Running Love Numbers of Charged Black Holes

TL;DR

The paper analyzes how quantum vacuum fluctuations induce tidal deformations on four-dimensional, unspinning charged black holes, introducing Love matrices to capture coupled gravitational and electromagnetic responses. It extends EFT-based treatments to both large black holes (EFT regime) and small magnetic black holes (strong-field regime), deriving coupled equations for metric and electromagnetic fluctuations in electric and magnetic backgrounds and computing the full Love matrices and their beta functions. A notable finding is a $Z_2$ symmetry relating electric and magnetic backgrounds despite electromagnetic duality breaking at the quantum level; in the strong-field regime, Love matrices run with the $U(1)$ beta function, while in the weak-field regime only a finite set of operators contribute for a given harmonic. The results establish a framework for using gravitational-wave observations to probe dark-sector physics via near-extremal magnetic black holes and provide a Green-function-based method to extract Love numbers and running coefficients from worldline EFT matching.

Abstract

Loops of virtual particles from the vacuum of quantum field theory (QFT) render black holes tidally deformable. We compute the static tidal response of unspinning charged black holes at arbitrary radius, using the perturbative formalism developed in 2501.18684. Since the gravitational and electromagnetic tidal responses mix, we generalize the notion of Love numbers to Love matrices. We derive the coupled equations of motion for the metric and electromagnetic fluctuations around purely electric and magnetic backgrounds. For large charged black holes, which are described by the Effective Field Theory (EFT) of gravity, we compute the full set of Love matrices induced by an arbitrary tower of $F^{2n}$ operators. We find that, although quantum corrections break electromagnetic duality, the Love matrices in electric and magnetic backgrounds are related by a $Z_2$ symmetry under electric-magnetic exchange. Going beyond EFT, we compute the Love matrices of small magnetic black holes. We show that the running of the Love matrices is governed by the running of the $U(1)$ gauge coupling, and we derive the correspondence between Love and $U(1)$ beta functions for arbitrary harmonics. The overall picture that emerges is that the QFT-induced tidal response of magnetic black holes saturates in the strong-field regime. These results imply that nearly-extremal magnetic black holes charged under an Abelian dark sector could be probed by gravitational-wave observations.