Using openGR for numerical relativity

Binary black hole mergers are the strongest expected producers of graviational radiation in the universe. Ground-based and proposed space-based gravitational wave detectors will benefit from simulations modeling the mergers and extracting the resulting gravitational waveforms. Producing templates of waveforms will both aid the likelihood of detection and the estimation of parameters (mass ratio, spin, etc.). openGR is modular, open framework development to carry out simulations of binary black hole mergers. While designed with the two-body problem in mind, openGR will evolve most general spacetimes. This work overviews the capabilities of openGR and the corresponding physics involved. openGR supports both excision and puncture methods. When excising the black hole, to date we have used only the weakly hyperbolic ADM formulation of the Einstein’s equations. As expected from a weakly hyperbolic system, instabilites arise and crash the code when simulating even just a single boosted black hole in Kerr-Schild coordinates. In contrast, successful mergers of two black holes have been achieved using the puncture method. We demonstrate such a simulation in Ch 8. In this case, we make use of a BSSN formulation of Einstein’s equations (a strongly hyperbolic system).