A new spin on black hole hair

TL;DR

This paper demonstrates that rotating black holes can support scalar hair, linking the existence of hairy black holes to superradiant instabilities and expanding understanding of black hole solutions in general relativity.

Contribution

It introduces a new class of rotating black holes with scalar hair, showing their relation to superradiant instabilities and clarifying conditions for scalar hair formation.

Findings

Hairy black holes exist for rotating, vacuum black holes.

Scalar hair is connected to superradiant instabilities.

Hairy solutions only occur if the black hole is spinning.

Abstract

We show that scalar hair can be added to rotating, vacuum black holes of general relativity. These hairy black holes (HBHs) clarify a lingering question concerning gravitational solitons: if a black hole can be added at the centre of a boson star, as it typically can for other solitons. We argue that it can, but only if it is spinning. The existence of such HBHs is related to the Kerr superradiant instability triggered by a massive scalar field. This connection leads to the following conjecture: a (hairless) black hole which is afflicted by the superradiant instability of a given field must allow hairy generalizations with that field.