Bulletin of the American Physical Society
APS April Meeting 2020
Volume 65, Number 2
Saturday–Tuesday, April 18–21, 2020; Washington D.C.
Session B16: Interface of Numerical Relativity and Classical General RelativityLive

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Sponsoring Units: DGRAV Chair: Beverly Berger, Stanford University Room: Virginia C 
Saturday, April 18, 2020 10:45AM  10:57AM Live 
B16.00001: Angular momentum at null infinity in EinsteinMaxwell theory Kartik Prabhu, Beatrice Bonga, Alexander Grant On Minkowski spacetime, the angular momentum flux through null infinity of Maxwell fields, computed using the stressenergy tensor, depends not only on the radiative degrees of freedom, but also on the Coulombic parts. This flux cannot be written as the change of an angular momentum charge computed purely on crosssections of null infinity. We investigate the angular momentum charge and flux in full EinsteinMaxwell theory. Using the prescription of Wald and Zoupas, we compute the charges associated with any BMS symmetry on crosssections of null infinity. The change of these charges along null infinity then provides a flux. For Lorentz symmetries, the Maxwell fields contribute an additional term to the charge on a crosssection, compared to the charge in vacuum general relativity. With this additional term, the flux associated with Lorentz symmetries, e.g. the angular momentum flux, is purely determined by the radiative degrees of freedom of the gravitational and Maxwell fields. In fact, the contribution to this flux by the Maxwell fields is given by the purely radiative Noether current flux and not by the stressenergy flux. [Preview Abstract] 
Saturday, April 18, 2020 10:57AM  11:09AM Live 
B16.00002: Persistent gravitational wave observables in nonlinear plane wave spacetimes Alexander Grant, Eanna Flanagan, Abraham Harte, David Nichols Persistent gravitational wave observables are generalizations of the gravitational wave memory effect that are nonlocal in time and nonzero in the presence of gravitational radiation. A natural class of spacetimes in which to discuss these observables is that of nonlinear plane waves, which are exact, radiative solutions to Einstein's equations. In this talk, we discuss a particular observable in these spacetimes: a holonomy involving a closed curve, which is known to contain information about the usual gravitational wave memory. For linearized plane waves, this observable is determined by just one, two, and three integrals of the Riemann tensor along a central worldline. At nonlinear order, we show that a similar result holds: this observable can be written in terms of two functions, which we call the transverse Jacobi propagators, and their first derivatives. These functions are related to the usual gravitational wave memory. [Preview Abstract] 
Saturday, April 18, 2020 11:09AM  11:21AM Live 
B16.00003: Numerical simulations of spacetime singularities David Garfinkle Numerical simulations are performed of the approach to the singularity in gravitational collapse. Comparisons are made between singularities in closed cosmologies and those in asymptotically flat spacetimes. Comparisons are also made between spacelike singularities and null singularities. [Preview Abstract] 
Saturday, April 18, 2020 11:21AM  11:33AM Live 
B16.00004: Selftorque and frame nutation in binary black hole simulations Aaron Zimmerman, Maria Jose Bustamante Rosell We investigate the precession of the spin of the smaller black hole in binary black hole simulations. By considering a sequence of binaries at higher mass ratios, we approach the limit of geodetic precession of a test spin. This precession is corrected by the ``selftorque'' due to the smaller black hole's own spacetime curvature. We find that the spins undergo spin nutations which are not described in conventional descriptions of spin precession, an effect which has been noticed previously in simulations. These nutations arise because the spins are not measured in a frame where the smaller hole is stationary. We develop a simple model for these frame nutations, extract the instantaneous spin precession rate, and compare our results to PN and extrememassratio approximations for the selftorque. [Preview Abstract] 
Saturday, April 18, 2020 11:33AM  11:45AM On Demand 
B16.00005: Spin SelfForce Kristian Mackewicz, Robert Wald We analyze the motion of charged and spinning bodies along the symmetry axis of a nonextremal KerrNewman black hole. If one treats the body as a test point particle of mass $m$, charge $q$, and spin $S$, then the first order area increase, $\delta A$, of the black hole can be made arbitrarily small by dropping the body into the black hole from sufficiently near the horizon. At second order, there may be effects quadratic in $q$ and $S$ on the energy delivered to the black hole. These effects are due to (i) the finite size of the body and (ii) selfforce corrections to the energy. We consider a charged and spinning body on the symmetry axis of a KerrNewman black hole, where the selfforce effects have not been calculated. After accounting for all finite size effects, we find that the condition $\delta^2 A \geq 0$ yields a nontrivial lower bound on the selfforce energy, $E_{SF}$, at the horizon. For an uncharged, spinning body on the axis of a Kerr black hole of mass $M$, the spin selfforce energy of the body at the horizon satisfies $E_{SF} \geq S^2/8M^3$. [Preview Abstract] 
Saturday, April 18, 2020 11:45AM  11:57AM Not Participating 
B16.00006: Supermomentum balance laws as a tool to improve gravitational waveforms Neev Khera, Abhay Ashtekar, Tommaso De Lorenzo, Badri Krishnan Current nonprecessing gravitational waveform models do not (reliably) incorporate the $(2,0)$ mode. This difficulty, in part, is due to the challenges of numerically extracting this mode accurately. However for future detectors this must be rectified. We present a technique based on supermomentum balance laws at null infinity which can be used to improve the $(2,0)$ mode waveforms. We apply this to SXS binary black hole simulations and find a significant improvement. [Preview Abstract] 
Saturday, April 18, 2020 11:57AM  12:09PM Not Participating 
B16.00007: Critical Phenomena in the Gravitational Collapse of Electromagnetic Waves Thomas Baumgarte, Carsten Gundlach, David Hilditch Critical phenomena in the collapse of vacuum gravitational waves remain mysterious even 25 years after they were first reported. This case differs qualitatively from other, better understood examples of critical collapse in that the critical solution cannot be spherically symmetric. I will report on critical phenomena in the gravitational collapse of electromagnetic waves, which also do not allow spherically symmetric solutions. Finetuning numerical evolution calculations to the blackhole threshold we find both approximate powerlaw scaling as well as a critical solution with approximately discrete selfsimilarity, but neither the powerlaw scaling nor the selfsimilarity are exact. The absence of an exactly discrete selfsimilarity might be caused by the interplay of electromagnetic and gravitational wave degrees of freedom, or the presence of higherorder angular multipoles. I will also discuss implications of these findings for the critical collapse of vacuum gravitational waves. [Preview Abstract] 
Saturday, April 18, 2020 12:09PM  12:21PM Not Participating 
B16.00008: Nearfield gravitational lensing with SpECTRE William Throwe The SpECTRE project (https://github.com/sxscollaboration/spectre) is primarily focused on solving elliptic and hyperbolic PDEs using discontinuous Galerkin methods. We present progress in applying SpECTRE to a different type of problem: tracing geodesics through a known spacetime. We give particular emphasis to the application to nearfield gravitational lensing by black hole systems. [Preview Abstract] 
Saturday, April 18, 2020 12:21PM  12:33PM Not Participating 
B16.00009: Pathologies of van Stockum dust/Tipler's time machine David Lindsay We describe external vacuum solutions for radial cutoff of ``van Stockum dust'', an infinitely long rotating matter solution of Einstein's equations of general relativity. These poorly explored spacetimes have been known for decades, but it seems that they have never been investigate in detail. They exhibit a number of exotic properties, which we described more fully in General Relativity and Gravitation (2016 48:121). Exotic properties include circular ``time travel'' in cylindrical shells alternating with shells permitting no time travel; there are infinitely many such shells. With sufficiently massive rotating columns, these shells get closer and closer together as one gets farther from the rotation axis. Also, a separate set of infinitely many cylindrical shells exists, having what might be termed ``extreme framedragging'', within which revolution is possible only in one direction; they alternate with ``normal'' shells allowing motion in either direction. Gravitational attraction and tides increase with distance from the matter column, and diverge at the ``edge of the universe,'' which is only a finite distance away  although its circumference is infinite; and its boundary is a circle, not a cylinder. [Preview Abstract] 
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