Non-linear tides and Gauss-Bonnet scalarization

TL;DR

This paper investigates the effects of non-linear gravitational tides on black holes within full General Relativity, revealing that static tidal interactions cannot induce Gauss-Bonnet scalarization and highlighting qualitative differences from linear theory.

Contribution

The study provides exact solutions for black holes in multipolar gravitational fields, demonstrating non-linear tide effects and their impact on Gauss-Bonnet curvature, which differ from linear perturbation predictions.

Findings

High/low tides can deviate from Newtonian patterns.

Gauss-Bonnet invariant remains non-negative in low tide regions.

Static, strong tides do not induce GB$^-$ scalarization.

Abstract

In linear perturbation theory, a static perturber in the vicinity of a Schwarzschild black hole (BH) enhances [suppresses] the Gauss-Bonnet (GB) curvature invariant, $\mathcal{R}_{\rm GB}$, in the high [low] tide regions. By analysing exact solutions of the vacuum Einstein field equations describing one or two BHs immersed in a multipolar gravitational field, which is locally free of pathologies, including conical singularities, we study the corresponding non-linear tides on a fiducial BH, in full General Relativity (GR). We show that the tidal field due to a far away, or close by, static BH creates high/low tides that can deviate not only quantitatively but also qualitatively from the weak field/Newtonian pattern. Remarkably, the suppression in low tide regions never makes $\mathcal{R}_{\rm GB}$ negative on the BH, even though the horizon Gaussian curvature may become negative; but $\mathcal{R}_{\rm GB}$ can vanish in a measure zero set, a feature qualitatively recovered in a Newtonian analogue model. Thus, purely gravitational, static, tidal interactions in GR, no matter how strong, cannot induce GB$^-$ scalarization. We also show that a close by BH produces noticeable asymmetric tides on another (fiducial) BH.