Bulletin of the American Physical Society
APS April Meeting 2022
Volume 67, Number 6
Saturday–Tuesday, April 9–12, 2022; New York
Session H16: Beyond Einstein Gravity: Theories, Modeling, and Tests IIRecordings Available
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Sponsoring Units: DGRAV Chair: Leah Jenks, Brown University Room: 16th Floor Sky Lobby |
Sunday, April 10, 2022 10:45AM - 10:57AM |
H16.00001: The Blandford–Znajek Process in scalar Gauss–Bonnet and dynamicalChern–Simons gravity Nicolas Patino, Jameson Dong, Yiqi Xie, Alejandro Cardenas-Avendano, Charles F Gammie, Nicolas Yunes The origin of the energy behind the observed relativistic jets in several accreting objects has been, so far, best explained by the Blandford–Znajek process. Since this process depends on astrophysics (through the magnetosphere) and the theory of gravity (through the exterior black hole spacetime metric), studying its observational consequences may provide another way to learn about gravity in the strong-field regime. How is the Blandford–Znajek process modified in quadratic gravity theories? In this talk, I will show analytical solutions to the Blandford–Znajek process in two well-motivated quadratic gravity theories: scalar Gauss–Bonnet and dynamical Chern–Simons. The mechanism was found assuming the split-monopole configuration and under the small-coupling and slow-rotation approximations. I will finish by comparing our results with the general relativity prediction and discussing what they imply to tests of quadratic gravity theories through the observations of active galactic nuclei. |
Sunday, April 10, 2022 10:57AM - 11:09AM |
H16.00002: How Do Black Holes Grow Hair? Abhishek Hegade K R, Nicolas Yunes, Helvi Witek, Elias R Most, Jorge Noronha Black holes in certain modified gravity theories that contain a scalar field coupled to curvature invariants are known to possess (monopole) scalar hair while non-black-hole spacetimes (like neutron stars) do not. Therefore, as a neutron star collapses to a black hole, scalar hair must grow until it settles to the stationary black hole solution with (monopole) hair. In this talk, I will describe this process in detail and show that the growth of scalar hair is tied to the appearance and growth of the event horizon (before an apparent horizon forms), which forces scalar modes that would otherwise (in the future) become divergent to be radiated away. I will then outline a proof of this result for a large class of modified theories. I will also present the results for the temporal evolution of the scalar field numerically in scalar Gauss-Bonnet gravity in a collapsing neutron star background as an example to explain the physical process by which the scalar hair grows.These results suggest that the emission of a burst of scalar field radiation is a necessary condition for black hole formation in a large class of modified theories of gravity. |
Sunday, April 10, 2022 11:09AM - 11:21AM |
H16.00003: Analytic Estimates for Quasi-normal Mode Frequencies of Black Holes in scalar Gauss-Bonnet Gravity Kent Yagi, Hector O Silva, Kostas Glampedakis, Albert T Bryant The ringdown part of a gravitational waveform from a binary black hole merger is characterized by quasi-normal mode (QNM) frequencies. Although one needs to rely on numerical calculations to find accurate QNM frequencies, there are several analytic techniques available to obtain physical implications such as the relation between QNM frequencies and properties of null geodesics. Here, we adopt an eikonal approximation to derive analytic estimates of QNM frequencies for black holes in scalar-Gauss-Bonnet gravity. In this theory, a scalar field is coupled to the metric through a quadratic curvature (Gauss-Bonnet invariant) and the theory is a generalization to Einstein-dilaton Gauss-Bonnet gravity motivated by string theory. We find that the eikonal machinery leads to axial perturbation modes deviating from the general relativistic results. We show that this result is in agreement with an analysis of unstable circular null orbits around black holes in this theory, allowing us to establish the geometrical optics-null geodesic correspondence for the axial modes. For the polar modes, the tensor and scalar perturbation equations are coupled and the scalar-Gauss-Bonnet corrections lift the general relativistic degeneracy between scalar and tensorial eikonal quasinormal modes. In general, our analytic, eikonal QNM frequencies agree with numerical results with an error of ~10% in the regime of a small coupling constant. |
Sunday, April 10, 2022 11:21AM - 11:33AM |
H16.00004: Approximate inspiral-merger-ringdown waveforms for binary-black-hole mergers in extensions of general relativity David A Nichols Gravitational-wave tests of general relativity (GR) rely on gravitational waveform models which are produced in either a specific modified theory of gravity or which are generated in a more theory-agnostic framework [often by adding a set of deviation parameters to some (semi-)analytical waveform family that reduces to the GR waveform when the parameters are set to zero]. Generating theory-specific inspiral-merger-ringdown (IMR) waveforms can be computationally intensive, but they give definite predictions for the modified theory. Theory-agnostic waveforms are often much faster to generate, but they do not necessarily correspond to a waveform that would be produced in any specific extension of GR. We describe an adaptation to modified theories of gravity of a hybrid post-Newtonian and black-hole perturbation method that was used in GR to explain the qualitative features and physics of binary-black-hole mergers. The method could be used to generate approximate IMR waveforms in extensions of GR, to explain their qualitative features, or to tailor the parameter space of theory-agnostic waveforms to better match those from specific extensions of GR. |
Sunday, April 10, 2022 11:33AM - 11:45AM |
H16.00005: Comparing Merger and Ringdown Waveforms for Modified Gravity Theories Gabriel S Bonilla Binary black hole mergers are prime candidates when looking for deviations from GR in the strong-field regime. The parameterized post-Einsteinian (ppE) formalism provides a framework for constructing frequency domain GR waveforms into modified waveforms that account for leading PN order deviations predicted by a beyond-GR theory. These corrections are computed assuming a quasicircular inspiral, making the correction applicable only in the inspiral regime. We extend the ppE correction into the merger and ringdown regimes where beyond-GR corrections may also be expected. We compare theories which predict stronger corrections in the early inspiral against theories the predict corrections at merger, and these effects on parameter estimation. |
Sunday, April 10, 2022 11:45AM - 11:57AM |
H16.00006: Exotic compact object search results using an expanded template bank Richard George, Horng Sheng Chia, Thomas Edwards, Christian Setzer, Cody Messick, Aaron Zimmerman, Adam Coogan, Katherine Freese Systems that contain exotic compact objects can go undetected by current template-based gravitational wave searches, especially for objects with large spin-induced quadrupole moments. This is particularly true for low mass binaries, where phase evolution due to these quadrupole moments can be large. To remedy this, we have have carried out a matched-filter search of public gravitational wave data using GstLAL, and using templates which account for large dimensionless quadrupole parameters. To make the search feasible, we constructed reduced-dimensional models with high effectualness for our templates, and implemented a novel approach to creating random template banks. I will discuss our methods and preliminary results. |
Sunday, April 10, 2022 11:57AM - 12:09PM |
H16.00007: Tidal Deformabilities of Neutron Stars in scalar-Gauss-Bonnet Gravity and Their Applications to Multimessenger Tests of Gravity Alexander G Saffer, Kent Yagi The spacetime surrounding compact objects such as neutron stars and black holes provides an excellent place to study gravity in the strong, non-linear, dynamical regime. Here, the effects of strong curvature can leave their imprint on observables which we may use to study gravity. Recently, NICER provided a mass and radius measurement of an isolated neutron star, while LIGO/Virgo measured the tidal deformability of neutron stars. These measurements can be used to test the relation between the tidal deformability and compactness of neutron stars that are known to be universal in general relativity. In this talk, I will discuss how we constructed tidally-deformed neutron star solutions and expanded upon the observations of NICER and LIGO/Virgo to investigate universal relations in scalar-Gauss Bonnet gravity. We find that the relation between the tidal deformability and compactness remains to be mostly universal for a fixed dimensionless coupling constant. For the equations of state considered, it remains inconclusive whether one can place a meaningful bounds on scalar Gauss-Bonnet gravity with the new universal relations. However, we found a new bound from the mass measurement of the pulsar J0740+6620 that is comparable to other existing bounds from black hole observations. |
Sunday, April 10, 2022 12:09PM - 12:21PM |
H16.00008: Inferring tidal deformability in the Black Hole compactness limit Carl-Johan O Haster, Cecilia Chirenti One of the distinguishing features between Black Holes and other categories of astrophysical compact objects is that the gravitational multipole moments of nonrotating Black Holes are unaffected by the strong tidal forces from a binary companion near their gravitational-wave induced merger. |
Sunday, April 10, 2022 12:21PM - 12:33PM |
H16.00009: Tidal Deformability of Neutron Stars in Scalar Tensor Theories of Gravitation Stephanie M Brown, Badri Krishnan Gravitational waves from compact binary coalescence are valuable for testing theories of gravity in the strong field regime. Gravitational waves from binary neutron stars also lead to stringent constraints on the equation of state of matter at extreme densities. In this work, we explicitly calculate the fully relativistic l ≥ 2 tidal love numbers for neutron stars in scalar tensor theories of gravitation. Combined with a family of nuclear equations of state based on chiral effective field theory, we explore how the mass, radius, and tidal deformability relations differ from those of general relativity. We aim to apply this to the binary neutron star merger event GW170817 in order to obtain observational constraints on neutron star radius in these modified theories of gravity. |
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