Backreaction of the ergoregion instability in rotating compact objects

Determine the nonlinear gravitational backreaction of the ergoregion instability on the spacetime of a rotating compact object, specifying how the growth of negative-Killing-energy modes trapped in the ergoregion modifies the object's geometry and dynamics.

Background

Ergoregions in sufficiently compact rotating spacetimes allow modes with negative Killing energy to be trapped, leading to the linear ergoregion instability in horizonless objects. While linear growth rates have been analyzed in various models, the nonlinear development and ultimate endstate of this instability have remained largely unexplored.

This paper studies a specific case—a rapidly spinning boson star perturbed by a massless vector field—using fully nonlinear numerical relativity to follow the instability’s evolution. Although concrete findings are presented for this model (including enhanced growth, burst-like emissions, turbulent cascades, and eventual black hole formation), the broader question of how the instability backreacts on generic rotating compact spacetimes motivates and frames the investigation.