Short-scale gravitational instability in a disordered Bose gas

We study numerically 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 stimulated gravitational scattering of the bosons. Our results imply that formation of Bose stars, in regions that have fallen out of the Hubble expansion, may be a quite general phenomenon, not requiring a large preexisting correlation length or any short-range interactions.