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.

Characteristic Evolution and Matching
Jeffrey Winicour
http://www.livingreviews.org/lrr-2001-3
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