Greybody factors in scalar-tensor gravity and beyond

TL;DR

This paper investigates how scalar-tensor theories beyond Horndeski modify black hole properties, analyzing scalar, vector, and gravitational perturbations to reveal distinct deviations from general relativity in greybody factors and quasinormal modes.

Contribution

It provides a comprehensive analysis of black hole perturbations in beyond Horndeski theories, highlighting novel modifications in greybody factors and quasinormal modes compared to GR.

Findings

Modified greybody factors and quasinormal frequencies in beyond Horndeski theories.

Distinct signatures in scalar and vector field propagation around black holes.

Altered damping times and oscillation frequencies linked to scalar hair parameters.

Abstract

In the framework of the beyond Horndeski action, we consider three subtheories that support scalarised black-hole solutions, and look for modified characteristics compared to GR. We first study the propagation of massless scalar and vector test fields in the fixed background of an analytical spherically symmetric black hole derived in the context of a parity-symmetric beyond Horndeski theory, and show that the profiles of the effective gravitational potentials, greybody factors, absorption cross sections and quasinormal frequencies exhibit distinct modifications as we move away from the GR limit. We then turn our attention to the perturbations of the gravitational field itself and adopt a full-theory analysis that takes into account the backreaction of the scalar field on the metric. Employing as background solutions scalarized black holes arising in the shift-symmetric Horndeski theory and in the quadratic-quartic-scalar-Gauss-Bonnet theory, we compute the greybody factors and quasinormal modes of the axial sector. In both theories, in direct correspondence to the form of the gravitational potential, which features multiple extremal points, we find modified (suppressed or nonmonotonic) greybody curves and altered quasinormal frequencies (smaller oscillating frequencies and larger damping times), especially as the hair-sourcing parameter increases.