Spinning black holes as cosmic string factories

Abstract

We consider the evolution of a cosmic string loop that is captured by a much more massive and compact black hole. We show that after several reconnections that produce ejections of smaller loops, the loop that remains bound to the black hole moves on a nearly-periodic non-self-intersecting trajectory, "the orbit". The orbit evolves due to an energy and angular momentum exchange between the loop and the spinning black hole. We show that such evolution is mathematically equivalent to a certain continuous deformation of an auxiliary closed curve in a 3-dimensional space; for zero black-hole spin this deformation is curve-shortening that has been extensively studied by mathematicians. The evolution features competing effects of loop growth by the superradiant extraction of the black-hole spin energy, and loop decay by the friction of the moving string against the horizon. A self-intersection of an auxiliary curve corresponds to a capture by the black hole of a new string segment and thus an addition of a new captured loop. Possible asymptotic states of such evolution are shown to be strong emitters of gravitational waves. Whether reconnections prevent reaching the asymptotic states remains to be explored. Additionally, the orbit's shape also evolves due to an emission of gravitational waves, and a recoil of the black hole that changes the orbit and likely leads to self-intersections. We argue that for a significant range of the dimensionless tension $\mu$, string loops are captured by supermassive black holes at the centers of galaxies. This strongly motivates further study of interaction between string loops and black holes, especially the influence of this process on the black hole spindown and on the production of gravitational waves by strings created in galactic nuclei. We also discuss potential loop captures by primordial black holes.