More Messy Dark Matter

Longtime readers know that I’m fascinated by the possibility that dark matter is “interesting.” Of course dark matter is by its very nature interesting, but I’m referring to the idea that the dark matter isn’t simply a single neutral particle with a negligible rate of interaction in the current universe, but rather a set of one or more kinds of particles with some noticeable forces acting between them. Friends of mine and I have investigated the idea of dark photons — dark matter being charged under a new “dark force” resembling ordinary electromagnetism. The next obvious step is dark atoms — two different kinds of charged dark particles that can join together in atom-like bound states. These ideas, it turns out, are fairly compatible with what we know about the dynamics of dark matter in the real universe.

Now a new paper by JiJi Fan, Andrey Katz, Lisa Randall, and Matthew Reece examines the particle physics and astrophysical consequences of a somewhat more elaborate version of this idea, which they call “Partially Interacting Dark Matter.” The idea is that most of the dark matter is vanilla and boring, but some fraction of it is atom-like. This has interesting implications for galaxies and small-scale structure. Here’s the abstract:

Double-Disk Dark Matter
JiJi Fan, Andrey Katz, Lisa Randall, Matthew Reece

Based on observational tests and constraints on halo structure, dark matter is generally taken to be cold and essentially collisionless. On the other hand, given the large number of particles and forces in the visible world, a more complex dark sector could be a reasonable or even likely possibility. This hypothesis leads to testable consequences, perhaps portending the discovery of a rich hidden world neighboring our own. We consider a scenario that readily satisfies current bounds that we call Partially Interacting Dark Matter (PIDM). This scenario contains self-interacting dark matter, but it is not the dominant component. Even if PIDM contains only a fraction of the net dark matter density, comparable to the baryonic fraction, the subdominant component’s interactions can lead to interesting and potentially observable consequences. Our primary focus will be the special case of Double-Disk Dark Matter (DDDM), in which self-interactions allow the dark matter to lose enough energy to lead to dynamics similar to those in the baryonic sector. We explore a simple model in which DDDM can cool efficiently and form a disk within galaxies, and we evaluate some of the possible observational signatures. The most prominent signal of such a scenario could be an enhanced indirect detection signature with a distinctive spatial distribution. Even though subdominant, the enhanced density at the center of the galaxy and possibly throughout the plane of the galaxy can lead to large boost factors, and could even explain a signature as large as the 130 GeV Fermi line. Such scenarios also predict additional dark radiation degrees of freedom that could soon be detectable and would influence the interpretation of future data, such as that from Planck and from the Gaia satellite. We consider this to be the first step toward exploring a rich array of new possibilities for dark matter dynamics.

Most investigations of dark matter indicate that it is spread much more tenuously through the universe than ordinary matter, which tends to clump together. The basic idea is illustrated in this artist’s conception of the dark matter halo associated with our Milky Way galaxy and its Magellanic Cloud satellites. (Update: oops, reading comprehension failure on my part. This is an artist’s conception of hot gas around the Milky Way, not dark matter, as Peter Edmonds pointed out on Twitter. But they look similar!)


There is a straightforward explanation for this behavior: ordinary matter feels the electromagnetic interaction, so atoms can bump into each other and release energy by radiating photons, which lets them “cool” and settle down into relatively dense clumps (like galaxies and even stars). Standard dark matter particles have very weak interactions indeed, so when they fall into a gravitational potential well they just zip through the other side without cooling, giving the dark matter distribution a much puffier profile.

Here Fan et al. are suggesting that part of the dark matter could form atoms and cool, allowing it to clump more efficiently in the centers of galaxies. This could lead to more frequent dark-matter annihilations than we would otherwise expect, which might be suggested by some tantalizing observational results (although that’s fairly tentative).

It’s fun to think about, although we’re far away from drawing any firm conclusions at the moment. But we won’t know how to test these ideas observationally unless we work out their predictions theoretically. It’s a complicated universe, we need to be prepared.

  1. Hi, Sean. Always interesting to read a such a digestible article on this subject.

    Does this interesting notion of (some family of) DM particles possibly interacting with each other in a galaxy’s gravity well suggest that the radiation emitted by the black hole at our galactic center might possibly contain some signature of DM?

    For that matter (uggh…!), could the twisted space-time constituting all black holes also be dark matter hangouts? In the sense that there would be an increased probability of DM particles interacting in the restricted space and slow time.

    In my mind, the closest analogy I can draw are two H atoms zipping around at Room Temp. and their mutual interaction (of any detectable kind) made more likely in any constrained space and near-infinite time.

  2. Anywhere the density of dark matter is high, you can get annihilation signatures. Near a black hole is certainly one possibility. (But you wouldn’t expect the density to be extremely high there, since the dark matter would ultimately fall into the hole.)

  3. ” Standard dark matter particles have very weak interactions indeed, so when they fall into a gravitational potential well they just zip through the other side without cooling, giving the dark matter distribution a much puffier profile.”

    Does this mean dark matter is turbulent?

    I truly enjoy elegant word-pictures that permit me to grok a concept without having to wade through the deep theory.

    Got one for the Higgs-weak interaction?

  4. Ah! So the task for experimentalists would be to determine where the DM density might be high.

    Last question (I promise)
    But how do we ‘know’ that annihilation signatures might be stronger if the DM density were higher? We are extrapolating based on high density interactions of regular matter? Isn’t it true that the only observations we’ve made are thus far gravity-related?

    What I mean is, is it worthwhile to speculate that high density of DM might neutralize any such effects – given that we know little of their local interactions – and that low density (long range) interactions might instead be the norm? Like DM affecting gravity at galactic scales. [Emergent phenomena, etc etc]

    I’m certain I missed some part of the DM conversation about expected local interactions; if so, feel free to dismiss this speculation! This is so fascinating.

  5. In this model, does the DDDM still interact with the ordinary dark matter via non-gravitational forces? If it only does via gravity, isn’t we really talking about two dark sectors?

    Still fascinating! Here we have actual evidence of whole sectors existing beyond our familiar world, and here there are people sprouting nonsense about astral planes and all that. Only if they’d just pause and actually look up, they’d realize the actual universe is just as cool if not even better.

    Are DDDM particles still their own anti-matter?

  6. The messiest thing about dark matter is its name, which hardwires explanations. If oceans had been called dark land, there would still be debate on how much dirt was in them.

  7. @ n. & Brett: If you think MOND makes more sense as an overall theory than dark matter does your not aware of the full story. MOND is very good at one specific thing (galaxy rotation), DM covers a broader range of issues &.

  8. Hi Sean,
    happy that I found you, after long research.

    I’m just a researcher that is looking in space and time for the ultimate answer.
    I would love to mention your articles and make a link exchange, if you want.

    Kind Regards,
    The Researcher

  9. Hi Sean,
    happy that I found you, after long research.

    I’m just a researcher that is looking in space and time for the ultimate answer.
    I would love to mention your articles and make a link exchange, if you want.

    Kind Regards,
    The Researcher

  10. @cr2 et al.: nobody seems to realize that these days, “MOND” also means “some more complex form of dark matter” than the usual WIMP, one that is fully empirically motivated because it is built to actually explain galaxy phenomenology (that’s a bottom-up rather than top-down approach). In that sense, it is perfectly sensible to vote for MOND, but only after one realizes that it is also another kind of “more messy dark matter”, but whose messiness can actually explain observations that (at the moment) dont make sense otherwise: there are a few examples of such MOND theories, the most promising one currently being, IMHO, , which passes the CMB and large-scale structure test at linear order with brillo, and might make some contrarian predictions on non-gaussianities which will be interesting to look for.

  11. Sort of off the subject: Sean- The new book is awesome, and very well written, I must say! Great job!

  12. Speaking of messy (dark) matter…why ya gotta be such a cockblock? We (respectfully) demand a teleportation machine. STAT.

  13. I could accept the existence of dark particles only if we live in a zero-dark-particle universe with an equal amount of dark particles and anti-dark particles.

  14. Sean:
    Permit me to offer a reasonably well-defined conjecture as to the nature of dark matter. Dark matter consists of 5 “ensembles” of “stuff,” with each ensemble being quite similar to the “baryonic-matter ensemble” of stuff.
    Indeed, “adequately physics-savvy beings,” (in any of the 6 ensembles) would …
    * Measure most properties of “their” elementary particles to be similar to properties we ascribe to baryonic matter. (Exceptions could include parity and handedness.)
    * Consider that there are 5 ensembles of “dark matter.” (Yes, we could be part of some beings’ “dark matter.”)
    The above conjecture results from a symmetry that I identify while attempting some research. I heard it said that Albert Einstein (while developing special relativity) posed a question somewhat to the effect of ‘what can I learn by imagining travelling along with a light wave?’ I explored quantum mechanics related to the same question. I find a quantum-mechanical representation for photon states. There is a 6-fold symmetry. I generalize (yes, there are more steps than I discuss here) to the notion of ensembles of similar stuff. Each ensemble has a complete set of traditional elementary particles (except “gravitons”) and does not share the corresponding boson-mediated forces with other ensembles. A group of 6 ensembles shares gravity, another type of zero-mass boson (which likely is responsible for the current increase in the rate of expansion of the universe), and some possible spin-2 relatives of the traditional weak-interaction bosons. (The spin-2 relatives may have been responsible for matter/anti-matter imbalance in baryonic matter – and in the other 5 ensembles mentioned above.)
    The research points to yet more symmetry and thereby indicates possibilities for 4 or 8 such groups of 6 ensembles. Presumably, 3 of the “other such groups” provide a candidate explanation for dark-energy density-of-the-universe “stuff.” No 2 such groups of 6 ensembles share gravity or “increase the rate of expansion” bosons. But, they do share some other conjectured bosons.
    For more information, perhaps you will want to consult the video at . It is (minus the first and last slides) a draft of the “Dark Matter Bosons and Fermions” talk I plan to give at the American Physics Society meeting in Denver in April. (I need to change a remark about neutrino masses to indicate the mass shown is one of a few candidates.) The book “Physics Small and Vast” provides more detail about the research.
    I would be happy to make your acquaintance and discuss this further.
    Also, permit me to note that you and I share linkage to Caltech. My wife Helen and I are life members of the Associates. She and I met as graduate students in the physics department at Berkeley. I had earned a B.S. in math from Caltech.

  15. Re: March 12, 2013 at 9:35 pm
    I have a naive question regarding very weakly interacting DM falling into a BH: wouldn’t some DM actuallly manage to go through? Say, if it has an impact parameter somehow smaller than the Schwarzschild radius. Classically I expect energy and momentum conservation to work, but I don’t know if these arguments still apply here.

  16. Dear Sean, are there any theories that describe dark matter not as ‘stuff’ but as ‘bumps’ in space itself, perhaps distributed in the same manner as the ‘bumps’ in the temperature variations of the cosmic background radiation ? If sou, could you provide some pointers ?