Short-scale gravitational instability in a disordered Bose gas

We numerically study the evolution of a self-gravitating nonequilibrium Bose gas contained in a fixed volume. We find that, even when the volume is small enough to prevent collisionless instability, a compact phase-correlated object (a Bose star) can form, by gravity alone, from a disordered initial state. We interpret this effect and the associated growth of the density contrast as consequences of a new type of gravitational instability---a nonlinear instability due to a stimulated gravitational scattering of the bosons. Our results imply that the formation of Bose stars, in regions that have fallen out of Hubble expansion, may be a quite general phenomenon, not requiring a large preexisting correlation length or any short-range interactions.