field being inhomogeneous space, or the non-uniform expansion. Another possible issue is that dark energy is thought to be increasing as space expands. That just doesn’t square with conservation of energy. Maybe a better approach is to say vacuum energy stays constant overall whilst space changes. Milgrom hints at this on page 5 of http://arxiv.org/abs/0912.2678 : “We see that the modification of GR entailed by MOND does not enter here by modifying the ‘elasticity’ of spacetime (except perhaps its strength), as is done in f (R) theories and the like.” I don’t think MOND is right, but I do think it’s on the right lines for the wrong reason, and you can have your cake and eat it by saying dark matter is essentially the same thing as dark energy. For a cherry on top go back to Einstein’s Leyden Address and see him talking about space as an “ether” then saying “The contrast between ether and matter would fade away”.

        1. You’ll choke if you try to have your cake and eat it too by saying “dark matter” is the essentially same thing as “dark energy”. The whole reason we give them completely different names is that they have completely different physical effects. Today dark energy and dark matter are the main components of the universe, but dark matter played an essential role much earlier in the universe, at a time when the dark energy was much less important.

          1. Your comment noted Matt. But I think you’re associating dark energy with accelerating expansion, not with conservation of energy and expansion accompanied by a diminishing spatial strong-force tension. Get some silly putty, stretch it out, watch it droop. And think about what sort of energy caused the initial expansion and dumped out as light and matter.

    2. “there’s something of a presumption that dark matter consists of particles.”

      “Dark matter” is a hypothesis that supposes that a variety of kinds of phenomena that astronomers observe can be described by a huge number of particles that interact gravitationally but are otherwise almost collisionless (with the “almost” part leaving room for variety). It is “presumed” because that is the nature of the hypothesis that is being tested. One can offer up another hypothesis that explains the phenomena attributed to dark matter with something other than massive particles (and such theories are out there in the academic literature), but then we wouldn’t call it a “dark matter” hypothesis – it would be an alternative hypothesis to dark matter. Direct dark matter searchers are one way out of many to test this dark matter hypothesis by seeing if the hypothesized particles actually exist in the most straightforward of ways.

  12. I have made dark matter in my coffee cup. I’ll publish right after I alert the press.

  13. I still find it disturbing that Newton’s good old gravitational force has the privilege of being explained by curving space-time, while the electroweak and nuclear forces have never been subsumed under any sort of geometry. That is only possible because everything with mass is affected equally by gravity, while the other forces act differently on different kinds of matter. Yet how unfair it seems to make gravity the queen of the air, pure in every way, while the other forces have to slug it out with each other on the ground!

    1. That’s actually not quite true. In some contexts, the other forces do get some kind of explanation in terms of warping of extra-dimensional space-time. That can happen in string theory, for instance. However, string theory also demonstrates clearly that even within string theory, not all forces need arise in this way. So we can’t assume anything one way or the other at this point about the non-gravitational forces; it’s just not known how they arise.

      1. Oh come on…Mr. Dratman is exactly right, and I humbly suggest that you are missing the point Matt. Yes, people have proposed from time to time that the other forces can be described by some form of geometrical space-time (though these attempts are far more complicated and thus less pleasing than Einstein’s description of gravitation). The point is that only gravity has mass as its “charge”, and mass (unlike electric charge for example) is not quantized – it does not come in discrete units. The point is that only gravity acts on everything equally (or so we currently believe). So gravity does sit like a queen…

        But what if it is not so…

        Look, once upon a time a man came up with a beautiful and simple equation for the spacing of the planets. It came to be known as Bodes Law. It seemed to work, and if I’m not mistaken (and I may be) it even predicted the location of the asteroid belt, many scientists swore by it. But eventually (I think with the discovery of Neptune) it was found to be wrong. Just a COINCIDENCE. Today, it is rightly regarded as complete nonsense.

        Here’s another…devout Moslem’s believe that God placed the Sun and the Moon in the heavens in such a way that when viewed from the Earth, they are, at special times – exactly the same size. Indeed, they regard anyone who does not see the obvious intelligent design in the glorious beauty of a total eclipse as a deluded fool. We (I shouldn’t speak for you guys – but I will) regard the fact that the Sun is about four hundred times the diameter of the moon and four hundred times its distance from the Earth as a COINCIDENCE. An amazing coincidence, but just a coincidence.

        So is it possible that the apparent equality between gravitational and inertial mass is, like the examples above, just a COINCIDENCE!?

        Perhaps, just perhaps, we will find that gravity is not a queen…and just perhaps, rather than trying to make the other forces like her, we will find a way to make her like the others.

        1. I rather think Prof. Strassler is right and YOU are missing the point.

          It’s hilarious, that certain keywords can not be mentioned in a physics blog (not even by Prof. Strassler) without eintailing strange ranting comments drifting off topic to religious believe issues which have nothing to do with science … 🙁

          That is why I once decided to read only Prof. Strassler’s original articles and stay away from the comments below them generally. I obviously forgot about my decision, so thanks for reminding me about it by your comment … :-/

        2. Matt is factually correct, but dratman’s question is also genuine. My view is as follow.

          a. All four forces are unified. This is a faith (or dream) of many, but it was already done in some models (of course, not shared by the majority thus far).

          b. These four forces can be classified into two groups.
          i. The envelope-forming type — it forms an envelope, defining a boundary for a system.

          ii. The envelope-cracking type — it cracks an envelope.
          With this classification, the electroweak unification of today is not done on the conceptual level (about the fundamental/emergent relation) but is on the “energy scale” level, that is, their coupling strengths are identical at an energy scale, called the weak scale.

          c. The four forces unification goes way beyond the coupling strengths but is about the fundamental/emergent relations. With this context, the easiest unification is between the electric and gravity forces, as both their charges share the same fundamental, as below.
          e (electric charge) = f (ħc), c is the light speed. Thus, e is the emergent of ħ, c.

          m (mass charge) = f (ħ/c), m is also the emergent of ħ, c.

          The fundamental for both strong and weak forces goes one step deeper (on the spacetime tier, sort of geometrical), but they cannot be discussed without alluding to a particular theory. Thus, I will not discuss this here.

      2. If you try to combine the effect of multiple space/time curvatures, won’t the curvatures contradict each other or something? One curvilinear geometry at a time sounds like more than enough for me. Unfortunately I never made it to GR or electroweak unification or QCD in college, but I would be surprised if you can just add up the curvature tensors from forces so different in range.

        Or can you?

    2. Dratman: you should read The Role of Potentials in Electromagnetism by Percy Hammond. See http://www.compumag.org/jsite/images/stories/newsletter/ICS-99-06-2-Hammond.pdf . Look at the sentence near the endnote: “We conclude that the field describes the curvature that characterizes the electromagnetic interaction.” Percy Hammond is co-author of the Geometry of Electromagnetic systems. The curvature of electromagnetism is akin to the curvature of frame dragging of gravitomagnetism, but much fiercer. Imagine the Earth was spinning equatorially at the speed of light, and spinning pole-over-pole at half the speed of light. Then reduce the Earth to a single electron and call it a spinor. Then look at Maxwell’s page title in On Physical Lines of Force and the Einstein-de Haas effect.

  14. Don’t you think that the replacement of UFOs and extraterrestrials in popular (science) magazines by Unidentified Digital Signals and dark matter (or energy) is an evidence for the raising of the average cultural level’s reader ? 😉

  15. There seems to be a fundamental problem with dark matter as it is presently conceived. To my way of thinking the fundamental problem is a gravitational/inertial mismatch. In order for dark-matter not to clump too much as permitted by h/mc (where m is its inertial mass) it needs to be of low mass – in the 10 to the -8 electron mass range or lower, but in order to do its job gravitationally it needs to be of high mass in the MeV range or higher. Now you can try to work your way around this with the invention of a fairly complex and ad-hoc ‘dark sector dynamics’ but reminds me of adding epicycles…

    Einstein notwithstanding (I know, GR so beautiful – it must be right, and of course let’s not forget the great retort “there is only one kind of mass”) I would push to perform experiments that test the fundamental assumption of the equivalence of inertial and gravitational mass. ‘MICROSCOPE’ an ESA satellite to be launched in 2016 (originally scheduled for launch back in 1992!) will test equivalence for normal matter to unprecedented accuracy. But even that is still a test of the equivalence of gravitational and inertial mass for ordinary matter…

    The current view of what’s inside the Nucleon notwithstanding (I know Matt, the Nucleon is chock full of antimatter) equivalence has yet to be tested for antimatter (originally proposed for positrons in the late 1960’s – but never done, for anti-protons in the late 1980’s – but never done, and now for anti-hydrogen – though not yet done. See AEGIS experiment). Also, it has yet to be tested for higher generation matter (muons, muon-neutrinos, strange quarks, etc). Just want to make sure we have all our ducks in a row before we go running half-cocked into the ‘dark sector’. There may be alternatives…

  16. One of the reasons we have so many “hints” or “anomalies” is simply that we are too easily triggered. Those three events are hardly a very significant signal. Is it 2 sigma, or maybe nearly 3? We all know that if you do a lot of experiments, that’s bound to happen every once in a while. If we didn’t know a signal should appear one day, and weren’t so eager to see it, no one but the people involved in the experiment would be spending a lot of time and energy on such an “excess”.

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