Hawking radiation as the cosmic censor

TL;DR

This paper demonstrates that Hawking radiation prevents extremal black holes from forming and overspinning smaller black holes, acting as a cosmic censor, but it may not prevent overspinning of larger black holes.

Contribution

It provides an analysis showing Hawking radiation's role in preventing extremality and overspinning, with estimates on mass limits where evaporation is effective.

Findings

Hawking radiation pushes black holes away from extremality.

Evaporation can prevent overspinning for black holes with mass up to 10^{17}-10^{18} g.

Overspinning of larger black holes remains possible despite evaporation.

Abstract

Hawking radiation acts as a cosmic censor since it carries away the angular momentum of the black hole, proportionally more than its mass. In this work we first show that an extremal black hole cannot exist since it will be pushed away from extremality by its own Hawking radiation, without being perturbed by any external effect. We evaluate the efficiency of Hawking radiation to prevent overspinning of black holes. We make an order of magnitude estimate to show that evaporation can prevent overspinning of black holes with an upper limit of mass $M\lesssim 10^{17}-10^{18}\rm{g}$, when we take the interaction period to be the age of the universe. Overspinning of black holes of higher masses by test fields remains possible, even if evaporation is taken into account. We also discuss the possibility to attribute a shorter interaction period for the problem which would reduce the effect of evaporation.