Understanding the "anti-kick" in the merger of binary black holes

TL;DR

This paper explains the 'antikick' phenomenon in binary black hole mergers by analyzing horizon curvature and radiation in Robinson-Trautman spacetimes, enhancing understanding of black hole recoil dynamics.

Contribution

It introduces a gauge-invariant method to analyze the antikick using Robinson-Trautman spacetimes, linking horizon curvature to recoil velocity.

Findings

Antikick correlates with anisotropic horizon curvature.

Radiation analysis in Robinson-Trautman spacetimes reveals recoil dynamics.

Provides a simplified model for black hole merger recoil understanding.

Abstract

The generation of a large recoil velocity from the inspiral and merger of binary black holes represents one of the most exciting results of numerical-relativity calculations. While many aspects of this process have been investigated and explained, the "antikick", namely the sudden deceleration after the merger, has not yet found a simple explanation. We show that the antikick can be understood in terms of the radiation from a deformed black hole where the anisotropic curvature distribution on the horizon correlates with the direction and intensity of the recoil. Our analysis is focussed on Robinson-Trautman spacetimes and allows us to measure both the energies and momenta radiated in a gauge-invariant manner. At the same time, this simpler setup provides the qualitative and quantitative features of merging black holes, opening the way to a deeper understanding of the nonlinear dynamics of black-hole spacetimes.