Bulletin of the American Physical Society
APS April Meeting 2020
Volume 65, Number 2
Saturday–Tuesday, April 18–21, 2020; Washington D.C.
Session T16: Approximate Methods in Gravitational AstrophysicsOn Demand

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Sponsoring Units: DGRAV Chair: Emanuele Berti, Johns Hopkins University Room: Virginia C 
Monday, April 20, 2020 3:30PM  3:42PM 
T16.00001: A Spheroidal Harmonic Picture for GWs from Astrophysical Sources I: BiOrthogonality Lionel London The study of isolated perturbed Kerr black holes plays an important role in gravitational wave (GW) signal modeling for coalescing binary black holes. In this, the natural multipolar structure of Kerr black holes has informed both signal models (Numerical Relativity and approximants) and related tests of General Relativity. In this talk we present new results on the multipolar structure of GWs from Kerr black holes; in particular, we demonstrate for the first time that the natural spherical harmonics for Kerr, the spheroidal harmonics, display biorthogonality and so obey a spectral theorem. We thereby present a method for the general calculation of spheroidal harmonic GW multipole moments. This marks a departure from the spherical multipole moments that are currently ubiquitous, despite being most appropriate for only nonspinning spacetimes. Noting that all astrophysical systems with angular momentum are asymptotically similar to the Kerr metric, we discuss possible applications for the spheroidal harmonic decomposition of gravitational waves from general astrophysical sources. [Preview Abstract] 
Monday, April 20, 2020 3:42PM  3:54PM On Demand 
T16.00002: Calculating the scalar selfforce experienced by extrememassratio binaries during $r\theta$resonances Zachary Nasipak, Charles R. Evans A vast majority of extrememassratio black hole binaries (EMRIs) will encounter at least one strong $r\theta$resonance as they evolve through LISA's passband. These resonances occur when the frequencies of the librating radial and polar motion of the EMRI's smaller body form a lowinteger ratio, and they drive significant `kicks' in the amount of energy and angular momentum that EMRIs radiate through gravitational waves. These kicks, if not properly accounted for, can amplify errors in modeled EMRI waveforms by factors of $\sim 100$. Despite the importance of modeling these resonant dynamics, researchers have not yet calculated the gravitational selfforce experienced by EMRIs during $r\theta$resonances. As a first step in quantifying these effects, we calculate the scalar selfforce (the scalar analog to the gravitational selfforce) experienced by a scalarcharged particle following an $r\theta$resonant geodesic around a Kerr black hole. We present how local and global radiationreaction effects vary with respect to initial conditions. We also demonstrate, numerically, that conservative selfforce effects do not contribute to the leadingorder evolution of the system, as hypothesized by previous researchers. [Preview Abstract] 
Monday, April 20, 2020 3:54PM  4:06PM 
T16.00003: An extendedbody approach to selfforce in curved spacetime Isaac Waldstein, David Brown Selfforce describes the effect of an object's own field on its motion. If we model a physical object as a point particle, then the selfforce diverges when evaluated at the particle's location. One way to avoid this difficulty is to model the physical object as an extended body with detailed internal structure. We consider an extended body, modeled as an elastic material, moving in an arbitrary background gravitational field. We express the spacetime coordinates of each point in the body in terms of the Fermi normal coordinates tied to a fiducial observer whose basis vectors are FermiWalker transported. We construct the action and equations of motion for the elastic body, expanding in powers of the spatial Fermi normal coordinates $\bar{x}^a$. A center of mass condition ties the motion of the fiducial observer to the motion of the elastic body. We show that (a) the elastic body follows a geodesic at zeroth order in $\bar{x}^a$ and (b) we recover the MathissonPapapetrouDixon (MPD) equations at first order in $\bar{x}^a$. In each case, the details of the body's elastic interactions are absorbed into the body's multipole moments. In future work, we will remove the restriction to background gravitational fields and derive the gravitational selfforce on the extended body. [Preview Abstract] 
Monday, April 20, 2020 4:06PM  4:18PM On Demand 
T16.00004: Second order perturbation of a Kerr black holes Justin Ripley, Elena Giorgi, Nicholas Loutrel, Frans Pretorius We present on progress to compute the second order metric perturbation of a Kerr black hole. Out motivation for pursuing this project includes (1) to understand the regime of applicability of the Teukolsky equation in describing the ringdown of Kerr black hole formed after the merger of two similar mass compact objects, and (2) to understand the proposed onset of "gravitational wave turbulence" around very rapidly spinning Kerr black holes. Our procedure for numerically computing the second order perturbation incorporates several observations made by researchers in the mathematical relativity community in their work towards a proof of the nonlinear stability of Kerr black holes. [Preview Abstract] 
Monday, April 20, 2020 4:18PM  4:30PM On Demand 
T16.00005: The Rotating Black Hole Interior: Insights from Gravitational Collapse in (2+1)D Alex Pandya, Frans Pretorius We address a hole in the space of existing toy models for the astrophysical black hole (BH) interior by simulating the gravitational collapse of \textit{rotating} matter in (2+1)D classical general relativity. We compare and contrast our timedependent numerical solution to the stationary analytic solution due to Ba\~nados, Teitelboim, and Zanelli (BTZ), as well as to the celebrated Kerr solution. We focus on three features in the dynamical case: the singularity structure, the regularity of the Cauchy horizon, and the geodesicfocusing effect first described by Marolf \& Ori. We observe the latter effect for the first time in a BH formed from gravitational collapse. We also find that curvature singularities form at the origin and Cauchy horizon for low spin, but disappear entirely for sufficiently high spins, signaling a violation of the $C^0$ and $C^2$ formulations of the strong cosmic censorship conjecture. [Preview Abstract] 
Monday, April 20, 2020 4:30PM  4:42PM Not Participating 
T16.00006: GaugeFixing Waveforms in Numerical Relativity Dante Iozzo, Michael Boyle, Saul Teukolsky Asymptotic waveforms of the gravitational wave strain and the Weyl scalars have infinitedimensional gauge freedoms expressed by the BMS group. In order to compare numerical relativity waveforms across different simulations, or even different grid resolutions of a simulation, it is critical to systematically understand and fix these gauge freedoms. This also will become an important factor for surrogate waveforms and phenomenological models that require numerical waveforms. We present preliminary results using a method for fixing these BMS gauge freedoms. [Preview Abstract] 
Monday, April 20, 2020 4:42PM  4:54PM On Demand 
T16.00007: Constraints on GW waveforms Tommaso De Lorenzo, Abhay Ashtekar, Neev Khera Gravitational waveforms for compact binary coalescences (CBCs) have been invaluable for detections by the LIGOVirgo collaboration. They are produced by powerful combinations of analytical approximations and numerical simulations. So far systematic errors arising from these approximations have been less than statistical errors. However, we are now entering an era of abundant detections, and the third generation observatories, as well as LISA, are on the horizon. Therefore, it is highly desirable to have more accurate waveforms. The goal of this talk is to show that full nonlinear general relativity (GR) imposes an infinite number of sharp constraints on the CBC waveforms. These can be used as additional measures both to evaluate the accuracy of candidate waveforms against exact GR, and to discriminate between various avenues currently used to generate waveforms. [Preview Abstract] 
Monday, April 20, 2020 4:54PM  5:06PM 
T16.00008: Relativistic Corrections for Asteroseismology to First PostNewtonian Order Reece Boston, Charles Evans In light of the precision available from the K2 and TESS missions in determining stellar pulsation periods, general relativistic effects are in principle measurable contributors in white dwarf light curves. We provide a demonstration of the appearance of general relativistic effects by considering initially simplified polytropic models for stars of mass and radius comparable to white dwarfs. Relativistic effects are calculated to sufficient accuracy using a first postNewtonian (1PN) correction to the standard Dziembowski form of the pulsation equations. Background models and linear nonradial pulsations are calculated in both the 1PN and Newtonian regimes and the periods of gmodes and pmodes are compared. Future work will include more astrophysically accurate white dwarf models with gmodes confined near the stellar surface. [Preview Abstract] 
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