Fully extremal black holes: a black hole graveyard?

TL;DR

This paper explores the possibility that extremal black holes, which are stable and have multiple horizons, could serve as the final state of black hole evolution, challenging the traditional view of complete evaporation.

Contribution

It introduces a new class of solutions with multiple extremal horizons, suggesting these could be the ultimate end-point of black hole evolution.

Findings

Extremal black holes may be stable endpoints of gravitational collapse.

Multiple extremal horizons can exist in spherical and axisymmetric solutions.

These configurations could represent the black hole graveyard, with long evolution timescales.

Abstract

While the standard point of view is that the ultimate endpoint of black hole evolution is determined by Hawking evaporation, there is a growing evidence that classical and semi-classical instabilities affect both black holes with inner horizons as well as their ultra-compact counterparts. In this essay we start from this evidence pointing towards extremal black holes as stable endpoints of the gravitational collapse, and develop a general class of spherical and axisymmetric solutions with multiple extremal horizons. Excluding more exotic possibilities, entailing regular cores supporting wormhole throats, we argue that these configuration could be the asymptotic graveyard, the end-point, of dynamical black hole evolution -- albeit the timescale of such evolution are still unclear and possibly long and compatible with current astrophysical observations.