Gravitational Radiation and Relativistic Astrophysics

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The Laser Interferometer Gravitational-Wave Observatory (LIGO) has detected gravitational waves (GWs) --ripples in space and time-- that were emitted by the collision of two black holes billions of light-years from Earth. The rate and accuracy of these detections will improve as LIGO is upgraded. By comparing the detected waves with detailed predictions of the waves, scientists can measure properties (locations, masses, spins) of the black holes and learn about how they might have formed and how they warp space and time as they collide. The predictions are made using the equations of General Relativity, written down by Einstein in 1915 but unsolvable (for colliding black holes) until about 2005 with the development of advanced methods and powerful supercomputers. This project supports theoretical work designed to underpin and improve LIGO's ability to extract from observed GWs the rich information that the waves carry. This includes the improvement and use of the Spectral Einstein Code (SpEC), currently the most accurate computer code for solving Einstein's equations for black hole binaries. SpEC will be used to carry out numerical solutions of black-hole collisions for the purpose of analyzing LIGO data. In addition, a new computer code SpECTRE will be developed; this is a next-generation upgrade of SpEC designed for new upcoming computer architectures and accuracies beyond those of current codes. SpECTRE will be used to predict gravitational waveforms from two colliding neutron stars and from a black hole colliding with a neutron star. Gravitational waves from neutron-star collisions are expected to be observed by LIGO, and can teach scientists about how matter behaves at ultra-high densities. This program will also serve as a training ground for young physicists and astrophysicists. The new code SpECTRE and its output will be publicly released. Group members will reach out to the general public through lectures, interactive web pages, and YouTube videos.By combining analytical techniques and numerical simulations with SpEC: (i) Gravitational-wave signals for black-hole binaries will be generated for use in LIGO data analysis, will be used to calibrate analytic models, and will be used to produce numerical surrogate models that can evaluate a single waveform in milliseconds while retaining the accuracy of full numerical simulations; and (ii) the dynamical behavior of highly curved spacetime will be explored via analytic, perturbative, and numerical approaches. The next-generation open-source code SpECTRE will be developed for numerical relativity simulations with matter and radiation. SpECTRE uses Discontinuous Galerkin finite element methods. It is designed for high accuracy and high scalability on current and future supercomputers by following a novel task-based parallelization paradigm. SpECTRE's initial version already implements general-relativistic magnetohydrodynamics. It will be upgraded to handle the dynamical space times of neutron star-neutron star and black hole-neutron star mergers, including nuclear-theory based hot equations of state and neutrinos. SpECTRE will be applied to studies of BH and accretion disk formation to understand dynamics and GW emission. It will be used for high-accuracy ultra-long neutron star inspiral simulations to predict GW signals and help LIGO constrain the nuclear equation of state.