Bulletin of the American Physical Society
APS April Meeting 2019
Volume 64, Number 3
Saturday–Tuesday, April 13–16, 2019; Denver, Colorado
Session B16: Modified Gravity 
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Sponsoring Units: DGRAV Chair: Leo Stein, University of Mississippi Room: Sheraton Grand Ballroom I 
Saturday, April 13, 2019 10:45AM  10:57AM 
B16.00001: Friedmannlike universes with torsion Dimitrios Kranas, Christos Tsagas, John David Barrow, Damianos Iosifidis We consider spatially homogeneous and isotropic cosmologies with nonzero torsion. Given the high symmetry of these universes, we adopt a specific form for the torsion tensor that preserves the homogeneity and isotropy of the spatial hypersurfaces. Employing both covariant and metricbased techniques, we derive the torsional analogs of the continuity, Friedmann and, Raychaudhuri equations. These formulae demonstrate how, by playing the role of the spatial curvature, or that of the cosmological constant, torsion can drastically change the standard evolution of Friedmannlike cosmologies. For instance, torsion alone can lead to exponential inflationary expansion. Also, in the presence of torsion, the Milne universe evolves very much like the de Sitter model. Turning our attention to static spacetimes, we find that there exist torsional analogs of the classic Einstein static universe, with all three types of spatial curvature. Finally, we use perturbative methods to test the stability of the static solution in the presence of torsion. Our results show that stability is possible, provided the torsion field and the universe's spatial curvature have the appropriate profiles. 
Saturday, April 13, 2019 10:57AM  11:09AM 
B16.00002: Parametrized PostEinsteinian Waveforms in Various Modified Theories of Gravity Sharaban Tahura, Kent Yagi Modified theories of gravity are important from both theoretical and observational point of view. Since most modified theories of gravity reduce to general relativity in the weakfield limit, strongfield tests are required to evaluate their feasibility. Gravitational wave observations can explore dynamical strongfield regime and hence are important in the context of testing modified theories of gravity. For the purpose of analysis, it is more efficient to take a modelindependent approach instead of testing each theory against observed data individually. Parametrized postEinsteinian formalism is one of such modelindependent formalisms to test gravity with gravitational wave observations and considers generic nonGR corrections to the gravitational wave amplitude and phase. In this talk, I will present parametrized postEinsteinian waveforms in various modified theories of gravity. In particular, I present not only phase corrections that were the main focus of previous literature, but also amplitude corrections in various theories for the first time. Using both amplitude and phase corrections, I will present the implications on fundamental pillars in general relativity from existing gravitational wave events like GW150914. 
Saturday, April 13, 2019 11:09AM  11:21AM 
B16.00003: Hyperbolicity in gravitational collapse in a modified gravity theory Justin L Ripley, Frans Pretorius We numerically study spherical gravitational collapse in a modified gravity theory, Truncated Einstein dilaton Gauss Bonnet (TEdGB) gravity. We find evidence that there are open sets of initial data for which the character of the system of equations changes from hyperbolic to elliptic type in a compact region of the spacetime. In these cases the evolution of the system, treated as a hyperbolic initial boundary value problem, develops a region where the equations of motion become illposed. No singularities or discontinuities are encountered on the corresponding effective "Cauchy horizon". Therefore it is not implausible that a wellposed formulation of TEdGB gravity (at least within spherical symmetry) may be possible if the equations are appropriately treated as mixedtype. 
Saturday, April 13, 2019 11:21AM  11:33AM 
B16.00004: Massive Gravity in (Anti) de Sitter Space Laura A Johnson, Kurt Hinterbichler, Claudia de Rham Massive gravity is a small deformation to general relativity, found by giving the graviton a mass. Given the mass range imposed by current experimental constraints, massive gravity will be indistinguishable from GR on shorter distance scales, like those relevant to the solar system, but for larger distances, like those on the order of the Hubble scale, the mass weakens the gravitational force leading to differences between the two theories. This makes massive gravity an appealing theory to explore, with the potential to give an accelerating universe without dark energy. In this work, we examine the properties of Massive Gravity in (anti) de Sitter. In de Sitter, the spacetime approximating our currently expanding universe, we examine cosmological and black hole solutions and their features. In the massless decoupling limit of massive gravity in anti de Sitter, a nonlinear Proca theory emerges, which has a new symmetry. We discuss the implications this has for the dual CFT theory through the AdS/CFT correspondence. 

B16.00005: ABSTRACT WITHDRAWN

Saturday, April 13, 2019 11:45AM  11:57AM 
B16.00006: Radial Acceleration and TullyFisher Relations in Conformal Gravity James G O'Brien, Thomas Chiarelli, Philip David Mannheim In 2016 McGaugh, Lelli and Schombert established a universal Radial Acceleration Relation for centripetal accelerations in spiral galaxies. Their work showed a strong correlation between observed centripetal accelerations and those predicted by luminous Newtonian matter alone. Through the use of the fitting function that they introduced, mass discrepancies in spiral galaxies can be constrained in a uniform manner that is completely determined by the baryons in the galaxies. Here we present a new empirical plot of the observed centripetal accelerations and the luminous Newtonian expectations, which more than doubles the number of observed data points considered by McGaugh et al. while retaining the Radial Acceleration Relation. If this relation is not to be due to dark matter, it would then have to be due to an alternate gravitational theory that departs from Newtonian gravity in some way. 
Saturday, April 13, 2019 11:57AM  12:09PM 
B16.00007: Limits on Leading Order Terms in $f(R,T)$ Gravity Taylor Ordines, Eric D Carlson We investigate the ideal gas approximation of Earth's atmosphere in the $f(R,T)$ modified theory of gravity, in which the gravitational Lagrangian is given by an arbitrary function of the Ricci scalar and the trace of the stressenergy tensor. We obtain a generic form for the gravitational field equations and derive the hydrostatic equation for Earth's atmosphere for leading order terms $f(R,T) = \frac{1}{16\pi} \left( R + 2\chi T \right)$. Based on the apparent accuracy of the 1976 U.S. Standard Atmosphere model, which varies no more than $10\%$ from observations, we find limits of $ \chi  \lesssim 1.9 \times 10^{13}$. 
Saturday, April 13, 2019 12:09PM  12:21PM 
B16.00008: Reexamining $f(R,T)$ Gravity Sarah B Fisher, Eric D Carlson In $f(R,T)$ gravity, the gravitational contribution to the action is generalized from $R$ to an arbitrary function of the curvature $R$ and the trace of the stressenergy tensor $T$. Such theories have been studied and limits place on the parameters using a variety of physical systems. But these studies rely on a nonphysical separation of the matter Lagrangian ${\cal L}_m$ from the gravitational contribution. We illustrate the artificial nature of this separation with a specific example and suggest a convention for avoiding such confusion in the future. 
Saturday, April 13, 2019 12:21PM  12:33PM 
B16.00009: Black Hole Hair Loss in Parity Violating Gravity Pratik K Wagle, Nicolas Yunes, David Garfinkle, Lydia Bieri The detection of gravitational waves by the LIGO/VIRGO collaboration has allowed for the first tests of Einstein's theory of general relativity in the extreme gravity regime where the interaction is simultaneously strong, nonlinear and dynamical. The presence of hairy black holes can be constrained by considering the rate at which binaries inspiral or the rate at which the gravitational wave frequency increases. Black holes with scalar hair would typically excite dipole radiation which in turn leads to a faster decay and frequency chirping. In this talk, I will provide a mathematical proof that the 1/r component of scalar charge sourced by an asymptotically flat, stationary and axisymmetric spacetime vanishes when considering parity violating extensions of Einstein's theory particularly with focus on the dynamical ChernSimons theory. 
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