# Black hole collisions, instabilities, and cosmic censorship violation at large D

## TL;DR

This paper investigates black hole collisions and instabilities in higher dimensions, providing evidence that such processes can violate cosmic censorship, and explores the evolution of rotating black bars and ultraspinning black holes.

## Contribution

It introduces an effective theory for large D black hole dynamics, demonstrating potential cosmic censorship violations and analyzing the evolution of black bars and ultraspinning black holes.

## Key findings

- Black hole collisions can lead to naked singularities via black bar instabilities.
- Ultraspinning black holes develop transient black rings and multi-pronged horizons.
- Spin-down via gravitational radiation is inefficient at large D, allowing instabilities to develop.

## Abstract

We study the evolution of black hole collisions and ultraspinning black hole instabilities in higher dimensions. These processes can be efficiently solved numerically in an effective theory in the limit of large number of dimensions D. We present evidence that they lead to violations of cosmic censorship. The post-merger evolution of the collision of two black holes with total angular momentum above a certain value is governed by the properties of a resonance-like intermediate state: a long-lived, rotating black bar, which pinches off towards a naked singularity due to an instability akin to that of black strings. We compute the radiative loss of spin for a rotating bar using the quadrupole formula at finite D, and argue that at large enough D ---very likely for $D\gtrsim 8$, but possibly down to D=6--- the spin-down is too inefficient to quench this instability. We also study the instabilities of ultraspinning black holes by solving numerically the time evolution of axisymmetric and non-axisymmetric perturbations. We demonstrate the development of transient black rings in the former case, and of multi-pronged horizons in the latter, which then proceed to pinch and, arguably, fragment into smaller black holes.