Toroidal Horizons in Binary Black Hole Mergers

TL;DR

This paper reports the first observation of a toroidal event horizon in binary black hole mergers, achieved through a novel coordinate transformation that reveals a transient toroidal topology during the merger process.

Contribution

The authors introduce a coordinate transformation that uncovers a toroidal event horizon in binary black hole simulations, confirming a long-standing theoretical prediction.

Findings

First binary black hole event horizon with toroidal topology

Coordinate transformation reveals transient toroidal phase

Toroidal horizons satisfy topological censorship by construction

Abstract

We find the first binary black hole event horizon with a toroidal topology. It had been predicted that generically the event horizons of merging black holes should briefly have a toroidal topology, but such a phase has never been seen prior to this work. In all previous binary black hole simulations, in the coordinate slicing used to evolve the black holes, the topology of the event horizon transitions directly from two spheres during the inspiral to a single sphere as the black holes merge. We present a coordinate transformation to a foliation of spacelike hypersurfaces that "cut a hole" through the event horizon surface, resulting in a toroidal event horizon. A torus could potentially provide a mechanism for violating topological censorship. However, these toroidal event horizons satisfy topological censorship by construction, because we can always trivially apply the inverse coordinate transformation to remove the topological feature.