Gauge Boundary conditions to mitigate center-of-mass drift in BBH simulations

TL;DR

This paper identifies gauge boundary conditions as the cause of center-of-mass drift in binary black hole simulations and proposes a modified boundary condition with a CoM correction to significantly reduce this artifact.

Contribution

The authors introduce a new boundary condition with an explicit CoM correction to mitigate gauge artifacts causing drift in NR BBH simulations.

Findings

The CoM drift is a gauge artifact caused by boundary reflections.

The new boundary condition reduces CoM drift by several orders of magnitude.

The modified boundary condition does not introduce physical artifacts.

Abstract

Long-term numerical relativity (NR) simulations of binary black hole (BBH) systems in the Spectral Einstein Code (SpEC) code exhibit an unexpected exponential drift of the center-of-mass (CoM) away from the simulation's origin. In our work, we analyze this phenomenon and demonstrate that it is not a physical effect but rather a manifestation of a gauge artifact. The origin of this drift is the reflection of the gauge waves off the outer boundary of the computational domain. These reflections are introduced by inaccuracies in the gauge boundary condition, specifically, the application of the Sommerfeld condition to the time derivative of the gauge fields. Such an approach fails to completely suppress or correctly absorb the outgoing modes, thereby generating artificial feedback into the simulation. To mitigate this problem, we introduce a modified boundary condition that incorporates an explicit CoM correction source term designed to counteract the CoM motion. Our numerical experiments, performed with the SpEC code, reveal that this new boundary treatment reduces the CoM drift by several orders of magnitude compared to the standard implementation, and does not introduce any unwanted physical artifacts.