The successful implementation of CCM for Einstein’s equations requires a well-posed initial-boundary value problem for the artificial outer boundary of the Cauchy evolution. This is particularly cogent for dealing with waveform extraction in the simulation of black holes by BSSN formulations. There is no well-posed outer boundary theory for the BSSN formulation and the strategy is to place the boundary out far enough so that it does no harm. The harmonic formulation has a simpler mathematical structure as a system of coupled quasilinear wave equations, which is more amenable to an analytic treatment.
Standard harmonic coordinates satisfy the covariant wave equation, whereby . For simplicity of discussion, I will set , although gauge forcing is an essential tool in simulating black holes .
When , Einstein’s equations reduce to the ten quasilinear wave equations
In the harmonic formalism, the constraints can be reduced to the harmonic coordinate conditions (64). For the resulting IBVP to be constraint preserving, these harmonic conditions must be built into the boundary condition. Numerous early attempts to accomplish this failed because Equation (64) contains derivatives tangent to the boundary, which do not fit into the standard methods for obtaining the necessary energy estimates. The use of pseudo-differential techniques developed for similar problems in elasticity theory has led to the first well-posed formulation of the IBVP for the harmonic Einstein equations . Subsequently, well-posedness was also obtained using energy estimates by means of a novel, non-conventional choice of the energy for the harmonic system . A Cauchy evolution code, the Abigel code, based upon a discretized version of these energy estimates was found to be stable, convergent and constraint preserving in nonlinear boundary tests . These results were confirmed using an independent harmonic code developed at the Albert Einstein Institute . A linearized version of the Abigel code has been used to successfully carry out CCM (see Section 5.8).
Given a well-posed IBVP, there is the additional complication of the correct specification of boundary data. Ideally, this data would be supplied by matching to a solution extending to infinity, e.g., by CCM. In the formulations of  and , the boundary conditions are of the Sommerfeld type for which homogeneous boundary data , i.e., zero boundary values, is a good approximation in the sense that the reflection coefficients for gravitational waves fall off as as the boundary radius . A second differential order boundary condition based upon requiring the Newman–Penrose  Weyl tensor component has also been shown to be well-posed by means of pseudo-differential techniques . For this condition, the reflection coefficients fall off at an addition power of . In the present state of the art of black-hole simulations, the condition comes closest to a satisfactory treatment of the outer boundary .
Living Rev. Relativity 15, (2012), 2
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