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

TL;DR

This paper provides numerical evidence for a non-axisymmetric gravitational 'hair' in extremal Kerr black holes, suggesting new observable features related to horizon perturbations using hyperboloidal foliations.

Contribution

It introduces a new gravitational 'charge' expression for extremal Kerr black holes and demonstrates its potential observability through numerical simulations of perturbations.

Findings

Numerical evidence supports the existence of non-axisymmetric gravitational 'hair'.

The conserved charge can be determined at the horizon and finite distances.

Hyperboloidal foliation effectively captures horizon perturbations.

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.