Numerical simulations of single and binary black holes in scalar-tensor theories: circumventing the no-hair theorem

TL;DR

This paper numerically investigates black hole dynamics in scalar-tensor theories, revealing scalar radiation emission and field relaxation, with implications for alternative gravity models and gravitational wave observations.

Contribution

It provides the first detailed numerical analysis of black hole and binary dynamics in scalar-tensor theories with scalar gradients, comparing results to analytical models.

Findings

Scalar fields relax to static configurations around black holes.

Accelerated black holes emit scalar radiation in a scalar-field gradient.

Binary black holes emit scalar radiation at twice their orbital frequency.

Abstract

Scalar-tensor theories are a compelling alternative to general relativity and one of the most accepted extensions of Einstein's theory. Black holes in these theories have no hair, but could grow "wigs" supported by time-dependent boundary conditions or spatial gradients. Time-dependent or spatially varying fields lead in general to nontrivial black hole dynamics, with potentially interesting experimental consequences. We carry out a numerical investigation of the dynamics of single and binary black holes in the presence of scalar fields. In particular we study gravitational and scalar radiation from black-hole binaries in a constant scalar-field gradient, and we compare our numerical findings to analytical models. In the single black hole case we find that, after a short transient, the scalar field relaxes to static configurations, in agreement with perturbative calculations. Furthermore we predict analytically (and verify numerically) that accelerated black holes in a scalar-field gradient emit scalar radiation. For a quasicircular black-hole binary, our analytical and numerical calculations show that the dominant component of the scalar radiation is emitted at twice the binary's orbital frequency.