Bulletin of the American Physical Society
APS April Meeting 2018
Volume 63, Number 4
Saturday–Tuesday, April 14–17, 2018; Columbus, Ohio
Session K13: Modified Gravity, Observables and Foundations |
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Sponsoring Units: DGRAV Chair: Nicolas Yunes, Montana State University Room: A224-225 |
Sunday, April 15, 2018 3:30PM - 3:42PM |
K13.00001: Hair Loss in Parity Violating Gravity Pratik Wagle, Nicolas Yunes, Lydia Bieri, David Garfinkle The recent detection of gravitational waves by the LIGO/Virgo collaboration has allowed for the first tests of Einstein's theory in the \textit{extreme gravity} regime, where the gravitational interaction is simultaneously strong, non-linear and dynamical. One such test concerns the rate at which binaries inspiral, or equivalently the rate at which the gravitational wave frequency increases, which can constrain the existence of hairy black holes. This is because black holes with scalar hair typically excite dipole radiation, which in turn leads to a faster decay rate and frequency chirping. In this talk, I will present a mathematical proof that scalar hair is \textit{not} sourced in vacuum, spherically symmetric spacetimes when considering extensions of Einstein's theory that break parity in gravity, focusing on dynamical Chern-Simons theory as a particular toy model. This result implies that the observational confirmation of the baldness of black holes cannot be used to constrain parity violation in gravity, unless the black holes observed are also spinning. [Preview Abstract] |
Sunday, April 15, 2018 3:42PM - 3:54PM |
K13.00002: The Illusion of the Ubiquity of Chaos Alejandro Cardenas-Avendano, Andres Gutierrez, Leonardo Pachon, Nicolas Yunes The recent gravitational wave observations by the LIGO/Virgo collaboration have allowed the first tests of General Relativity (GR) in the extreme gravity regime, when comparable-mass black holes and neutron stars collide. In addition, future space-based detectors, such as LISA, will allow tests of GR with gravitational waves emitted when a small black hole falls into a supermassive one in an extreme mass-ratio inspiral (EMRI). One particular test that cannot be carried out cleanly with ground-based instruments is the search for chaos, which is expected to be absent in two-body encounters described by GR. In this talk, I will discuss whether chaos is present in parity-violating modified theories of gravity, focusing in particular on dynamical Chern-Simons gravity. Given the fact that for such modified theories, exact solutions for isolated, spinning black holes are not yet available, I will show how the use of approximate solutions may lead one to believe that chaos is present, when in reality such chaotic behavior is an artifact of the truncation of the solution. I will present a method that allows us to identify whether numerical signatures of chaos are real or not. The studies presented here begin to lay the foundations for chaotic tests of GR with LISA observations of EMRIs. [Preview Abstract] |
Sunday, April 15, 2018 3:54PM - 4:06PM |
K13.00003: Deformations of extremal black holes in GR and from stringy interactions Baoyi Chen, Leo Stein Linear perturbation theory is a powerful toolkit for studying black hole spacetimes. However, the perturbation equations are hard to solve unless we can use separation of variables. In the Kerr spacetime, metric perturbations do not separate, but curvature perturbations do. The cost of curvature perturbations is a very complicated metric-reconstruction procedure. This procedure can be avoided using a symmetry-adapted choice of basis functions in highly symmetric spacetimes, such as near-horizon extremal Kerr. We focus on this spacetime, and (i) construct the symmetry-adapted basis functions; (ii) show their orthogonality; (iii) show that they lead to separation of variables of the scalar, Maxwell, and metric perturbation equations; and (iv) solve for the deformations to the near-horizon extremal Kerr metric due to two example string-inspired beyond-GR theories: Einstein-dilaton-Gauss-Bonnet, and dynamical Chern-Simons theory, in the weak-coupling limit. We find that the EdGB metric deformation has a curvature singularity, while the dCS metric is regular. From these solutions we compute orbital frequencies, horizon areas, and entropies. This sets the stage for analytically understanding the microscopic origin of black hole entropy in beyond-GR theories. [Preview Abstract] |
Sunday, April 15, 2018 4:06PM - 4:18PM |
K13.00004: Evanescent dark energy and dark matter Maurice Van Putten The cosmological distributions of dark energy and dark matter are identified with evanescent waves crossing the cosmological horizon. A detailed confrontation with cosmological data on the Hubble parameter gives $H_0=74.29\pm2.6$, consistent with Anderson & Riess (2017) and Guidorzi et al.(2017) from GW170817. This result derives from $\Lambda = \omega_0^2$ derived from the eigenfrequency $\omega_0 = \sqrt{1-q}H$, where $q$ is the deceleration parameter, based on the Gauss-Bonnet theorem. A further detailed consideration of galaxy dynamics against data from Lellie et al.(2016) and Genzel et al.(2017) points to a dark matter mass of less than $10^{-30}$eV. (Based on van Putten, 2017, ApJ, 848, 28; ApJ 837, 22.) [Preview Abstract] |
Sunday, April 15, 2018 4:18PM - 4:30PM |
K13.00005: Self-interactions in a model with composite gravitons Christopher Carone We consider a generally covariant theory of scalars minimally coupled to an auxiliary background metric, subject to the constraint of vanishing energy-momentum tensor. Eliminating the auxiliary metric leads to a non-polynomial, metric-independent action for the scalar fields. In the limit of a large number of physical scalars, a composite, massless spin-2 state, the graviton, may identified in a two-into-two scalar scattering process. We show that the emergent graviton's cubic self-interactions are consistent with those of Einstein's general relativity, up to higher-derivative corrections. We also demonstrate in a theory with more than one type of scalar that the composite graviton coupling is universal. [Preview Abstract] |
Sunday, April 15, 2018 4:30PM - 4:42PM |
K13.00006: Curved Backgrounds in Emergent Gravity Yiyu Zhou, Joshua Erlich, Shikha Chaurasia Field theories that are generally covariant but nongravitational at tree level typically give rise to an emergent gravitational interaction whose strength depends on a physical regulator. We consider emergent gravity models in which scalar fields assume the role of clock and rulers, addressing the problem of time in quantum gravity. We discuss the possibility of nontrivial dynamics for clock and ruler fields, and describe some of the consequences of the dynamics for the emergent gravitational theory. [Preview Abstract] |
Sunday, April 15, 2018 4:42PM - 4:54PM |
K13.00007: The Space-Time Between Einstein and Kaluza-Klein: Further Developments Charles Vuille The idea of a multilinear metric can be generalized to elements of a subalgebra of the space of tensor multinomials, where the latter is dense in the space of smooth nonlinear differential operators. Whereas Kaluza-Klein theories increase the number of dimensions of space-time in order to accommodate more physical fields, here the spacetime remains four-dimensional, but the type of operator generalized to those that are non-multilinear. Using a small subalgebra of this space, the metric and curvature can be generalized in a way that explicitly contains Einstein's original theory and Maxwell's equations, as well as a scalar field. Thus it bears a resemblance to Kaluza-Klein and Brans-Dicke theories, but has a different underlying mathematical structure. The theory and attempts at the Lagrangian formulation, together with some possible consequences, will be presented. [Preview Abstract] |
Sunday, April 15, 2018 4:54PM - 5:06PM |
K13.00008: Flat-Space Holography and Stress Tensor of Kerr Black Hole Yousef Izadi, Reza Fareghbal, Omid Baghchesaraei According to the AdS/CFT correspondence, gravity in an asymptotically AdS spacetimes has a conformal field theory dual. One of the perplexing concepts in theoretical physics is obtaining a comprehensive understanding of holography. To this end, it is of interest to explore whether holography exists beyond the known example of the AdS/CFT correspondence and for spacetimes other than the AdS. A correspondence has recently been proposed between asymptotically flat spacetimes and a contracted conformal field theory and it is known as Flat/CCFT. This correspondence will be reviewed. Then using Flat/CCFT dictionary by taking the flat limit from quasi-local stress tensor of four-dimensional Kerr-AdS black hole, a stress tensor for Kerr black hole in Boyer-Lindquist coordinate can be calculated. The proposed stress tensor yields the correct values for the mass and angular momentum of the Kerr black hole in spatial infinity and it is another confirmation for Flat/CCFT proposal. [Preview Abstract] |
Sunday, April 15, 2018 5:06PM - 5:18PM |
K13.00009: Vacuum energy in asymptotically flat spacetimes in 3 dimensions. Debraj Roy, Olivera Miskovic, Rodrigo Olea The vacuum energy limit of asymptotically flat spacetimes is calculated, based on the evaluation of the Noether charges. The action formulation used is three-dimensional asymptotically flat gravity based on a Chern–Simons formulation in the Poincaré group, corresponding to an Einstein–Hilbert term in the bulk, plus half of the Gibbons-Hawking term at the boundary. This is required for a well-defined and finite action principle under the conditions adopted. We obtain that the vacuum energy of this space has the same value as the one of the asymptotically flat limit of three-dimensional anti-de Sitter space. We also comment on an approach based on holonomies of the Chern-Simons connection used to model the solutions being considered. [Preview Abstract] |
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