Gravitational collapse and formation of universal horizons in Einstein-aether theory

TL;DR

This paper numerically demonstrates the formation of universal horizons during gravitational collapse in Einstein-aether theory, showing their stability and evolution into causal boundaries even for high-speed excitations.

Contribution

It provides the first numerical evidence of universal horizons forming in Einstein-aether theory during scalar field collapse, including their dynamic behavior and stability.

Findings

Universal horizons can form during gravitational collapse in Einstein-aether theory.

The spacetime outside horizons becomes static and well-behaved.

The outermost dynamical universal horizon evolves into the causal boundary.

Abstract

We numerically study the gravitational collapse of a massless scalar field with spherical symmetry in Einstein-aether theory, and show that apparent, spin-0 and dynamical universal horizons (dUHs) can be all formed. The spacetime and the aether field are well-behaved and regular, including regions nearby these horizons (but away from the center of spherical symmetry). The spacetime outside the apparent and spin-0 horizons settles down to a static configuration, and some of such resulting static black holes were already found numerically in the literature. On the other hand, the proper distance of the outermost dUH from the apparent (or spin-0) horizon keeps increasing on $\mbox{\ae}$ther-orthogonal time slices. This indicates that the outermost dUH is evolving into the causal boundary, even for excitations with large speeds of propagation.