A conservation-based method for simulating the inspiral of binary black holes

TL;DR

This paper introduces a conservation-based method to simulate the inspiral of binary black holes, effectively bridging scattering experiments and N-body simulations, and reveals eccentricity growth in unequal mass binaries.

Contribution

The paper presents a novel, efficient approach that conserves energy and angular momentum to model black hole binary evolution across different scales.

Findings

Equal mass binaries tend to stall at circular orbits.

Unequal mass binaries exhibit runaway eccentricity growth.

Eccentricity growth accelerates black hole coalescence, often within a Hubble time.

Abstract

We present a new approach to studying the evolution of massive black hole binaries in a stellar environment. By imposing conservation of total energy and angular momentum in scattering experiments, we find the dissipation forces that are exerted on the black holes by the stars, and thus obtain the decaying path of the binary from the classical dynamical friction regime down to subparsec scales. Our scheme lies between scattering experiments and N-body simulations. While still resolving collisions between stars and black holes, it is fast enough and allows to use a large enough number of particles to reach a smooth and convergent result. We studied both an equal mass and a 10:1 mass ratio binaries under various initial conditions. We show that while an equal mass binary stalls at a nearly circular orbit, a runaway growth of eccentricity occurs in the unequal mass case. This effect reduces the timescale for black hole coalescence through gravitational radiation to well below the Hubble time, even in spherical and gasless systems formed by dry mergers.