Session C08: Tests of General Relativity

Testing General Relativity with black hole X-ray data: a progress report

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 6-7 years have seen significant advancements in the study of the strong-field regime, which can now be tested with gravitational waves, X-ray data, and mm VLBI observations. In my talk, I will summarize the state-of-the-art of the tests of General Relativity with black hole X-ray data, showing the latest results, comparing its constraints on new physics with those from other techniques, and discussing the accuracy of these measurements and the criteria to select sources and observations suitable for these tests.   

Constraining gravity theories with neutron star mass-radius observations, or not

The structure of a compact object in an alternative theory is different from the case in general relativity (GR), hence any measurement of these structures can, in principle, be used to test deviations from GR. A recent concrete example is the spontaneous scalarization scenario of Damour and Esposito-Farese (DEF), where part of the parameter space has been ruled out via Bayesian inference by comparing the simultaneous mass and radius measurements of neutron stars with theoretical predictions. Here, we extend this work to generalizations of the DEF model as well as the scalar-Gauss-Bonnet theories which are known to feature spontaneous scalarization. We find that, unlike the original DEF model, the current mass-radius data set does not provide significant constraints in these cases. This is sometimes due to the fact that the theory closely resembles GR, and sometimes due to uncertainties in the data and modeling. We discuss how these results can and cannot be improved with future observations, and plans to adapt the same method to other alternative theories of gravitation.   

Dark Matter or Regular Matter in Neutron Stars? How to tell the difference from the coalescence of compact objects

The mirror Twin Higgs model is a candidate for strongly self-interacting dark matter. Since the model is a nearly identical copy of the Standard Model (SM), one expects that this model acquires interactions similar to the SM. Therefore, a neutron star entirely composed of mirror dark matter is possible. Our previous study showed mirror neutron stars are distinguishable from SM neutron stars. In this presentation, I will focus on two scenarios that produce a mirror matter admixed neutron star: mirror matter captured by a neutron star and the coalescence of a mirror neutron star and a SM neutron star. I will discuss the properties of the admixed star and the distinguishability from the observation perspective.   

Extensions of Inspiral and Merger Waveforms for Modified Gravity Theories

Binary black hole mergers are prime candidates when looking for deviations from GR in the strong-field regime. The parameterized post-Einsteinian (ppE) framework produces waveforms that account for leading PN order deviations predicted by a beyond-GR theory from GR waveforms. Previous work extended the ppE framework into the merger and ringdown regimes where beyond-GR corrections may also be expected. We explore how more recently developed waveform approximants inform further extensions one might make to the ppE framework.   

Converging to the wrong answer: robustness of theory-specific gravitational wave tests of GR

When using Bayesian inference on gravitational wave data to test specific modified theories, one must ensure that the parameter space has been thoroughly explored so that the high likelihood regions have all been identified. Degeneracies between modified theory coupling constants (especially when there are more than one) and astrophysical parameters (like masses and spins) can mislead one to believe a thorough exploration has been accomplished. We will here describe a gravitational wave test of a particular modified theory of gravity, Einstein-æther theory, and the challenges that arise when trying to constrain the parameters of this theory. We will also describe a method to overcome such challenges to reach robust conclusions.   

When can we ignore eccentricity when testing general relativity with gravitational waves?

Detections of gravitational waves (GW) emitted from binary black hole (BBH) coalescences allow us to probe the strong-field dynamics of general relativity (GR). One can compare the observed GW signals with theoretical waveform models to constrain possible deviations from GR. Any physics that is not included in these waveform models might show up as apparent GR deviations. The waveform models used in current tests of GR describe binaries on quasicircular orbits, since most of the binaries detected by ground-based gravitational wave detectors are expected to have negligible eccentricities. Thus, a signal from an eccentric binary in GR is likely to show up as a deviation from GR in the current implementation of these tests. We study the response of four standard tests of GR to numerically simulated eccentric BBH signals in the LIGO-Virgo network. Specifically, we consider a test for the consistency between the low- and high-frequency parts of the signal; two tests that introduce parameterized modifications to the phase of the signal (e.g., in the post-Newtonian coefficients); and a test for dispersive propagation effects.We find that signals having larger eccentricities (~0.1) when entering the detector's sensitive band lead to very significant false GR deviations in most tests for the high-SNR cases we consider, while signals having smaller eccentricities (~0.05) lead to significant deviations in some tests. Thus, it will be necessary to exclude the possibility of an eccentric binary in order to make any claim about detecting a deviation from GR.   

Higher Harmonic Spin-Precession Gravitational Waveforms in Dynamical Chern-Simons

In dynamical Chern-Simons (dCS) theory the no-hair theorem of vacuum solutions to general relativity (GR) is violated. Within GR multipolar structures of the exterior spacetime are fixed to depend only on the mass and spin of a Kerr black hole (the no-hair theorem). Yet, in parity-violating theories, such as dCS, these structures have alternative multipolar arrangements. Here the beyond-GR corrections in dCS have dependencies on the relation between the spin of the compact objects and orbital angular momentum of the binary system. In the non-spinning limit, corrections to GR occur at higher post-Newtonian order. Thus, to identify leading-order, beyond-GR corrections in realistic black hole binaries a spin-precession parameterized post-Einsteinian (ppE) waveform is necessary. This will lead to precession phase and spectral amplitude modulations, along with beyond-GR corrections, in typical ppE waveforms that would also require higher harmonics. This talk will discuss a model incorporating higher harmonics in existing spin-precession waveforms to perform tests of GR with current and future detectors.   

Probing Non-Axisymmetric Violations of the No Hair Theorems with Gravitational Waves

Black holes in general relativity obey the no hair theorems, which has strong implications on the orbital and precessional dynamics of binary black holes. However, this need not be true for black holes in modified theories of gravity, or for exotic compact objects, the latter of which are not necessarily axisymmetric. We here show how non-axisymmetry modifies the precession dynamics of a compact binary inspiral, and consequently, how such effects modify the observed gravitational waveform. We further discuss how well future detectors can constrain such effects through gravitational wave observations of precessing binary black holes.   

Probing the Effects of Nonviolent Nonlocality with Gravitational Waves

Measurement of gravitational waves can give precision tests of the nature of black holes and compact objects. Gidding’s nonviolent nonlocality is motivated by the information paradox and allows the information to escape via soft modes in a black hole atmosphere. Furthermore, these soft modes could exist at distances of around a Schwarzschild radius beyond the horizon. In this talk, we will discuss how to probe soft metric fluctuations near the horizon by observing the inspiral of a binary black hole. We use an effective one body model and evaluate the dynamics with added noisy metric fluctuations. We find that during the late inspiral-merger, the binary exhibits random dephasing. We also find that the parameterized post-Einsteinian coefficients will be randomly distributed depending on the noise realization, from which we constrain the size of the metric fluctuations by combining many events.