Bulletin of the American Physical Society
APS April Meeting 2022
Volume 67, Number 6
Saturday–Tuesday, April 9–12, 2022; New York
Session Y16: Beyond Einstein Gravity: Theories, Modeling, and Tests IRecordings Available
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Sponsoring Units: DGRAV Chair: Shohreh Gholizadeh Siahmazgi, Wake Forest University Room: Sky Lobby |
Tuesday, April 12, 2022 1:30PM - 1:42PM |
Y16.00001: Extended dRGT massive gravity via cubic Galileon Yousef Izadi, Amin Rezaei Akbarieh, Shahabeddin M Aslmarand, Sobhan Kazempour, Lijing Shao There are many models that describe graviton and its interactions. However, finding a stable and observationally consistent theory that explains a massive spin-2 field remains a major concern. Recently, de Rham, Gabadadze, and Tolley constructed a ghost-free nonlinear massive gravity theory called dRGT massive gravity. Although this avoids the Boulware-Deser ghost associated with the nonlinear Pauli-Fierz theory, the dRGT massive gravity has instabilities in FLRW homogeneous spacetime. One way to resolve this problem is to couple an additional scalar field to the background, known as quasi-dilaton massive gravity. In this talk, we introduce an extension to dRGT massive gravity by a generalized quasi-dilaton theory in which the scalar field coupled to the graviton is a cubic Galileon and present the cosmology of this theory. We analyze the self-accelerating background solutions to explain the late-time acceleration of the Universe. We examine the cubic Galileon massive gravity via the latest Union2 Type Ia Supernovae (SNIa) dataset and show that the model is compatible with the observations. We also present a tensor perturbation analysis and obtain the dispersion relation of gravitational waves in this theory. |
Tuesday, April 12, 2022 1:42PM - 1:54PM |
Y16.00002: Cosmic Acceleration from String Induced Galileons Tucker D Manton, Damien A Easson, Andrew Svesko It has been shown a specific Horndeski theory of gravity arises from a consistent Kaluza-Klein reduction of the gravi-dilaton sector of the low-energy effective heterotic string action with a first α' correction. Here we provide a first investigation of the cosmological solutions to the lower dimensional theory by constructing exact solutions to the field equations in various frames. At tree level, in string frame, we find the duality symmetry of the parent theory is unaltered by dimensional reduction, leading to standard bouncing and pre-Big Bang scenario solutions. For α'≠0, we uncover exact models exhibiting cosmic acceleration, where we find interesting physics concerning the number of noncompact dimensions. Certain stability requirements demand the inclusion of a dilaton potential and for stable solutions, the gravitational wave speed exhibits damped oscillations about unity with initial amplitudes that are O(α'). |
Tuesday, April 12, 2022 1:54PM - 2:06PM |
Y16.00003: Square Peg in a Circular Hole: Choosing the Right Ansatz for Isolated Black Holes in Generic Gravitational Theories Yiqi Xie, Jun Zhang, Hector O Silva, Claudia de Rham, Helvi Witek, Nicolas Yunes The metric of a spacetime can be greatly simplified if the spacetime is circular. In this talk I will show that in generic effective theories of gravity, the spacetime of a stationary, axisymmetric, and asymptotically flat solution must be circular if the solution can be obtained as a perturbation of the general relativistic solution. This result applies to a broad class of gravitational theories that include arbitrary scalars and vectors in their light sector, so long as their nonstandard kinetic terms and nonmininal couplings to gravity are treated perturbatively. |
Tuesday, April 12, 2022 2:06PM - 2:18PM |
Y16.00004: Perturbations of spinning black holes beyond General Relativity: Modified Teukolsky equation I 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 us to test relativistic gravity in its strong, dynamical and nonlinear regime, thus opening a new arena to confront general relativity (and modifications thereof) against observations. We consider a theory that modifies general relativity by introducing a scalar field coupled to a parity-violating curvature term, known as dynamical Chern-Simons gravity. In this theory, spinning black holes are different from their general relativistic counterparts, and can thus serve as probes of this theory. We modify the Teukolsky formalism to obtain a set of linear coupled differential equations that describe dynamical gravitational and scalar perturbations of a rotating black hole in dynamical Chern-Simons gravity to leading order in spin. In this talk, I will present the modified Teukolsky formalism and the equations describing the evolution of dynamical gravitational and scalar perturbations. Additionally, I will describe the calculation of the black hole's quasi-normal mode frequencies to leading order in spin, and compare these to previously obtained results. This formalism lays down the foundations for the general extension of the calculation of quasi-normal mode frequencies for black holes that rotate arbitrarily fast in dynamical Chern-Simons gravity, therefore extending results valid in Petrov Type D backgrounds to Petrov type I backgrounds. |
Tuesday, April 12, 2022 2:18PM - 2:30PM |
Y16.00005: Perturbations of spinning black holes beyond General Relativity: Modified Teukolsky equation II Dongjun Li, Pratik K Wagle, Yanbei Chen, Nicolas Yunes Linear gravitational perturbations of Kerr black holes in general relativity are most efficiently treated by the Teukolsky formalism, which leads to single decoupled equations for Weyl scalars Ψ0 and Ψ4. These equations are further separable into radial and angular equations. The standard derivation of the Teukolsky equation in general relativity required the background spacetime to be algebraically special (Petrov type D). In beyond-General-Relativity (bGR) theories, for example, dynamical Chern-Simons (dCS) and Einstein-dilaton Gauss-Bonnet (EdGB) theories, spacetimes of rotating black holes are not type-D, but type-I instead. This lack of symmetry creates potential difficulties computing gravitational waveforms in bGR theories. In this work, for any stationary background spacetime with an order epsilon deviation from a Petrov type-D spacetime, we obtain a single decoupled modified Teukolsky equation for the perturbative Ψ0 (and Ψ4) of that spacetime --- accurate up to linear order in epsilon. This equation may also have a source term on the right-hand side due to matter (including, e.g., dCS and EdGB scalar-field) perturbations. Our derivation is an extension to Chandrasekhar's alternative derivation of the Teukolsky equation and his metric reconstruction procedure (both originally formulated for the Kerr spacetime). For demonstration, we apply our formalism to perturbations of slowly-rotating black holes in dCS gravity. In this case, Ψ0 (and Ψ4) are decoupled from all other space-time degrees of freedom, and only couple to the dCS scalar field, which arises due to the source term of the modified Teukolsky equation. |
Tuesday, April 12, 2022 2:30PM - 2:42PM |
Y16.00006: Black holes beyond General Relativity: stability and nonlinear evolution Aaron Held, Hyun Lim, Jun Zhang The dawn of gravitational-wave astronomy provides a unique opportunity to test General Relativity (GR) in the strong-field regime. Going beyond null tests, I will present recent progress to obtain strong-field predictions in Quadratic Gravity, i.e., for the leading-order EFT-corrections to GR. |
Tuesday, April 12, 2022 2:42PM - 2:54PM |
Y16.00007: Compact Binary Systems in Einstein-Aether Gravity Fatemeh Taherasghari, Clifford M Will In this work we use the post-Minkowskian approach to analyze the Einstein-Aether(AE)/Khronometric theories of gravity. These theories add a long-range vector field to the spacetime metric. The vector field is constrained to have unit norm in AE theory. This theory has been constrained using solar system and binary pulsar observations, but we want to study strong-field constraints using gravitational waves. We constrain one of the free parameters of the theory to guarantee that light and tensor gravitational waves propagate at the same speed, as seen in the multimessenger event GW170817. This leaves us with a three-parameter subset of the theory. We study the PN expansion of Einstein-Aether gravity using direct integration of the relaxed Einstein equations and we employ the usual conventions for treating non-spinning compact objects whose masses are sensitive to the vector field. We have found a field redefinition that converts the field equations into decoupled, flat-spacetime wave equations for each of the metric and vector perturbation components with right-hand sides that contain matter terms and field terms that are quadratic and higher order in the small perturbations, precisely paralleling the “relaxed Einstein equations” of general relativity. We will report on progress toward obtaining the near-zone metric and the equations of motion for compact binaries to 2.5 PN order. |
Tuesday, April 12, 2022 2:54PM - 3:06PM |
Y16.00008: Constructing a waveform template for Einstein-æther theory Kristen Schumacher, Scott E Perkins, Ashley Shaw, Kent Yagi, Nicolas Yunes Our ability to test gravity with gravitational wave detectors is limited by our ability to accurately construct a gravitational waveform template. Thus, it is important to construct such templates for modified theories of gravity. Einstein-æther theory is a particularly interesting modified theory of gravity because it is the most generic Lorentz-violating theory one can construct with one additional vector field and its first derivatives. As well as modifying the amplitude and phase of the gravitational wave, this theory contains scalar and vector polarizations in addition to the tensor polarizations of general relativity. Therefore, any waveform model that can describe gravitational waves in this theory must include extra polarizations in the detector response function, which can travel with speeds different than that of light. In this talk, I will describe the construction of such a waveform through modification of the general relativity IMRPhenomD_NRTidalv2 model (used by the LIGO/VIRGO Collaboration) and explain how recent calculations of the sensitivity for neutron stars in the theory make this possible. I will outline the current constraints on the theory and place further constraints with gravitational wave data. |
Tuesday, April 12, 2022 3:06PM - 3:18PM |
Y16.00009: Moiré Gravity Alireza Parhizkar, Victor M Galitski Twisted bilayer graphene is a rich condensed matter system, which allows to tune energy scales and electronic correlations. The low-energy physics of the resulting moiré structure can be mathematically described in terms of a diffeomorphism in a continuum formulation. We stress that twisting is just one example of moiré diffeomorphisms. Another particularly simple and experimentally relevant transformation is a homogeneous isomorphic strain of one of the layers, which gives rise to a nearly-identical moiré pattern (rotated by 90○ relative to the twisted structure). We further observe that low-energy physics of the strained bilayer graphene takes the form of a theory of fermions tunneling between two curved space-times. Conformal transformation of the metrics results in emergent “moiré energy scales”, which can be tuned to be much lower than those in the native theory. This observation generalizes to an arbitrary space-time dimension with or without an underlying lattice or periodicity and suggests a family of toy models of “moiré gravity” with low emergent energy scales. Motivated by these analogies, we present an explicit toy construction of moiré gravity, where the effective cosmological constant can be made arbitrarily small. |
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