(H)airborne: two spinning black holes balanced by their synchronised scalar hair

TL;DR

This paper demonstrates the existence of asymptotically flat, non-singular solutions in General Relativity coupled with a massive scalar field, describing two spinning black holes in equilibrium thanks to scalar hair, extending the understanding of black hole configurations.

Contribution

It introduces novel equilibrium solutions of two spinning black holes with scalar hair, balancing mutual attraction through a scalar dipolar field, which was not previously known.

Findings

Two black holes in equilibrium with scalar hair are possible.

Solutions bifurcate from dipolar spinning boson stars.

Equilibrium is maintained by the scalar environment acting as a dipolar field.

Abstract

General Relativity minimally coupled to a massive, free, complex scalar field, is shown to allow asymptotically flat solutions, non-singular on and outside the event horizon, describing two spinning black holes (2sBHs) in equilibrium, with co-axial, aligned angular momenta. The 2sBHs configurations bifurcate from solutions describing dipolar spinning boson stars. The BHs emerge at equilibrium points diagnosed by a test particle analysis and illustrated by a Newtonian analogue. The individual BH "charges" are mass and angular momentum only. Equilibrium is due to the scalar environment, acting as a (compact) dipolar field, providing a lift against their mutual attraction, making the 2sBHs (h)airborne. We explore the 2sBHs domain of solutions and its main features.