Numerical Evidence for Non-Axisymmetric Gravitational “Hair” for Extremal Kerr Black Hole Spacetimes with Hyperboloidal Foliations

Abstract

Various generalizations of the scalar, axisymmetric “horizon hair” for extremal black holes have recently appeared in the literature. In this paper, we propose an expression for a non-axisymmetric gravitational “charge” and its potentially observable imprint at a finite distance from the horizon (Ori-coefficient) in extremal Kerr black hole backgrounds. Using a hyperboloidal foliation, we offer strong and robust numerical evidence for the potential existence of this horizon hair and its properties. Specifically, we consider the time evolution of horizon penetrating, quadrupolar and (subdominant) octupolar gravitational perturbations with compact support on extremal Kerr (EK) spacetime. We do this by numerically solving the Teukolsky equation and determining the conserved charge values on the horizon and at a finite distance from the black hole.