Session S17: Tests of General Relativity with Gravitational Waves II

Live

While the test particle's geodesic motion around a Kerr black hole is fully integrable, it may not be in modified theories of gravity. What are the observational consequences of observing an EMRI system that is not integrable through gravitational waves? In this talk, I will show how a truncated integrable system leads to chaotic behavior imprinted in the motion's fundamental frequencies' temporal evolution, and discuss its detectability with the future space-based detector LISA. To show how chaos affects the gravitational waves emitted, we have systematically analyzed the Fourier transform of approximate gravitational waveforms computed in the semi-relativistic approximation, augmented with gravitational wave dissipation, on a slowly-rotating Kerr background. Since signatures of chaotic dynamics in gravitational waves have been suggested to test general relativity in the strong field, if these effects are not correctly modeled and understood, they may undermine present proposals to verify the no-hair theorem's assumptions.   

Live

The theory of General Relativity has successfully passed a large number of observational tests. The theory has been extensively tested in the weak-field regime with experiments in the Solar System and observations of binary pulsars. The past five years have seen tremendous progress in the study of the strong-field regime, which can now be tested with gravitational waves, X-ray data, and mm and sub-mm Very Long Baseline Interferometry observations. In my talk, I will summarize the state-of-the-art of the tests of General Relativity with black hole X-ray data, discussing its recent progress and future developments.   

Live

Gravitational wave astronomy is allowing for a host of exciting new tests of fundamental physics, especially with regards to the gravitational interaction. To be better informed and prepared for the future, the scientific community needs accurate predictions concerning the effectiveness of various proposed detector networks. The efficacy of these tests of gravity depends sensitively on many details, but particularly on the distributions of merging binary black holes in Nature and the sensitivity and location of detectors over the next several decades. In this presentation, I will discuss a suite of simulations we have constructed to address this need for accurate modeling. To establish the accuracy of these simulations, the various components involved will be outlined, including the different population models we have used, the detector networks under consideration, and the statistical methodology employed. Finally, a brief summary of the most interesting results will be presented. These conclusions relate to the importance of future detector upgrades and proposed facilities, the importance of single high-SNR events as opposed to large catalogs of comparable-SNR events, and how these deductions change when considering specific theories as opposed to generic parameterizations.   

Live

The detection of gravitational waves from compact binary mergers by the LIGO/Virgo collaboration has, for the first time, allowed us to test general relativity in the strong-field regime and to place new constraints on extensions to Einstein's theory. Here we consider a theory which modifies general relativity by introducing a scalar field coupled to a parity violating Chern-Simons term known as dynamical Chern-Simons gravity. This theory predicts rotating black holes solutions which are different from those of general relativity and could consequently leave imprints in gravitational wave signals. Here we study the linear perturbations of slowly rotating black holes in dynamical Chern-Simons gravity. Working in an effective-field theory approach, we analyze their stability and calculate their quasinormal mode spectra. I will present a brief summary of these studies, their implications and discuss how this could impact the currently placed constraints on dynamical Chern-Simons gravity.   

Live

General Relativity predicts that gravitational waves (GWs) travel at the speed of light c. Bounds on the speed with which GWs propagate can be expressed in the context of the time of flight for the graviton, the particle which carries the gravitational force. Experimental measurements can be performed with LISA to compare light propagation speed vs GW propagation speed. This would be done utilizing eclipsing white dwarf binaries such as J0651 or ZTF J1539. The phase of the light curve would be compared to the GW phase determined by LISA to see if a phase difference is present. A phase difference would imply a non-zero graviton mass and thus favor certain quantum theories of gravity. No phase difference would be consistent with a graviton mass of zero, be in agreement with General Relativity and additionally would constrain the possible maximum mass within certain bounds. Simulated galaxies using the COSMIC population synthesis code are used to produce simulated LISA catalogues of white dwarf binaries. The properties of the simulated catalogue are used to model the accuracy of GW and electromagnetic observations to understand how well the graviton mass can be bounded from expected observations.   

Live

The spacetime surrounding compact objects provides an excellent place to study gravity in the strong, non-linear, dynamical regime. Here, the effects of strong curvature can leave their imprint on observables which we may use to study gravity. Recently, the Neutron Star Interior Composition ExploreR (NICER) provided mass and radius relations of an isolated neutron star (NS). These measurements, combined with tidal deformability gleaned from GW170817 have aided in the understanding of neutron stars (NS) in general relativity (GR). However, modified theories of gravity may behave differently in the strong field regime, leading to results which may differ from those in Einstein's theory. Here, we focus on comparing the results mentioned with new theoretical corrections to GR obtained from scalar-Gauss-Bonnet (sGB) gravity. Our goal is to determine whether the mass-radius relations as well as the Love-compactness relations can help constrain sGB given the observations from NICER and LIGO. In this talk, I will present the results of our study of sGB gravity and how these compare with the information gathered from NICER and LIGO. In addition to this, I will discuss the usefulness of this approach in placing constraints on sGB theory.   

Live

The observation of GW170817 binary neutron star (BNS) merger event has imposed strong bounds on the speed of gravitational waves (GWs) locally, inferring that the speed of GWs propagation is equal to the speed of light. Future GW detectors designer projects will be able to detect many coalescences of BNS at high $z$, such as the third generation of the ground GW detector called Einstein Telescope (ET) and the space-based detector deci-hertz interferometer gravitational wave observatory (DECIGO). In this paper, we relax the condition $c_T/c = 1$ to investigate modified GW propagation where the speed of GWs propagation is not necessarily equal to the speed of light. Also, we consider the possibility for the running of the Planck mass corrections on modified GW propagation. We parametrize both corrections in terms of an effective GW luminosity distance and we perform a forecast analysis using standard siren events from BNS mergers, within the sensitivity predicted for the ET and DECIGO. We find at high $z$ very strong forecast bounds on the running of the Planck mass, namely $\mathcal{O}(10^{-1})$ and $\mathcal{O}(10^{-2})$ from ET and DECIGO, respectively. Possible anomalies on GW propagation are bound to $|c_T/c - 1| \leq 10^{-2} \,\,\, (10^{-2})$ from ET (DECIGO), respectively.   

Live

Exotic compact objects (ECOs) describe a generic class of hypothetical horizonless regular astrophysical objects that have been considered as alternatives to classical black holes. Such objects are expected in theories that, motivated by quantum gravity modifications, predict horizonless objects as the final stage of gravitational collapse. It is theoretically predicted that the gravitational waveform resulting from the coalescence of two ECOs will present secondary pulses after the ringdown, called "echoes". We study the detectability of the first echo in a GW150914-like event, taking into account the binary mass-ratio in the inspiral (responsible for the initial excitation) and a physically motivated prescription for the ECO surface reflectivity (the dominant contributor to the echo amplitude). We find that the threshold for detectability could be achieved during O4, the next LIGO/Virgo/KAGRA observing run.   

Live

At sufficiently high energy scales, corrections to GR will appear that favor alternate theories of gravity. The parameterized post-Einsteinian (ppE) formalism provides a framework for modifying frequency domain GR waveforms into ppE-corrected waveforms that account for leading PN order deviations predicted by a beyond-GR theory. These corrections are computed assuming a quasicircular inspiral, making the applicable only in the inspiral regime. We examine various procedures that can be used to extend the inspiral correction into the merger and ringdown regimes with minimal assumptions, as well as the effect of these assumptions on parameter estimation efforts making use of ppE waveforms as matched filter templates.