Black Hole Spin Axis in Numerical Relativity

TL;DR

This paper reviews and compares different methods for defining the spin axis of black holes in numerical relativity, proposing improved measures that align better with theoretical expectations.

Contribution

It analyzes existing spin axis definitions in numerical relativity, identifies discrepancies, and introduces new measures that improve consistency with post-Newtonian theory.

Findings

Existing measures show qualitative differences from post-Newtonian expectations.

New measures provide better agreement with theoretical predictions.

Comparison of spin axis definitions enhances understanding of black hole dynamics.

Abstract

Colliding black holes are systems of profound interest in both gravitational wave astronomy and in gravitation theory, and a variety of methods have been developed for modeling their dynamics in detail. The features of these dynamics are determined by the masses of the holes and by the magnitudes and axes of their spins. While masses and spin magnitudes can be defined in reasonably unambiguous ways, the spin axis is a concept which despite great physical importance is seriously undermined by the coordinate freedom of general relativity. Despite a great wealth of detailed numerical simulations of generic spinning black hole collisions, very little attention has gone into defining or justifying the definitions of the spin axis used in the numerical relativity literature. In this paper, we summarize and contrast the various spin direction measures available in the SpEC code, including a comparison with a method common in other codes, we explain why these measures have shown qualitatively different nutation features than one would expect from post-Newtonian theory, and we derive and implement new measures that give much better agreement.