Session T15: Tests of Modified Gravity

On Demand

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 Newton-Raphson method to solve the full nonlinear modified Einstein's Equations on a two-dimensional 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 scalar-Gauss-Bonnet gravity with a linear (linear scalar-Gauss-Bonnet) and an exponential (Einstein-dilaton-Gauss-Bonnet) 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.   

Not Participating

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 Kerr-Schild coordinates, discuss its Carter type, and investigate the existence of a new Killing tensor that would generate a fourth constant of the motion.   

Projected Constraints on Einstein-dilaton-Gauss-Bonnet Gravity with NICER Observations

Due to unprecedented resolution of the Neutron Star Interior Composition Explorer (NICER), we can detect X-rays 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 Einstein-dilaton-Gauss-Bonnet gravity to a degree comparable to other gravitational wave constraints.   

On Demand

The parameterized post-Einsteinian (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 post-Keplerian 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.   

On Demand

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 strong-field regime of neutrons stars and another in the radiative regime of gravitational waves: the post-Tolman-Oppenheimer-Volkoff and the parametrized-post-Einsteinian formalisms. This unified framework open the doors towards theory-independent tests of gravity using both astronomical observations of neutron stars and gravitational wave observations.   

On Demand

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.   

On Demand

$f(R,T)$ 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 stress-energy 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.   

On Demand

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 scalar-tensor theories by numerically evolving the scalar field around a spherical matter distribution. A non-negligible 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.   

Gravitation with a non-symmetric metric tensor

General relativity with a non-symmetric metric tensor is examined. It is shown the non-symmetric 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.   

On Demand

Electromagnetic observations of the radiation emitted by an accretion disk around a black hole, as well as gravitational-wave observations of coalescing binaries, can be used to probe strong-field gravity. In this talk, I will compare the constraints that these two types of observations can impose on theory-agnostic, 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 X-ray 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 X-ray observations in 2034.