Bulletin of the American Physical Society
APS April Meeting 2020
Volume 65, Number 2
Saturday–Tuesday, April 18–21, 2020; Washington D.C.
Session T15: Tests of Modified GravityOn Demand

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Sponsoring Units: DGRAV Chair: David Garfinkle, Oakland University Room: Virginia B 
Monday, April 20, 2020 3:30PM  3:42PM On Demand 
T15.00001: Numerical Black Hole Solutions in Modified Gravity Theories: Axial Symmetry Case Andrew Sullivan, Nicolas Yunes, Thomas Sotiriou We extend recently developed numerical code to obtain stationary, axisymmetric solutions that describe rotating black hole spacetimes in a wide class of modified theories of gravity. The code utilizes a relaxed NewtonRaphson method to solve the full nonlinear modified Einstein's Equations on a twodimensional grid with a Newton polynomial finite difference discretization scheme. We validate this code by considering static and axisymmetric black holes in General Relativity. We obtain rotating black hole solutions in scalarGaussBonnet gravity with a linear (linear scalarGaussBonnet) and an exponential (EinsteindilatonGaussBonnet) coupling and compare them to known perturbative solutions. From these numerical solutions, we construct a fitted analytical model and study observable properties calculated from this model and the numerical results. [Preview Abstract] 
Monday, April 20, 2020 3:42PM  3:54PM Not Participating 
T15.00002: Principal Null Directions and Symmetries of Black Holes in Dynamical ChernSimons Gravity Caroline Owen, NicolĂˇs Yunes The gravitational waves emitted when a small compact object falls into a supermassive black hole depend sensitively on the orbital trajectory of the small object, which in turn depends on the properties of the spacetime generated by the supermassive black hole. The ability to test general relativity in this extreme gravity regime using gravitational waves motivates the mathematical study of the symmetries of black holes in modified theories of gravity. While the Kerr metric for a spinning black hole in general relativity possesses two distinct principal null directions, the still unknown analogous exact solution in dynamical Chern Simons gravity is thought to possess four. In this talk, I will present all principal null directions of such modified black holes in the slow rotation approximation. These directions will then allow us to write the metric in KerrSchild coordinates, discuss its Carter type, and investigate the existence of a new Killing tensor that would generate a fourth constant of the motion. [Preview Abstract] 
Monday, April 20, 2020 3:54PM  4:06PM 
T15.00003: Projected Constraints on EinsteindilatonGaussBonnet Gravity with NICER Observations Hung Tan, Hector Silva, Nicolas Yunes Due to unprecedented resolution of the Neutron Star Interior Composition Explorer (NICER), we can detect Xrays emitted from hot spots on the surface of certain neutron stars, and extract vital information, helping us understand what happens to matter when nuclear saturation density is exceeded. Besides the interior of neutron stars, we can also use NICER data to constrain parameters of modified theories of gravity, and determine how close to Nature the predictions of GR are. In this talk, I will explain how we can use Bayesian techniques to strongly constrain EinsteindilatonGaussBonnet gravity to a degree comparable to other gravitational wave constraints. [Preview Abstract] 
Monday, April 20, 2020 4:06PM  4:18PM On Demand 
T15.00004: Constraining the parameterized postEinsteinian framework with binary pulsar observations Remya Nair, Nicolas Yunes The parameterized postEinsteinian (ppE) framework provides a model independent way of searching for generic deviations from general relativity. In this talk we will present our bounds on the ppE parameters obtained from the postKeplerian measurements of binary pulsars and also discuss the effects of mass priors on these bounds. Our results can be further used as informative priors for other Bayesian studies, e.g. those involving measurements from gravitational wave observations. [Preview Abstract] 
Monday, April 20, 2020 4:18PM  4:30PM On Demand 
T15.00005: More than the sum of its parts: combining parameterized tests of extreme gravity Hector Silva, Nicolas Yunes The advent of the gravitational wave era is offering us a new arena to test Einstein's theory of General Relativity and to confront modifications to it against observations. On the one hand, these tests can place General Relativity on a firmer experimental ground, while on the other, any hint of a disagreement would imply a major revision in our understanding of gravity. In this presentation, we discuss recent results on a program to connect two parametrized formalism to test General Relativity, one in the strongfield regime of neutrons stars and another in the radiative regime of gravitational waves: the postTolmanOppenheimerVolkoff and the parametrizedpostEinsteinian formalisms. This unified framework open the doors towards theoryindependent tests of gravity using both astronomical observations of neutron stars and gravitational wave observations. [Preview Abstract] 
Monday, April 20, 2020 4:30PM  4:42PM On Demand 
T15.00006: Testing general relativity with black holes using Xray observations Cosimo Bambi Einstein's theory of general relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in weak gravitational fields. However, the strong field regime is still largely unexplored, and there are many modified and alternative theories that have the same predictions as Einstein's gravity for weak fields and present deviations only when gravity becomes strong. Astrophysical black holes are ideal laboratories for testing gravity in the strong field regime. In this talk, I will present the XSPEC models {\sc relxill\_nk} and {\sc nkbb}, which are designed for testing the metric around black holes by fitting, respectively, the reflection and the thermal component of the accretion disk. I will also show current constraints on possible new physics from the analysis of a few sources with these models. [Preview Abstract] 
Monday, April 20, 2020 4:42PM  4:54PM On Demand 
T15.00007: Perfect Fluids Analyzed in Separable $f(R,T)$ Gravity Sarah Fisher, Eric Carlson $f(R,T)$ gravity is a generalization of gravity where 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 $T$. We argue that whenever this function is separable and can be written in the form $f(R,T)=f_1(R) +f_2(T)$, the $f_2$ term can always be absorbed into the matter Lagrangian ${\cal L}_m$. How this is done is explicitly demonstrated for an arbitrary perfect fluid. [Preview Abstract] 
Monday, April 20, 2020 4:54PM  5:06PM On Demand 
T15.00008: Local Dynamics of Cosmological Scalar Fields Yukei Murakami, Dario Bettoni, Lavinia Heisenberg, Tomi Koivisto, Miguel ZumalacĂˇrregui Since the discovery of accelerating expansion of the universe, there have been numerous theories that attempt to explain this surprising phenomenon. Theories of Dark Energy can be tested on cosmological scales, but also on small scales, including Solar System tests and laboratory experiments on Earth. When analyzing theories in which dark energy is produced by a scalar field, it is usual to assume that the time variation of the scalar field in the Solar System is the same as the global, cosmological value. We test this hypothesis on a simple class of disformally coupled scalartensor theories by numerically evolving the scalar field around a spherical matter distribution. A nonnegligible disformal coupling significantly affects the relationship between the local (within the dense region) and global (asymptotically) regimes. The observed contrast in the local and global behavior suggests the need to revise usual assumptions about the evolution of scalar fields, as well as the constraints derived using those assumptions. [Preview Abstract] 
Monday, April 20, 2020 5:06PM  5:18PM 
T15.00009: Gravitation with a nonsymmetric metric tensor Richard Hammond General relativity with a nonsymmetric metric tensor is examined. It is shown the nonsymmetric part, in the weak field limit, is the potential of torsion, the origin of which is intrinsic spin. The torsion is of the string theory type, but gauge invariance is lost unless the electromagnetic field is introduced to preserve gauge invariance. [Preview Abstract] 
Monday, April 20, 2020 5:18PM  5:30PM On Demand 
T15.00010: GravitationalWave Versus XRay Tests of StrongField Gravity Alejandro CardenasAvendano, Sourabh Nampalliwar, Nicol\'as Yunes Electromagnetic observations of the radiation emitted by an accretion disk around a black hole, as well as gravitationalwave observations of coalescing binaries, can be used to probe strongfield gravity. In this talk, I will compare the constraints that these two types of observations can impose on theoryagnostic, parametric deviations from the Schwarzschild metric. I will show that current gravitational wave observations have already placed constraints on the metric deformation parameters than are slightly more stringent than what can be achieved with Xray instruments in the near future. Moreover, I will show that future gravitational wave observations with aLIGO at design sensitivity by 2026 will be even more stringent, in particular becoming stronger that constraints achievable with future ATHENA Xray observations in 2034. [Preview Abstract] 
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