## 4.3 Analytic-Numerical Matching for Waves

One of the earliest computational applications of null coordinates in a matching scheme was a hybrid numerical-analytical version, by Anderson and Hobill, for the test problem of 1D scalar waves [5, 6, 7]. Although the nonlinear near field cannot be treated analytically in a general relativistic radiation problem, the far field can be handled by a perturbative expansion. Their strategy was to match an inner numerical solution to an approximate outer analytic solution.

The initial conditions for the exterior solution were fixed by requiring that the interior sources be stationary prior to some fixed time. As a result, the exterior analytic solution is causal in the sense that it is stationary in the past of some null cone. This effectively introduces a condition that eliminates extraneous incoming radiation from the system in a physically plausible way and determines the exterior solution uniquely. An overlap region was introduced between the numerical interior and the analytic exterior. In the overlap, the numerical solution was matched to the causal analytic solution, resulting in an evolution that was everywhere causally meaningful.

This is the physically correct approach to a system which is stationary prior to a fixed time but is nontrivial to generalize, say, to the problem of radiation from an orbiting binary. Anderson and Hobill first tackled the 1D model problem of an oscillator coupled to a spherically symmetric, flat space, massless scalar field. The numerical results were in excellent agreement with the exact analytic solution (which could be obtained globally for this problem).

They extended the model to include spherical scalar waves propagating in a spherically symmetric curved background. This introduces backscattering which obscures the concept of a purely outgoing wave. No exact solution exists to this problem so that an approximation method was necessary to determine the exterior analytic solution. The approximation was based upon an expansion parameter controlling the amount of backscatter. In the 0th approximation, the scale parameter vanishes and the problem reduces to the flat space case which can be solved exactly. The flat space Green function was then used to generate higher order corrections.

A key ingredient of this perturbative scheme is that the wave equation is solved in retarded null coordinates (u, r) for the curved space metric, so that the proper causality is built into the Green function at each order of approximation. The transformation from null coordinates (u, r) to Cauchy coordinates (t, r) is known analytically for this problem. This allows a simple match between the null and Cauchy solutions at the boundary of the Cauchy grid. Their scheme is efficient and leads to consistent results in the region that the numerical and analytic solutions overlap. It is capable of handling both strong fields and fast motions.

Later, a global, characteristic, numerical study of the self-gravitating version of this problem, by Gómez and Winicour, confirmed that the use of the true null cones is essential in getting the correct radiated waveform [77]. For quasi-periodic radiation, the phase of the waveform is particular sensitive to the truncation of the outer region at a finite boundary. Although a perturbative estimate would indicate an O (M / R) error, this error accumulates over many cycles to produce an error of order in the phase.

Anderson and Hobill proposed that their method be extended to general relativity by matching a numerical solution to an analytic 1/ r expansion in null coordinates. However, the only analytic-numerical matching schemes that have been implemented in general relativity have been based upon perturbations of a Schwarzschild background using the standard Schwarzschild time slicing [1, 3, 2, 142]. It would be interesting to compare results with an analytic-numeric matching scheme based upon the true null cones. However the original proposal by Anderson and Hobill has not been carried out.

 Characteristic Evolution and Matching Jeffrey Winicour http://www.livingreviews.org/lrr-2001-3 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de