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.