Numerical and analytical methods for asymptotically flat spacetimes

TL;DR

This paper reviews numerical relativity methods for simulating asymptotically flat spacetimes, introducing boundary conditions and hyperboloidal approaches to efficiently handle infinite domains and extract gravitational radiation.

Contribution

It compares boundary condition techniques and develops hyperboloidal methods for stable, accurate simulations of spacetimes with gravitational radiation at null infinity.

Findings

Constraint-preserving boundary conditions improve stability

Hyperboloidal evolution accurately captures gravitational waves

Late-time tails of matter fields are successfully simulated

Abstract

This article begins with a brief introduction to numerical relativity aimed at readers who have a background in applied mathematics but not necessarily in general relativity. I then introduce and summarise my work on the problem of treating asymptotically flat spacetimes of infinite extent with finite computational resources. Two different approaches are considered. The first approach is the standard one and is based on evolution on Cauchy hypersurfaces with artificial timelike boundary. The well posedness of a set of constraint-preserving boundary conditions for the Einstein equations in generalised harmonic gauge is analysed, their numerical performance is compared with various alternate methods, and improved absorbing boundary conditions are constructed and implemented. In the second approach, one solves the Einstein equations on hyperboloidal (asymptotically characteristic) hypersurfaces. These are conformally compactified towards future null infinity, where gravitational radiation is defined in an unambiguous way. We show how the formally singular terms arising in a $3+1$ reduction of the equations can be evaluated at future null infinity, present stable numerical evolutions of vacuum axisymmetric black hole spacetimes and study late-time power-law tails of matter fields in spherical symmetry.