Bulletin of the American Physical Society
APS April Meeting 2016
Volume 61, Number 6
Saturday–Tuesday, April 16–19, 2016; Salt Lake City, Utah
Session H18: Alternative Theories of Gravity |
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Sponsoring Units: GGR Chair: Ted Jacobson, University of Maryland Room: 251F |
Sunday, April 17, 2016 8:30AM - 8:42AM |
H18.00001: Are Scalar-Tensor Theories Ruled Out by Solar System Observations? David Anderson, Nicol\'as Yunes, Enrico Barausse With the advent of new observations that probe the strong-field regime, it is a crucial time to determine which extensions of General Relativity are worth constraining. Scalar-tensor theories are both simple and well-motivated alternatives, and some lead to an excitation of the scalar in the strong field (scalarization), while reducing to Einstein's theory in the weak field. Previous studies, however, have shown that, upon cosmological evolution, the scalar field is typically driven to values that are ruled out today by Solar System observations. In this talk, I resolve this issue by modifying scalar-tensor theories in such a way so that the scalar field is driven to values that do satisfy weak-field tests today after cosmological evolution. In addition, these theories preserve scalarization (spontaneous, dynamical, induced) in the strong field regime, for example allowing neutron stars to deviate significantly from the predictions of General Relativity. [Preview Abstract] |
Sunday, April 17, 2016 8:42AM - 8:54AM |
H18.00002: The Lore of the Hair Nicolas Yunes, Kent Yagi, Leo Stein Stars can be hairy beasts, especially in theories that go beyond Einstein's. In the latter, a scalar field can be sourced and anchored to a neutron star, and if the later is in a binary system, the scalar field will emit dipole radiation. This radiation removes energy from the binary, forcing the orbit to adiabatically decay much more rapidly than due to the emission of gravitational waves as predicted in General Relativity. The detailed radio observation of binary pulsars has constrained the orbital decay of compact binaries stringently, so much so that theories that predict neutron stars with scalar hair are believed to be essentially ruled out. In this talk I will explain why this ``lore'' is actually incorrect, providing a counter-example in which scalar hair is sourced by neutron stars, yet dipole radiation is absent. I will then describe what binary systems need to be observed to constrain such theories with future astrophysical observations. [Preview Abstract] |
Sunday, April 17, 2016 8:54AM - 9:06AM |
H18.00003: Kludge modified gravity inspiral-merger-ringdown waveforms: Testing gravitational-wave tests of general relativity Nathan Johnson-McDaniel, Abhirup Ghosh, Archisman Ghosh, Anuradha Samajdar, Parameswaran Ajith, Walter Del Pozzo We describe a variety of self-consistent modifications of the effective-one-body framework that yield kludge modified gravity inspiral-merger-ringdown (IMR) waveforms. These waveforms do not correspond to any particular modified theory of gravity, but offer parametrized deviations from general relativity in various regimes. They can thus be used to test the performance of various gravitational wave tests of general relativity (GR). As an example, we introduce the IMR consistency test, which tests for consistency between the estimations of the final mass and spin from the inspiral and merger-ringdown portions of a binary black hole waveform. We show that for reasonable source parameters and SNRs in Advanced LIGO, this test is able to detect a deviation from GR with high confidence for certain modifications of the GR energy flux that are not constrained by observations of the double pulsar. We also consider the performance of a parameterized test of GR on these kludge modified gravity waveforms. [Preview Abstract] |
Sunday, April 17, 2016 9:06AM - 9:18AM |
H18.00004: Recent Advances in Conformal Gravity James O'Brien, Spasen Chaykov In recent years, significant advances have been made in alternative gravitational theories. Although MOND remains the leading candidate among the alternative models, Conformal Gravity has been studied by Mannheim and O'Brien to solve the rotation curve problem without the need for dark matter. Recently, Mannheim, O'Brien and Chaykov have begun solving other gravitational questions in Conformal Gravity. In this presentation, we highlight the new work of Conformal Gravity's application to random motions of clusters (the original Zwicky problem), gravitational bending of light, gravitational lensing and a very recent survey of dwarf galaxy rotation curves. We will show in each case that Conformal Gravity can provide an accurate explanation and prediction of the data without the need for dark matter. Coupled with the fact that Conformal Gravity is a fully re-normalizable metric theory of gravity, these results help to push Conformal Gravity onto a competitive stage against other alternative models. [Preview Abstract] |
Sunday, April 17, 2016 9:18AM - 9:30AM |
H18.00005: Scalar Gravitational Waves Emil Mottola General Relativity receives quantum corrections relevant at macroscopic distance scales and near event horizons. These arise from the conformal scalar degree of freedom in the extended effective field theory (EFT) of gravity generated by the trace anomaly of massless quantum fields in curved space. Linearized around flat space this quantum scalar degree of freedom combines with the conformal part of the metric and predicts the existence of scalar spin-0 "breather" propagating gravitational waves in addition to the transverse tensor spin-2 waves of classical General Relativity. Estimates of the expected strength of scalar gravitational radiation from compact astrophysical sources are given. [Preview Abstract] |
Sunday, April 17, 2016 9:30AM - 9:42AM |
H18.00006: Extremal Black Holes in Dynamical Chern-Simons Gravity Leo Stein, Robert McNees, Nicolas Yunes Analytic solutions are rare and precious in general relativity, even more so in theories beyond GR. From them we can calculate observational signals of beyond-GR physics, but also learn about the analytic structure of theories, which can't be done with numerics. Despite over a decade of searching, nobody has yet succeeded in finding an analytic form for a generically rotating black hole in dynamical Chern-Simons gravity (dCS), the simplest beyond-GR theory which includes a parity-odd interaction. In this talk we present progress towards a full solution by considering the extremal limit. We treat dCS in the decoupling limit, taking corrections as small perturbations away from GR. We perturb about extremal Kerr, and develop mathematical insight into the techniques needed to construct solutions for general spin. We find closed-form analytic expressions for the scalar field and a formal solution for the trace of the metric deformation, both in Legendre series. The first three (four) modes of the series of the scalar (metric trace) suffice to ensure a fidelity of over 99\% relative to numerical solutions. The monopole of the metric trace contains a logarithmic divergence at the extremal Kerr horizon, which is likely hidden behind the perturbed dCS horizon. [Preview Abstract] |
Sunday, April 17, 2016 9:42AM - 9:54AM |
H18.00007: Parity horizons and chronology protection in shape dynamics Gabriel Herczeg Shape dynamics is a classical theory of gravity closely related to the canonical formulation of general relativity but which possesses "relativity of size" rather than "relativity of time". I introduce the notion of a parity horizon, and show that many simple solutions of shape dynamics possess them. I show that the event horizons of the known asymptotically flat black hole solutions of shape dynamics are parity horizons and that this notion of parity implies that these horizons possess a corresponding CPT invariance. I present three new solutions of shape dynamics with parity horizons and find that not only event horizons become parity horizons in shape dynamics, but observer-dependent horizons and Cauchy horizons do as well. The fact that Cauchy horizons become (singular) parity horizons suggests a general chronology protection mechanism in shape dynamics that prevents the formation of closed time-like curves. [Preview Abstract] |
Sunday, April 17, 2016 9:54AM - 10:06AM |
H18.00008: Derivation of Einstein--Cartan theory from general relativity Richard Petti This article presents a derivation of Einstein--Cartan theory from general relativity with no additional assumptions or parameters. The derivation begins with distributions of Kerr masses that converge to a continuum with constant densities of mass, momentum, and angular momentum. The limit includes torsion and the spin-torsion relationship of Einstein--Cartan theory. The construction of curvature and torsion is equivalent to definition of curvature with Cartan forms on fiber bundles. Advantages of Einstein--Cartan theory include accommodating exchange of classical intrinsic and orbital angular momentum and generation of inflation-like expansion in high density cosmological models. [Preview Abstract] |
Sunday, April 17, 2016 10:06AM - 10:18AM |
H18.00009: Determining Exact Solutions in a Class of Non-Riemannian Theories of Gravitation P.F. Kelly Long ago, Einstein proposed a non-Riemannian extension of general relativity founded upon the notion of a non-symmetric metric.\footnote{A.~Einstein, {\it The Meaning of Relativity}, $5^{\text{{th}}}$ ed., (Princeton, 1955).} Variations on this theme have been reconsidered a number of times since then, but each time interest has waned, owing, in part, to a paucity of phenomenologically viable exact solutions of these theories. Recent progress in the determination of isotropic exact solutions of an alternative theory of gravitation of this type will be described. [Preview Abstract] |
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