General Relativity Seminar

Speaker: Suman Kundu (Weizmann Institute)

Title: ‘Grey Galaxy’ as the endpoint of the Kerr-AdS super radiant blackhole

Abstract: Kerr AdS$_{d+1}$ black holes for $d\geq 3$ suffer from classical superradiant instabilities over a range of masses near extremality. We conjecture that these instabilities settle down into Grey Galaxies (GG)s – a new class of solutions to Einstein’s equations which we construct for $d=3$. Grey Galaxies consist of an $\omega=1$ black hole in the `centre’ of $AdS$, surrounded by a uniformly thick and very large disk of thermal bulk matter that revolves around the centre of AdS at the speed of light. The parametrically low energy density and parametrically large radius of the gas disk are inversely related; as a consequence, the gas carries a finite fraction of the total energy. Grey Galaxy saddles exist at masses that extend all the way down to the unitarity bound. Their thermodynamics is that of a weakly interacting mix of Kerr AdS black holes and the gas. In addition to a smooth piece, the boundary stress tensor of these solutions includes a contribution from a delta function localized at the `equator’ of the boundary sphere, a term which may be used as an order parameter that sharply distinguishes GG solutions from ordinary Kerr-Black hole saddles. We also construct `Revolving Black Hole (RBH) saddles’,  macroscopically charged $SO(d,2)$ descendants of AdS-Kerr solutions, that describe black holes revolving around the centre of $AdS$, at the fixed radial location but in a quantum wave function in the angular directions. RBH saddles turn out to be (marginally) entropically subdominant to GG saddles. We argue that supersymmetric versions of RBH saddles exist and have interesting consequences for the spectrum of SUSY states in, e.g.  ${\cal N}=4$ Yang-Mills theory.