Theoretical Studies in Gravitation and Astrophysics

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This award supports research in relativity and relativistic astrophysics and it addresses the priority areas of NSF's "Windows on the Universe" Big Idea. The LIGO/Virgo Scientific Collaboration has compiled an impressive list of detected gravitational wave (GW) events consisting of several dozen binary black hole (BHBH) mergers, a couple of binary neutron star (NSNS) mergers and several likely black hole--neutron star (BHNS) mergers. These detections mark the beginning of the era of GW astronomy. This research project spans several problems involving general relativity (GR), the generation of GWs, relativistic hydrodynamics, and relativistic magnetohydrodynamics. A common thread uniting the different theoretical topics is the crucial role of gravitation, especially relativistic gravitation. Compact objects (black holes, neutron stars and white dwarfs) provide the principal forum, and the dynamics of matter in a strong gravitational field is a major theme. Some of the topics for investigation include the inspiral and coalescence of compact binaries (BHBHs, NSNSs and BHNSs); the generation of GWs from merging binaries and other promising astrophysical sources, and their counterpart electromagnetic (EM) signals; gravitational collapse; the stability of rotating, relativistic stars and the evolution and final fate of unstable stars; gamma-ray burst sources (GRBs); the formation and growth of supermassive black holes (SMBHs) from the magnetorotational collapse of supermassive stars (SMSs) and other scenarios; circumbinary disks around merging binary SMBHs in the cores of galaxies and quasars; and the profiles and observable signatures of dark matter around SMBHs in galaxy cores, including the Milky Way, and the dynamical evolution of clusters containing DM, stars and SMBHs. The results have important implications for astronomical observations, including those collected by and/or planned for GW interferometers.Most of these topics represent long-standing, fundamental problems in theoretical physics requiring large-scale computation for solution. Hence the approach involves to a significant degree large-scale simulations on supercomputers, in addition to analytical modeling. The key tool will be the robust and well-tested Illinois general relativistic, magnetohydrodynamic (GRMHD) code. The simulations solve Einstein's field equations of GR for gravity coupled to the equations of relativistic MHD for the fluid and Maxwell's equations for the EM fields. These equations constitute highly nonlinear, coupled partial differential equations in 3+1 dimensions that are solved by finite-differencing. The Illinois GRMHD code employs the Baumgarte-Shapiro-Shibata-Nakamura (BSSN) technique with moving puncture gauge conditions and adaptive moving mesh refinement to solve the field equations and a high-resolution, shock capturing scheme for the MHD. The problems to be tackled comprise both initial value and evolution computations and treat vacuum spacetimes containing black holes, as well as spacetimes containing realistic matter sources, magnetic fields and both EM and neutrino radiation ("multimessenger astronomy"). The research and outreach activities supported by this grant help promote the use of computers and visualization tools at all levels of education, as well as the public awareness of some the latest and most exciting developments in gravitational physics and astrophysics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.