Dynamical friction in a superfluid

Several modern theories hypothesize that dark matter is some sort of superfluid. If true, one key distinction from particle dark matter would be in its dynamical friction, a process by which a massive perturber moving through a cloud of matter is slowed by the gravitational attraction to its own wake. I will discuss a general approach to calculating the steady-state dynamical friction of a perturber moving through a superfluid condensate. This approach incorporates the effect of the Jeans instability of the gas cloud, as well as the “quantum pressure” of the gas. We show this in two equivalent ways: (i) via a familiar approach in which one linearizes the fluid equations, and (ii) via a novel quasiparticle description of phonon radiation. Surprisingly, we find that including the Jeans instability can result in non-zero subsonic dynamical friction.