DFG-NSF: Gravitational Waves from Neutron Star and Black Hole Mergers

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The first direct detection of gravitational waves emitted from merging black holes by LIGO was a historic breakthrough in gravitational physics. Since we can now detect and study gravitational waves, a new window to the universe has been opened. Observations from a variety of other astrophysical sources are anticipated. One particularly promising source is when two neutron stars orbit around each other and merge. As long as the two compact objects are widely separated, post-Newtonian calculations can give highly accurate and astrophysically realistic predictions about the orbits and the gravitational waves emitted. Yet, as the two objects get close, fully non-linear numerical computer simulations of the Einstein equations are required to make predictions about the final part of the inspiral and subsequent merger. This project is aimed at gaining a detailed understanding of such astrophysical scenarios by performing numerical simulations. The plan is to use and improve the highly efficient BAM computer code, which has been developed by the numerical relativity groups at the University of Jena in Germany and the group at Florida Atlantic University (FAU). Such theoretical predictions of the emitted gravitational waves are important to extract the maximum amount of information from observed gravitational wave signals. This award is co-funded by the Office of International Science and Engineering.Expecting the detection of more gravitational wave signals from black hole binaries and anticipating the detection of binaries containing neutron stars, the goal is to significantly enhance the theoretical understanding of such systems in terms of physics, in particular with regard to neutron star spins. This project proposes to explore astrophysically realistic compact-object binaries. In particular, addressing several key physics issues such as: (i) How does neutron star spin, mass ratio and eccentricity in binaries affect the evolution of the inspiral, the merger, and final object after merger? (ii) Study the gravitational waves emitted by such binaries: Investigate spectra of the waves and create hybrid waveforms using analytic approximations such as the effective-one-body approach. (iii) How can magnetic fields in neutron star initial data be included in a consistent and realistic way? (iv) How can neutron star evolutions be improved by removing the artificial numerical atmosphere, that is currently used instead of true vacuum between the stars? (v) How can we use hyperboloidal foliations to obtain more accurate gravitational waves? By providing gravitational wave templates, the proposed activities are critical for maximizing the science output of gravitational wave detectors such as LIGO. Neutrons star mergers and the resulting disk dynamics will shed light on the question whether such mergers are engines for Gamma Ray Bursts, and help determine the currently unknown equation of state at supranuclear densities. This research will be carried out in close collaboration with the relativity group at the University of Jena led by Dr. Bruegmann. Regular visits by faculty, postdocs and students from both institutions are planned. This exchange will have educational benefits for postdocs and students from both FAU and the University of Jena.