Probing the puncture for black hole simulations

TL;DR

This paper investigates the behavior of perturbations near the puncture boundary in black hole simulations, demonstrating that no pathological reflections occur due to mode propagation differences, supported by numerical experiments.

Contribution

It clarifies how different perturbative modes propagate in puncture black hole simulations, explaining the absence of boundary reflections and improving understanding of numerical stability.

Findings

Perturbations in physical and conformal geometries propagate differently.

Modes in the physical geometry are directed away from the boundary.

Numerical experiments confirm the absence of pathological reflections.

Abstract

With the puncture method for black hole simulations, the second infinity of a wormhole geometry is compactified to a single "puncture point" on the computational grid. The region surrounding the puncture quickly evolves to a trumpet geometry. The computational grid covers only a portion of the trumpet throat. It ends at a boundary whose location depends on resolution. This raises the possibility that perturbations in the trumpet geometry could propagate down the trumpet throat, reflect from the puncture boundary, and return to the black hole exterior with a resolution--dependent time delay. Such pathological behavior is not observed. This is explained by the observation that some perturbative modes propagate in the conformal geometry, others propagate in the physical geometry. The puncture boundary exists only in the physical geometry. The modes that propagate in the physical geometry are always directed away from the computational domain at the puncture boundary. The finite difference stencils ensure that these modes are advected through the boundary with no coupling to the modes that propagate in the conformal geometry. These results are supported by numerical experiments with a code that evolves spherically symmetric gravitational fields with standard Cartesian finite difference stencils. The code uses the Baumgarte--Shapiro--Shibata--Nakamura formulation of Einstein's equations with 1+log slicing and gamma--driver shift conditions.