CCM also has the potential to handle the two black holes inside the Cauchy region. As described earlier with respect to Fig. 5, an ingoing characteristic code can evolve a moving black hole with long term stability [73, 70]. This means that CCM might also be able to provide the inner boundary condition for Cauchy evolution once stable matching has been accomplished. In this approach, the interior boundary of the Cauchy evolution is located outside the apparent horizon and matched to a characteristic evolution based upon ingoing null cones. The inner boundary for the characteristic evolution is a trapped or marginally trapped surface, whose interior is excised from the evolution.
In addition to restricting the Cauchy evolution to the region
outside the black holes, this strategy offers several other
advantages. Although finding a marginally trapped surface on the
ingoing null hypersurfaces remains an elliptic problem, there is
a natural radial coordinate system
to facilitate its solution. Motion of the black hole through the
grid reduces to a one-dimensional radial problem, leaving the
angular grid intact and thus reducing the computational
complexity of excising the inner singular region. (The angular
coordinates can even rotate relative to the Cauchy coordinates in
order to accommodate spinning black holes.) The chief danger in
this approach is that a caustic might be encountered on the
ingoing null hypersurface before entering the trapped region.
This is a gauge problem whose solution lies in choosing the right
location and geometry of the surface across which the Cauchy and
characteristic evolutions are matched. There is a great deal of
flexibility here because the characteristic initial data can be
posed without constraints.
This global strategy is tailor-made to treat two black holes in the co-orbiting gauge, as illustrated in Fig. 6 . Two disjoint characteristic evolutions based upon ingoing null cones are matched across worldtubes to a central Cauchy region. The interior boundary of each of these interior characteristic regions border a trapped surface. At the outer boundary of the Cauchy region, a matched characteristic evolution based upon outgoing null hypersurfaces propagates the radiation to infinity.
Present characteristic and Cauchy codes can handle the individual pieces of this problem. Their unification appears to offer the best chance for simulating the inspiral and merger of two black holes. The CCM module is in place and calibrated for accuracy. The one missing ingredient is its long term stability, which would make future reviews of this subject very exciting.
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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 |