Observing the Momentum Density of Waves in Black Hole Mergers*

Modeling gravitational waves with supercomputer simulations allows us to compare models to detected waveforms and to check whether they are consistent with Einstein's predictions. The louder those detected signals are (i.e., the higher the signal-to-nosie ratio), the more accurate the numerical relativity model must be, because numerical errors larger than experimental errors lead to biased interpretations of gravitational-wave observations. Future detectors with higher sensitivity, such as Cosmic Explorer, Einstein Telescope, and LISA, will be much more sensitive than today’s detectors and thus will require better accuracy from numerical relativity models. SpECTRE is a next-generation numerical relativity code aiming to model gravitational waves emitted by black holes and neutron stars with much higher accuracy, by using novel techniques that scale well, to make effective use of exascale computing facilities. One physical quantity of interest that SpECTRE aims to compute is the recoil (“kick”) of the remnant. After a binary black hole merger, the momentum released through gravitational waves causes the merged black hole to experience a recoil velocity. As a first step toward computing the recoil of a binary-black-hole merger, I enabled SpECTRE to compute the momentum density for a scalar wave, a simple test case. I will present and discuss calculations of the momentum density of a scalar wave with SpECTRE.