Spontaneous ghostification: how a dying black hole comes back as a naked singularity

TL;DR

This paper explores how quantum effects in high-energy gravity can cause a black hole to evolve into a stable naked singularity, challenging traditional views on black hole endpoints and cosmic censorship.

Contribution

It proposes that ghost instabilities at the end of black hole evaporation can lead to a natural transition into a stable naked singularity within modified gravity theories.

Findings

Quantum ghost destabilizes Schwarzschild black hole

Final stages of evaporation trigger phase transition

Resulting naked singularity is stable and avoids pathologies

Abstract

A quantum ghost that destabilizes the Schwarzschild solution, transforming it into a naked singularity, may seem like a physicist's worst nightmare. However, we argue that this scenario represents the natural evolution of a black hole within a conservative high-energy gravity framework and may, in fact, be a desirable outcome. Quadratic curvature terms typically appear as corrections to the Einstein-Hilbert action at high energies; nonetheless, such theories are generally considered incomplete due to the presence of ghost particles at the quantum level, which can spoil vacuum stability. We argue that this instability can only be triggered at the final stages of black hole evaporation, starting a phase transition-like process that alters the nature of the spacetime, similarly to spontaneous scalarization. We propose that the endpoint is a stable, exotic naked singularity, possible only in modified gravity theories, and avoids some of the pathological features associated with standard naked singularities.