Hyperbolic formulations and numerical relativity II: Asymptotically constrained systems of the Einstein equations

TL;DR

This paper explores hyperbolic formulations of Einstein's equations for numerical relativity, introducing the $\

Contribution

It proposes a new 'adjusted system' mechanism that enhances stability by adding constraint terms with optimized multipliers, improving robustness in simulations.

Findings

The $\

The adjusted system reduces numerical errors by controlling eigenvalues of constraint propagation.

The methods are validated in Maxwell and Ashtekar's formulations, demonstrating improved stability.

Abstract

We study asymptotically constrained systems for numerical integration of the Einstein equations, which are intended to be robust against perturbative errors for the free evolution of the initial data. First, we examine the previously proposed "$\lambda$-system", which introduces artificial flows to constraint surfaces based on the symmetric hyperbolic formulation. We show that this system works as expected for the wave propagation problem in the Maxwell system and in general relativity using Ashtekar's connection formulation. Second, we propose a new mechanism to control the stability, which we call the ``adjusted system". This is simply obtained by adding constraint terms in the dynamical equations and adjusting its multipliers. We explain why a particular choice of multiplier reduces the numerical errors from non-positive or pure-imaginary eigenvalues of the adjusted constraint propagation equations. This ``adjusted system" is also tested in the Maxwell system and in the Ashtekar's system. This mechanism affects more than the system's symmetric hyperbolicity.