Session U14: Tests of General Relativity and Gravitation

Measuring Gravitational-Wave Polarizations with the Stochastic Background

The observation of gravitational waves by the LIGO and Virgo experiments has enabled many novel tests of general relativity (GR). One test of GR that has so far proven difficult, however, is verification of the polarization of gravitational waves. While GR predicts only two gravitational-wave polarizations (the plus and cross modes), generic metric theories can allow up to four additional polarizations. Current detectors, though, have limited ability to discern the polarization of transient signals, like those from neutron star and black hole mergers. In this talk, I will discuss an alternative means of revealing gravitational-wave polarizations -- observation of the stochastic gravitational-wave background. In particular, I will describe methods with which to detect the stochastic background and directly measure its polarization content.   

Testing astrophysical black holes using X-ray reflection spectroscopy

Einstein's gravity has been extensively tested in the weak field regime, mainly with experiments in the Solar System and observations of binary pulsars, and current data well agree with theoretical predictions. On the contrary, strong gravity is largely unexplored and there are a number of theories beyond Einstein's gravity having the same predictions for weak fields and presenting deviations only when gravity becomes strong. The best laboratory for testing strong gravity is the spacetime around astrophysical black holes. X-ray reflection spectroscopy can be a powerful tool to probe the strong gravity region around astrophysical black holes and test the nature of these objects. In this talk, I will present \verb7RELXILL_NK7, which is the first XSPEC reflection model to test Einstein's gravity in the strong field regime, and I will show some preliminary results from {\it XMM-Newton}, {\it NuSTAR}, and {\it Suzaku} data.   

Tests of General Relativity Through Gravitational Waves

General Relativity and the Standard Model are two fundamental, successful theories in physics. Yet the inability so far to successfully combine these two theories provides motivation to search for violations or extensions that may help lead to their unification. In particular, we discuss tests of Lorentz symmetry with gravitational waves from LIGO/VIRGO data.   

Constraining Einstein-\AE{}ther Theory with Eccentric Gravitational Waves

With the recent detection of gravitational waves from a variety of compact binaries, new ways to test modified gravity are quickly being developed. One interesting possibility is to constrain the violation of Lorentz symmetry in the gravitational sector, an effect that is inherent in Einstein-\AE{}ther theory through a dynamical \ae{}ther field that selects a preferred direction in spacetime. Developing eccentric waveform models in this theory will allow us to test its predictions against observations. In this talk, I will discuss how we have approached the development of these waveforms and what the results will mean for the future of Einstein-\AE{}ther theory.   

Do Solar System Observations Kill Scalarization in Neutron Stars?

Scalar-tensor theories of gravity are some of the most studied alternatives to Einstein’s General Relativity (GR). Scalar-tensor theories are both simple, well motivated, and conveniently believed to satisfy weak field tests while allowing for substantial deviations from GR in the strong field regime, particularly around neutron stars. In this talk, I will discuss how the cosmological evolution of the scalar field leads to modified observables today that are at odds with Solar System tests precisely in the regime of parameter space in which strong field deviations would occur.   

Testing Gravity on Cosmological Scales using Strong Gravitational Lensing

Time delays of strongly lensed quasars have long been used to measure either the mass of the lensing galaxy or cluster, or the Hubble parameter, after assuming a value for one or the other from independent measurements. Given the growing number of precise time-delay measurements, we study the possibility of strong lensing unveiling finer information, namely serving as a test of Einstein gravity on cosmological scales. In this context, we present a phenomenological model of gravitational screening, where the classic solar system PPN curvature parameter gamma is promoted to a step function in space, dividing the galactic or cluster dark matter halo into an inner GR region, and an outer non-GR region extending to extra-galactic space. We confront our model with available data, and pave the way for upcoming observations and more sophisticated analyses.   

Phenomenology of Holometer Cross-Correlations in Relational Emergent Space-Time

A Lorentz invariant framework is presented of exotic cross-correlations in the signals of two separate Michelson interferometers with bent arms, associated with the emergence of space-time and inertial frames from a Planck scale quantum system. Space-time relationships are modeled as antisymmetric cross-covariances on past and future light cones between world lines of Planck bandwidth in proper time, arising from nonlocal entanglement in geometrical states. Causal diamonds in a flat space-time are normalized to have the same holographic information content as black hole event horizons. The phenomenology produces a unique signature: a mostly imaginary broad-band cross-spectrum that is acausal in standard physics, with a frequency response derived from the layout and causal structure of the physical system. The framework will be used to interpret data from the reconfigured Fermilab Holometer and guide conceptual design of future experiments.   

The Gravitational Compass: new space-based detection methods for circularly polarized gravitational waves

We investigate the sensitivity of space-based gravitational wave observatories based on the platonic solids -- tetrahedron, cube and octohedron -- to a circular polarization of an isotropic stochastic gravitational wave background (ISGWB) as a function of frequency. For such a detector geometry, each vertex is a drag-free satellite and each edge is a laser interferometer arm. Extrapolating the noise model from LISA to these new geometries, we find that these designs can lead to an increase in sensitivity to an ISGWB.   

Expected Along-Track Geopotential Resolution for the GRACE Follow-On Mission.

The GRACE Follow-On Mission (GFO) is scheduled for launch early in 2018. Changes in the separation between the 2 satellites will be measured to determine variations along track in the geopotential height. The microwave system for measuring the separation will have about the same accuracy as for the GRACE Mission, but there also will be an experimental laser interferometry system with roughly a factor 20 lower measurement noise requirement. And the expected low frequency noise level from the accelerometers on the GFO satellites is about a factor 3 lower than for GRACE. However, the accuracy of global maps of the geopotential every 30 days is not expected to improve much because of the time variations in the geopotential during that time. Still, the improved along-track geopotential variation accuracy is expected to be scientifically valuable. This is partly because it permits analyses of time variations at short wavelengths from other data sources to be evaluated. The accuracy improvement expected for along-track wavelengths of roughly 2,000 km and shorter will be described. The analysis has been done using only frequencies of 1.25 cy/rev and lower in correcting for the acceleration noise, and 4 rev data arcs.