On Toroidal Horizons in Binary Black Hole Inspirals

TL;DR

This study investigates the topology of event horizons in binary black hole mergers, revealing that the observed spherical shape results from the specific time slicing used in simulations, not an inherent property of the merger.

Contribution

The paper demonstrates that apparent toroidal horizons in binary black hole simulations are artifacts of time slicing choices, emphasizing the importance of slicing in horizon topology analysis.

Findings

Event horizon cross section appears spherical in simulations.

Time slicing influences the observed topology of the event horizon.

No toroidal horizons observed under the specific slicing used.

Abstract

We examine the structure of the event horizon for numerical simulations of two black holes that begin in a quasicircular orbit, inspiral, and finally merge. We find that the spatial cross section of the merged event horizon has spherical topology (to the limit of our resolution), despite the expectation that generic binary black hole mergers in the absence of symmetries should result in an event horizon that briefly has a toroidal cross section. Using insight gained from our numerical simulations, we investigate how the choice of time slicing affects both the spatial cross section of the event horizon and the locus of points at which generators of the event horizon cross. To ensure the robustness of our conclusions, our results are checked at multiple numerical resolutions. 3D visualization data for these resolutions are available for public access online. We find that the structure of the horizon generators in our simulations is consistent with expectations, and the lack of toroidal horizons in our simulations is due to our choice of time slicing.