"Numerical relativity in the multi-messenger astrophysics era" by Vassilios Mewes (RIT)
The detection of gravitational waves emitted in binary black hole mergers by LIGO-VIRGO has opened a new window into the universe, and the simultaneous detection of the electromagnetic counterparts observed in the binary neutron star merger GW170817 has marked the beginning of multi-messenger astronomy. Numerical relativity, in the form of both vacuum and general relativistic (magneto)hydrodynamic simulations of these systems is an indispensable tool in correctly predicting both spacetime and matter field evolutions during these extremely energetic and luminous events. Of particular interest in multi-messenger astronomy is the evolution of the matter in the post-merger remnant following a binary neutron star or black hole neutron star merger, as well the gas dynamics in the late stages of supermassive binary black hole inspiral. I will present a novel framework for highly accurate numerical relativity in spherical/curvilinear coordinates within the Einstein Toolkit. I will describe our implementation of the vacuum BSSN equations in these coordinates, and describe its extension to the GRMHD equations. Azimuthal flows and angular momentum transport are more accurately modeled in spherical coordinates, which are less diffusive than Cartesian coordinates, as they are better adapted to the approximate symmetries of the problem. Combining spherical coordinates with traditional moving mesh refinement, this framework will also enable more accurate simulations of high mass ratio binary black holes, which are of great importance to LISA as well as third generation LIGO-VIRGO observations.