Bulletin of the American Physical Society
APS April Meeting 2018
Volume 63, Number 4
Saturday–Tuesday, April 14–17, 2018; Columbus, Ohio
Session B13: Alternative Theories of Gravity |
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Sponsoring Units: DGRAV Chair: Leo Stein, California Institute of Technology Room: A224-225 |
Saturday, April 14, 2018 10:45AM - 10:57AM |
B13.00001: Gravitational-wave polarizations beyond general relativity: recent results and future prospects Maximiliano Isi Polarizations are a fundamental property of the geometry of gravitational waves (GWs), determining the directions in which space is stretched and squeezed as the waves whiz by. With the detection of GW170814, we have obtained some direct evidence that GW polarizations are as Einstein predicted---but there is much more to be learned with future measurements. In this talk, I will review the basics of GW polarizations, summarize the implications of recent compact-binary detections, and outline future ways to study this basic property of gravity with ground-based detectors. [Preview Abstract] |
Saturday, April 14, 2018 10:57AM - 11:09AM |
B13.00002: Towards Scalar-Tensor Waveforms at Second post-Newtonian Order Anna Heffernan, Ryan Lang, Clifford Will With the birth of multi-messenger gravitational wave astronomy last year, we witnessed the first gravitational wave detection of a neutron star binary. Such a system will allow definitive tests of scalar-tensor theory in the strong regime - a test which will require scalar-tensor waveforms. Due to the non-vanishing dipole radiation emitted by the scalar field, the required waveforms are currently only known to 1PN order. By calculating the scalar field and equations of motion to 3PN order we can deliver the full 2PN requisite waveforms. We report on the on-going progress of this project. [Preview Abstract] |
Saturday, April 14, 2018 11:09AM - 11:21AM |
B13.00003: Eccentricity Matters: Improvement of Constraints on Jordan-Brans-Dicke-Fierz Theory with Gravitational Waves from Eccentric Compact Binary Inspirals Sizheng Ma, Nicolas Yunes Recent gravitational wave observations have allowed stringent new constraints on modifications to General Relativity in the extreme gravity regime.Â Although these observations were consistent with compact binaries with no orbital eccentricity, gravitational waves emitted in mildly eccentric binaries will probably be detected once detectors reach their design sensitivity. In this talk, I will show explicitly how orbital eccentricity can greatly enhance our ability to constrain deviations from General Relativity. Focusing on scalar-tensor theories, I will first present a closed-form frequency-domain model for gravitational waves emitted in the inspiral of mixed compact binaries. I will then use this model to estimate the accuracy to which the theory can be constrained through Fisher analysis, using both ground- and space-based detectors. [Preview Abstract] |
Saturday, April 14, 2018 11:21AM - 11:33AM |
B13.00004: Spin Precession in Dynamical Chern-Simons Gravity: A Complete Picture Nicholas Loutrel, Nicolas Yunes, Takahiro Tanaka Gravitational interactions between black holes in a binary system induce precession of the black holesâ€™ spin angular momenta and the orbital angular momentum. This effect is imprinted in an amplitude modulation of the observed gravitational wave signal from the binary, which may allow us to probe the fundamental nature of the gravitational interaction and constrain modified theories of gravity that have evaded current observations. One such theory, dynamical Chern-Simons gravity, modifies spinning black holes through the presence of additional scalar degrees of freedom. In this talk, I will present the modified spin precession equations in this theory, calculated using effective field theory and the post-Newtonian formalism. I will further comment on the existence of conserved quantities and the prospects of constructing spin precessing gravitational waveforms for use in constraining the theory. [Preview Abstract] |
Saturday, April 14, 2018 11:33AM - 11:45AM |
B13.00005: Slowly-Rotating Neutron Stars in Massive Bigravity Andrew Sullivan, Nicolas Yunes We study slowly-rotating neutron stars in ghost-free massive bigravity. This theory modifies general relativity by introducing a second, auxiliary but dynamical tensor field that non-linearly couples to matter through the physical metric tensor. We expand the field equations to linear order in slow rotation and numerically construct solutions in the interior and exterior of the star with a set of realistic equations of state. We calculate the physical mass function and find that this function asymptotes to a constant a distance away from the surface, whose magnitude is controlled by the ratio of gravitational constants. The Vainshtein-like radius at which the physical and auxiliary mass functions asymptote to is controlled by the graviton mass scaling parameter, and outside this radius, bigravity modifications are suppressed. We also calculate the frame-dragging metric function and find that bigravity modifications are typically small in the entire range of coupling parameters explored. We finally calculate both the mass-radius and the moment of inertia-mass relations for a wide range of coupling parameters and find that both the graviton mass scaling parameter and the ratio of the gravitational constants introduce large modifications to both. [Preview Abstract] |
Saturday, April 14, 2018 11:45AM - 11:57AM |
B13.00006: Regularization of instabilities in gravity theories Fethi M Ramazanoglu Known experimental bounds require any alternative theory of gravity to closely imitate general relativity (GR) in the weak-field regime. On the other hand, the limited precision of gravitational wave parameter estimation prefers modifications with large deviations in strong fields for ease of detectability. Spontaneous scalarization phenomenon in scalar-tensor theories is a well-known alternative to GR that satisfies both criteria. In this case, neutron stars grow large scalar fields due to a tachyonic instability that lead to order-of-unity deviations from GR, whereas they die off away from the star satisfying weak field limits. I will explain how the underlying mechanism of spontaneous scalarization, an instability regularized by nonlinear effects, can be generalized to other fields such as vectors, and other instabilities such as ghosts. This family of theories has the desired behavior in both weak and strong fields, and makes connections to Horndeski theories and massive gravity. These novel theories have the appeal of near-future detectability using gravitational waves, and I will discuss how compact object mergers can be used to test them. [Preview Abstract] |
Saturday, April 14, 2018 11:57AM - 12:09PM |
B13.00007: Quasi-normal modes of black holes in scalar-tensor theories with non-minimal derivative couplings Ruifeng Dong, Jeremy Sakstein, Dejan Stojkovic We study the quasi-normal modes of asymptotically anti-de Sitter black holes in a class of shift-symmetric Horndeski theories where a gravitational scalar is derivatively coupled to the Einstein tensor. The space-time differs from exact Schwarzschild-anti-de Sitter, resulting in a different effective potential for the quasi-normal modes and a different spectrum. We numerically compute this spectrum for a massless test scalar coupled both minimally to the metric, and non-minimally to the gravitational scalar. We find interesting differences from the Schwarzschild-anti-de Sitter black hole found in general relativity. [Preview Abstract] |
Saturday, April 14, 2018 12:09PM - 12:21PM |
B13.00008: A Conformal Gravity Approach to Universal Centripetal Accelerations of Spiral Galaxies James O'Brien, Thomas Chiarelli, Philip Mannheim In a recent article, McGaugh et al. explored the universal nature of centripetal accelerations of spiral galaxies as a new natural law. Their work showed a strong correlation between observed centripetal accelerations and those predicted by luminous matter alone. They explore a fitting function, which can serve to constrain the amount of dark matter in spiral galaxies in a uniform manner, which is completely determined by the baryons. Another possible explanation explored in the paper, is that new physics could be responsible for the close correlation between observation and luminous matter alone. In this work, we show that conformal gravity, a fourth order renormalizable metric theory of gravity, which has enjoyed success in fitting galactic rotation curves can provide a solution to the universal centripetal accelerations observed by McGaugh et al., without the need for any dark matter. [Preview Abstract] |
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