Bulletin of the American Physical Society
APS April Meeting 2023
Volume 68, Number 6
Minneapolis, Minnesota (Apr 15-18)
Virtual (Apr 24-26); Time Zone: Central Time
Session M09: Theories Beyond General Relativity |
Hide Abstracts |
Sponsoring Units: DGRAV Chair: Aaron Zimmerman, University of Texas at Austin Room: Conrad B/C - 2nd Floor |
Monday, April 17, 2023 10:45AM - 10:57AM |
M09.00001: A tensorial approach to a beyond-GR Modified Teukolsky Equation Asad Hussain, Aaron Zimmerman The increasing precision of gravitational wave detectors has enabled even more precise tests of general relativity, including spectroscopic tests of black holes through the measurement of their quasinormal modes (QNMs). These spectroscopic tests ideally compare the QNM frequencies to predictions from theories beyond GR, where the lack of a separable wave equation prevents computing quasinormal modes. We approach this problem by directly analyzing the tensorial form of the equations of motion in general beyond-GR theories, then later projecting to a new shifted Teukoslky equation. We also show how this can be used to compute the shifts in the gravitational ringdown spectrum in beyond-GR theories. |
Monday, April 17, 2023 10:57AM - 11:09AM |
M09.00002: Perturbations of spinning black holes in dynamical Chern Simons gravity I: Parity breaking Dongjun Li, Asad Hussain, Pratik K Wagle, Yanbei Chen, Nicolas Yunes, Aaron Zimmerman The extension of Teukolsky formalism from General Relativity (GR) to modified gravity allows the study of gravitational perturbations of black holes with general spin in a wide class of beyond-GR (bGR) theories. Modified Teukolsky equations were found for bGR Petrov type I spacetimes which deviate not significantly from Petrov type D spacetimes in GR. This modified Teukolsky formalism can potentially be used to study these parity-breaking bGR theories, where the quasinormal modes (QNMs) of even and odd parity modes are shifted differently, such as dynamical Chern-Simons (dCS) and Einstein-dilaton Gauss-Bonnet (EdGB) theories. However, unlike metric perturbations, the (modified) Teukolsky equations do not naturally have definite parity. In this work, we first introduce how the definite-parity modes of Teukolsky equations are defined in GR and then show that this definition can be extended to modified gravity. We then derive the equations of motion of these definite-parity modes from the modified Teukolsky equations. We observe that parity is generally broken since the source terms in the modified Teukolsky equations usually mix modes with different frequencies. As a demonstration, we compare the parity-breaking structure of several simple examples in dCS and EdGB to the results using metric perturbations. We also briefly illustrate how to evaluate the shift of QNMs of these definite-parity modes within our modified Teukolsky formalism. |
Monday, April 17, 2023 11:09AM - 11:21AM |
M09.00003: Perturbations of spinning black holes in dynamical Chern-Simons gravity II: master equations Pratik K Wagle, Dongjun Li, Nicolas Yunes, Yanbei Chen The detection of gravitational waves from compact binary mergers by the LIGO/Virgo collaboration has, for the first time, allowed for tests of relativistic gravity in the strong, dynamical and nonlinear regime. Outside Einstein's relativity, spinning black holes may be different from their general relativistic counterparts, and therefore their merger may lead to a modified ringdown spectrum. Last year, we developed a new formalism to derive a modified Teukolsky equation, i.e., a set of linear, decoupled differential equations that describe dynamical perturbations of non-Kerr black holes for the radiative Newman-Penrose scalars $Psi_0$ and $Psi_4$. In this talk, I will focus on: (1) applying our new formalism to slowly rotating black holes in dynamical Chern-Simons gravity; (2) present the master equation in the null tetrad basis; (3) discuss the application of metric reconstruction and perturbation scheme procedures involved in decomposing the master equation to the coordinate basis; (4) and lastly, present the master equations in terms of the radial and angular coordinates. These equations can then be integrated to obtain the QNM spectra. Our work enables the calculation of the QNM spectra, thus laying the foundations to study the gravitational waves emitted in the ringdown phase of black hole coalescence in modified gravity for black holes of any spin. |
Monday, April 17, 2023 11:21AM - 11:33AM |
M09.00004: Pathology of self-interacting vectors Fethi M Ramazanoglu, Andrew Coates Various groups have recently shown that theories of vector fields with couplings beyond the Proca mass, i.e., self-interacting vectors, are pathological. The problem manifests when the field norm reaches high-but-finite values, beyond which well-posed time evolution ceases to exist. The underlying reason is that the dynamics of the field is not governed by the spacetime metric, but by an effective one. The latter metric depends on the vector field itself, hence can become singular even when the spacetime is regular. This result suggests a reevaluation of fundamental theories that features such vector fields, which are numerous in gravity and cosmology. We will detail the underlying reason for the ill-posed behavior and the subtleties in its identification. We will also discuss how it can be avoided using an effective field theory picture. |
Monday, April 17, 2023 11:33AM - 11:45AM |
M09.00005: Equations of motion of Compact Binary Systems in Einstein-Aether Gravity Fatemeh Taherasghari, Clifford M Will In this study, we analyze the Einstein-Aether(AE)/Khronometric theories of gravity using the post-Minkowskian approach.These theories extend the spacetime metric by adding a long-range vector field. In AE theory, the vector field is constrained to have a unit norm. Solar system and binary pulsar observations have been used previously to constrain this theory; here we intend to explore strong-field constraints using gravitational waves. To ensure that light and tensor gravitational waves propagate at the same speed as observed in the multimessenger event GW170817, we constrain one of the theory’s free parameters. This leaves us with a three-parameter subset of the theory. We adapt the formalism of the Direct Integration of the Relaxed Einstein Equations to these theories and obtain the post-Newtonian (PN) expansion of the fields to 2.5 PN order. We use the convention devised by Eardley to treat non-spinning compact objects with masses that are sensitive to the vector field. We have generalized the Eardley approach to a wide class of alternative theories of gravity with a range of scalar, vector and tensor auxiliary field couplings to the masses. We will report on progress toward obtaining the near-zone metric and the equations of motion for compact binaries to 2.5 PN order. |
Monday, April 17, 2023 11:45AM - 11:57AM |
M09.00006: Gravitational-wave energy and other fluxes in ghost-free bigravity Leo C Stein, Alexander M Grant, Alexander G Saffer, Shammi Tahura Energy and angular momentum fluxes are key ingredients for making binary waveforms. Identifying the appropriate energy functional is unclear in Hassan-Rosen bigravity, a ghost-free theory with one massive and one massless graviton. The difficulty arises from new degrees of freedom and length scales not present in GR, rendering an Isaacson-style averaging calculation ambiguous. We compute the energy carried by gravitational waves (GWs) in bigravity starting from the action, using the canonical current formalism. The canonical current agrees with other common energy calculations in GR, and is unambiguous (modulo boundary terms). This makes it a good choice for quantifying the GW energy in bigravity or any diff-invariant theory of gravity. Our calculation opens the door for future waveform modeling in bigravity to correctly include backreaction due to emission of gravitational waves. |
Monday, April 17, 2023 11:57AM - 12:09PM |
M09.00007: Where and why does Einstein-scalar-Gauss-Bonnet theory break down? Abhishek Hegade K R, Nicolas Yunes, Justin L Ripley In this talk, I will present a systematic exploration of the loss of predictivity in Einstein-scalar-Gauss-Bonnet (ESGB) gravity in spherical symmetry. I will first present a gauge covariant method of characterizing the breakdown of the hyperbolicity of the equations of motion in the theory. With this formalism, I will show that strong geodesic focusing leads to the breakdown of hyperbolicity, and the latter is unrelated to the violation of the null convergence condition. I then present numerical studies of the loss hyperbolicity of the equations during gravitational collapse for a version of the theory that admits "spontaneously scalarized'' black holes. I will devise a "phase space'' model to describe the end states for a given class of initial data. Using this phase space picture, I will demonstrate that the theory remains predictive (hyperbolic) for a range of GB couplings. The range of couplings, however, is small, and thus, the presence of "spontaneously scalarized'' solutions requires fine-tuning of initial data. These results, therefore, cast doubt as to whether scalarized black hole solutions can be realistically realized in Nature even if ESGB gravity happened to be the correct gravitational description. |
Monday, April 17, 2023 12:09PM - 12:21PM |
M09.00008: Lyapunov Exponents in Modified Gravity Alexander Deich, Nicolas Yunes, Charles F Gammie Light-rings are key targets for near-future space-based VLBI missions. The ratio of flux measured between successive light-rings is characterized by the Lyapunov exponents of the corresponding nearly-bound null geodesics. Therefore, understanding Lyapunov exponents in this environment is of crucial importance to understanding black hole observations in general, and in particular, they offer a route for constraining modified theories of gravity. While recent work has made significant progress in describing these geodesics for Kerr, a theory-agnostic description is complicated by the fact that Lyapunov exponents are time-parameterization dependent, which necessitates care when comparing the Lyapunov exponents of null geodesics in two different theories. In this work, we present a numerical framework for comparing the Lyapunov exponents of null geodesics in Kerr with those in arbitrarily modified theories, and then present the results of calculating the Lyapunov exponents for null geodesics in two particular effective theories, sGB and dCS. |
Monday, April 17, 2023 12:21PM - 12:33PM |
M09.00009: Regularizing Parameterized Kerr Spacetime Samantha Lomuscio Probing gravity in extreme environments with strong and dynamic gravitational fields, such as those around rotating black holes, can allow for small deviations from General Relativity (GR) to be amplified and detected. Various parameterized Kerr spacetimes have been proposed to perform strong-field tests with black hole observations in a theory-agnostic way. These spacetimes consist of arbitrary functions of the radial coordinate that capture deviations from Kerr in GR. Practically, one expands these functions about infinity and truncates to extract a finite number of deviation parameters. We find this truncation can introduce pathologies such as nonphysical divergences. To overcome this, we treat the non-GR deviations as small perturbations in one of the parameterized Kerr spacetime, expand, and keep to linear order in the deviation. We then map black hole solutions in several example non-GR theories to the refined parameterized metric and quantify how well the latter can recover the former with a root mean square error analysis. We find that the new expansions of the functions have remedied the fictitious divergences seen with the original expansion. Additionally, we find overall the parameterized metric does fairly well at recovering the beyond-GR metrics, the root-mean-square error is smaller than about 0.1 in most cases, and can even recover some non-GR solutions exactly. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700