Dynamical friction in a superfluid
Several modern theories hypothesize that dark matter behaves as a superfluid. If true, one key distinction from particle dark matter would be in its dynamical friction, a process by which a massive object 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 small but heavy object 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. I will show this in two equivalent ways: (i) via a familiar procedure in which one linearizes the fluid equations, and (ii) via a novel quasiparticle description of phonon radiation. Surprisingly, we will find that including the Jeans instability can result in non-zero subsonic dynamical friction.