Dynamical Love numbers of black holes: Theory and gravitational waveforms

TL;DR

This paper investigates the dynamical tidal Love numbers of black holes, revealing a finite conservative correction at second order in frequency that suggests a potential dynamical deformation of the black hole geometry, with minimal observable impact on gravitational waves.

Contribution

It introduces a response function for dynamical tidal perturbations of black holes that is gauge-invariant and computes the conservative correction at quadratic order in frequency.

Findings

Finite, nonvanishing conservative correction at second order in frequency.

Dynamical tidal effects enter gravitational wave phase at eighth post-Newtonian order.

Corrections are too small to be observed with future gravitational wave detectors.

Abstract

In General Relativity, the static tidal Love numbers of black holes vanish identically. Whether this remains true for time-dependent tidal fields -- i.e., in the case of dynamical tidal Love numbers -- is an open question, complicated by subtle issues in the definition and computation of the tidal response at finite frequency. In this work, we analyze the dynamical tidal perturbations of a Schwarzschild black hole to quadratic order in the tidal frequency. By employing the Teukolsky formalism in advanced null coordinates, which are regular at the horizon, we obtain a particularly clean perturbative scheme. Furthermore, we introduce a response function based on the full solution of the perturbation equation which does not depend on any arbitrary constant. Our analysis recovers known results for the dissipative response at linear order and the logarithmic running at quadratic order, associated with scale dependence in the effective theory. In addition, we find a finite, nonvanishing conservative correction at second order in frequency, thereby possibly demonstrating a genuine dynamical deformation of the black hole geometry. Although removing any ambiguity in the dynamical tidal response would require a matching with some gauge-invariant coefficient, we assess the impact of these effects on the gravitational-wave phase. These contributions enter at eighth post-Newtonian order, and can be expressed in terms of generic $\mathcal{O}(1)$ coefficients, which have to be matched to the perturbative result. Regardless of the matching ambiguities, we argue that such corrections are too small to be observable even with future-generation gravitational wave detectors. Moreover, the corresponding phase shifts are degenerate with unknown point-particle contributions entering at the same post-Newtonian order.