Stars, Black Holes, and Disks: Dense Matter Phenomena in 2D Numerical Relativity

This award supports research in relativity and relativistic astrophysics and it addresses the priority areas of NSF's "Windows on the Universe" Big Idea. 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.

Such a binary system emits gravitational waves as the neutron star and its companion spiral together and electromagnetic waves (visible light, infrared, gamma rays, etc.) after the merger. Both signals are sometimes detectable and in some cases have been detected. Numerical simulations that include general relativity are the only way to predict how merger processes unfold, and they've gotten pretty good at modeling the merger and the first few hundredths of a second afterward.

The post-merger evolution lasts around a couple of seconds. Although this does not sound long, it is too long to be accessible to 3D numerical simulations, especially because the presence of small-scale turbulent flows makes the grids needed large and the simulations expensive. This project will probe these later times with numerical relativity, following the story of the merger through to completion, when nothing but a stable, quiescent black hole or neutron star remains.

The problems of large grid and long time requirements will be dealt with by carrying out 2D simulations (using the rough symmetry about a rotation axis) and carefully-designed models of the effect of turbulent flows on scales to small to be directly captured on the 2D grids. These simulations will include a high degree of realism in the treatment of magnetic fields and neutrino radiation.

The techniques and code developed to carry them out will be applicable to other interesting systems with near-axisymmetry, such as collapsing stars.

This award supports long-time (multi-second) studies of these strong-gravity, dense-matter systems using the Spectral Einstein Code (SpEC), a high-accuracy numerical relativity code that includes magnetohydrodynamics, nuclear microphysics, and neutrino transport. SpEC's 2D radiation magnetohydrodynamics code will be improved to include 2D spacetime evolution, Monte Carlo neutrino transport, and a mean-field/large-eddy incorporation of the effects of turbulence in the azimuthal direction.

Long-time 2D simulations will extend existing compact binary merger simulations for multiple seconds, yielding accurate predictions about disk winds, winds from and possible delayed collapse of a stellar remnant, and GRB energetics. Finally, these numerical techniques will be applied to the first SpEC simulations of black hole formation and growth in a post-collapse, long-duration GRB scenario.

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.