Binary black hole coalescence in the large-mass-ratio limit: the hyperboloidal layer method and waveforms at null infinity

TL;DR

This paper introduces the hyperboloidal layer method for computing gravitational waveforms at null infinity from large-mass-ratio black hole binaries, revealing phase differences and improving waveform extrapolation reliability.

Contribution

It presents the first application of the hyperboloidal layer method to black hole binary waveforms, enabling direct extraction at null infinity and comparison with finite-radius results.

Findings

Significant phase differences between finite-radius and null-infinity waveforms.

Validation of the extrapolation procedure used in numerical relativity.

Excellent agreement between EOB angular momentum loss and flux at null infinity.

Abstract

We compute and analyze the gravitational waveform emitted to future null infinity by a system of two black holes in the large mass ratio limit. We consider the transition from the quasi-adiabatic inspiral to plunge, merger, and ringdown. The relative dynamics is driven by a leading order in the mass ratio, 5PN-resummed, effective-one-body (EOB), analytic radiation reaction. To compute the waveforms we solve the Regge-Wheeler-Zerilli equations in the time-domain on a spacelike foliation which coincides with the standard Schwarzschild foliation in the region including the motion of the small black hole, and is globally hyperboloidal, allowing us to include future null infinity in the computational domain by compactification. This method is called the hyperboloidal layer method, and is discussed here for the first time in a study of the gravitational radiation emitted by black hole binaries. We consider binaries characterized by five mass ratios, $\nu=10^{-2,-3,-4,-5,-6}$, that are primary targets of space-based or third-generation gravitational wave detectors. We show significative phase differences between finite-radius and null-infinity waveforms. We test, in our context, the reliability of the extrapolation procedure routinely applied to numerical relativity waveforms. We present an updated calculation of the gravitational recoil imparted to the merger remnant by the gravitational wave emission. As a self consistency test of the method, we show an excellent fractional agreement (even during the plunge) between the 5PN EOB-resummed mechanical angular momentum loss and the gravitational wave angular momentum flux computed at null infinity. New results concerning the radiation emitted from unstable circular orbits are also presented.