Dynamical, Strong-field Gravity

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The research goals of this project are focused on understanding the strong-field regime of Einstein's theory of general relativity. This encompasses both astrophysical and theoretical aspects of general relativity. On the astrophysical side, the main effort is numerical simulations of binary black hole, black hole-neutron star and binary neutron star collisions. This is important to support the field of gravitational wave astronomy recently ushered in by LIGO's detection of the collision of two black holes. Numerical models of such sources are needed to aid in detection, and are crucial to decipher details of observed signals. On the theoretical side, there are many outstanding questions about the nature of spacetime in extreme situations. The one that will be the main focus here is the ultra-relativistic limit of black hole collisions; namely, what happens when two black holes collide at the speed of light. Speculation about the outcome began in the 1960's when Penrose used this scenario to help formulate his cosmic censorship conjecture, stating all singularities in spacetime are hidden behind event horizons. Numerical methods are now giving us the tools to definitively tackle this regime of general relativity. The pursuit of these projects will involve graduate students, undergraduates and postdoctoral fellows. They will be trained to do leading scientific research, become knowledgeable in corresponding areas of physics, and adept in high-performance computing and numerical methods. These skills are invaluable to many professions, and would thus also benefit and further the development of those students and postdocs that subsequently wish to pursue careers outside academia.A specific list of gravitational wave source modeling projects that will be pursued are (1) understanding the consequences of neutron star spin in binary neutron star and black hole-neutron star mergers, (2) improving estimates of heavy element yields and corresponding electromagnetic counterpart emission in collisions involving neutron stars when there is significant ejected material, (3) beginning to include neutrino and radiation transport, and improving our models of magnetic fields in our evolution code, relevant for mergers involving neutron stars, (4) continued development of the parameterized post Einsteinian (ppE) framework for eccentric binaries to allow such events, if observed, to be used to test general relativity, and (5) explore binary mergers in alternative theories of gravity and/or exotic alternatives to black holes within general relativity (such as boson stars), to allow the merger event recently detected by LIGO and future detections to constrain/rule-out/discover these alternatives. Regarding the ultra-relativistic collision problem, the route to approaching this limit will be by developing a code to study the collision of distributions of null dust. Such distributions can be tailored to approach both the point-particle and plane-parallel wave collision limits in a non-singular manner amenable to full numerical treatment.