First order hyperbolic formalism for Numerical Relativity

TL;DR

This paper presents a first order hyperbolic formulation of Einstein's evolution equations, enabling more accurate and stable numerical simulations in general relativity by leveraging the system's causal structure and constraint equations.

Contribution

It introduces a hyperbolic first order system for Einstein's equations applicable to various slicings, improving numerical stability and boundary condition handling in numerical relativity.

Findings

The system is hyperbolic for many common slicings.

Elimination of certain terms reduces numerical inaccuracies.

The formulation facilitates advanced numerical methods and better boundary treatments.

Abstract

The causal structure of Einstein's evolution equations is considered. We show that in general they can be written as a first order system of balance laws for any choice of slicing or shift. We also show how certain terms in the evolution equations, that can lead to numerical inaccuracies, can be eliminated by using the Hamiltonian constraint. Furthermore, we show that the entire system is hyperbolic when the time coordinate is chosen in an invariant algebraic way, and for any fixed choice of the shift. This is achieved by using the momentum constraints in such as way that no additional space or time derivatives of the equations need to be computed. The slicings that allow hyperbolicity in this formulation belong to a large class, including harmonic, maximal, and many others that have been commonly used in numerical relativity. We provide details of some of the advanced numerical methods that this formulation of the equations allows, and we also discuss certain advantages that a hyperbolic formulation provides when treating boundary conditions.