Nonlinear evolution of the ergoregion instability: Turbulence, bursts of radiation, and black hole formation

TL;DR

This paper studies the nonlinear evolution of ergoregion instability in rotating boson stars, revealing turbulence, bursts of gravitational radiation, and eventual black hole formation through numerical simulations.

Contribution

It provides the first detailed numerical analysis of the nonlinear dynamics and gravitational wave signatures of ergoregion instability leading to black hole formation.

Findings

Nonlinear evolution causes the star to become more gravitationally bound.

Turbulent cascade develops in the unstable mode and emitted radiation.

Gravitational wave signals show bursts with increasing amplitude before black hole formation.

Abstract

Spacetimes with an ergoregion that is not connected to a horizon are linearly unstable. While the linear regime has been studied in a number of settings, little is known about the nonlinear evolution of this ergoregion instability. Here, we investigate this by numerically evolving the unstable growth of a massless vector field in a rapidly spinning boson star in full general relativity. We find that the backreaction of the instability causes the star to become more gravitationally bound, accelerating the growth, and eventually leading to black hole formation. During the nonlinear growth phase, small scale features develop in the unstable mode and emitted radiation as nonlinear gravitational interactions mediate a direct turbulent cascade. The gravitational wave signal exhibits bursts, akin to so-called gravitational wave echoes, with increasing amplitude towards black hole formation.