Description
Several modern theories hypothesize that dark matter condenses to a superfluid phase around galaxies. If true, one key distinction from particle dark matter is 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 describe the steady-state dynamical friction of a perturber moving through a superfluid condensate. Crucially, I will account for the tachyonic gravitational mass of sound waves (a consequence of the Jeans instability of the fluid cloud), as well as the “quantum pressure” of the gas. I will 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. Although subsonic perturbers are ordinarily unable to experience a drag force in a superfluid, surprisingly we will find that the Jeans instability modifies the dispersion relation enough to result in small, but non-vanishing, subsonic dynamical friction. This effect may be a key observable in distinguishing between particle and fluid models of dark matter.
