Mergers, Stars, and Disks: Dense Matter Phenomena in Numerical Relativity

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A great deal can be learned about the extremes of strong gravity, about denser-than-nuclei matter, and even about the origin of heavy elements by studying the collisions of neutron stars with each other or with black holes. One such merger of two neutron stars has recently been observed in gravitational waves and across the electromagnetic spectrum. Astronomers now know that such events happen and that they release tremendous amounts of radiation and material, but exactly how this release happens in a merger aftermath remains unclear. Numerical simulations that include general relativity are the only way to predict how merger processes unfold, but these simulations are made incredibly difficult by the presence of turbulence, which creates fluid flow and magnetic field features across a wide range of length scales. Small-scale flows do drive changes in the merger remnant at large scales and so cannot be ignored. Although computer simulations cannot yet directly capture all of this dynamics, a great deal can be learned with current computing capabilities by parameterized exploration of unresolved small-scale effects and post-merger initial states. This project will pursue these strategies to shed light on how these exotic and violent events produce their outflows and radiation. Carrying out these simulations will involve training both graduate and undergraduate students on techniques of computer modeling in relativistic astrophysics. The resulting gravitational wave and nuclear debris data will be made available to all astronomers working to detect and characterize these events.The Washington State University will study three core problems of numerical relativity--inspiraling binaries, differentially rotating neutron stars, and hyperaccreting black holes--using the Spectral Einstein Code (SpEC), a high-accuracy numerical relativity code that includes magnetohydrodynamics, nuclear microphysics, and neutrino transport. Inspiral simulations will study equation of state effects on gravitational waves, and improvements to evolution algorithms and smooth, causal equation of state parameterizations will be explored. Post-merger dynamics of NSNS systems will be modeled, focusing on the case of a hypermassive neutron star remnant. Simulations will track the secular evolution to collapse under the influence of neutrino radiation, large-scale magnetic fields, and subgrid-scale turbulence. Finally, neutrino effects around black hole-torus systems will be analyzed using neutrino transport methods.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.