Rotating black holes and black bars at large D

Abstract

We propose and demonstrate a new and efficient approach to investigate black hole dynamics in the limit of large number of dimensions $D$. The basic idea is that an asymptotically flat black brane evolving under the Gregory-Laflamme instability forms lumps that closely resemble a localized black hole. In this manner, the large-$D$ effective equations for extended black branes can be used to study localized black holes. We show that these equations have exact solutions for black-hole-like lumps on the brane, which correctly capture the main properties of Schwarzschild and Myers-Perry black holes at large $D$, including their slow quasinormal modes and the ultraspinning instabilities (axisymmetric or not) at large angular momenta. Furthermore, we obtain a novel class of rotating `black bar' solutions, which are stationary when $D\to\infty$, and are long-lived when $D$ is finite but large, since their gravitational wave emission is strongly suppressed. The leading large $D$ approximation reproduces to per-cent level accuracy previous numerical calculations of the bar-mode growth rate in $D=6,7$.