Session C13: Tests of General Relativity

Impact of selection biases on tests of general relativity with gravitational-wave inspirals

Tests of general relativity with gravitational wave observations from merging compact binaries continue to confirm Einstein's theory of gravity with increasing precision. However, these tests have so far only been applied to signals that were first confidently detected by matched-filter searches assuming general relativity templates. This raises the question of selection biases: what is the largest deviation from general relativity that current searches can detect, and are current constraints on such deviations necessarily narrow because they are based on signals that were detected by templated searches in the first place? In this paper, we estimate the impact of selection effects for tests of the inspiral phase evolution of compact binary signals with a simplified version of the GstLAL search pipeline. We find that selection biases affect the search for very large values of the deviation parameters, much larger than the constraints implied by the detected signals. Therefore, combined population constraints from confidently detected events are mostly unaffected by selection biases, with the largest effect being a broadening at the ∼ 10% level for the −1PN term. These findings suggest that current population constraints on the inspiral phase are robust without factoring in selection biases. Our study does not rule out a disjoint, undetectable binary population with large deviations from general relativity, or stronger selection effects in other tests or search procedures.   

Fortifying gravitational-wave tests of general relativity against astrophysical assumptions

Tests of general relativity with gravitational-wave sources often rely on inferring both the degree to which the signal deviates from general relativity and the astrophysical properties of the source. Due to the nature of the gravitational-wave signal, the inferred deviations and astrophysical parameters can be highly correlated. As a consequence, prior assumptions about the astrophysical population impact the inferred deviation. Therefore, information about the underlying astrophysical population must be jointly inferred within these tests of general relativity to avoid biases. In this study, we present a framework to mitigate these biases and directly apply the method to constrain the graviton mass and post-Newtonian deviations from binary black hole mergers observed by the LIGO-Virgo-KAGRA Collaboration.   

Assessing biases in testing general relativity due to Type II lensing of binary black hole signals

The detection of gravitational waves (GW) originating from binary black hole (BBH) coalescences has emerged as a powerful tool for probing the strong-field dynamics of general relativity (GR). In this study, we investigate biases from strong gravitational lensing on tests of GR with GW signals, since this effect is not accounted for in the current implementation of these tests. In the geometric optics approximation, strong lensing produces three image types (Type I, Type II, and Type III). Our focus is on Type II signals, where the introduced phase shift is non-degenerate with the parameters of quasicircular binaries if there are non-negligible higher modes and/or precession. We assess the response of four standard tests of GR to simulated Type II lensed BBH signals. Specifically, we consider a test for consistency between the low- and high-frequency parts of the signal, two tests that constrain parameterized modifications to the signal phase (e.g., in the post-Newtonian coefficients), and a test for dispersive propagation effects. We investigate the binary parameters for which Type II lensing leads to significant false GR deviations when observed with near-future ground-based GW detectors.   

Neural Post-Einsteinian Framework for Efficient Theory-Agnostic Tests of General Relativity with Gravitational Waves

The parametrized post-Einsteinian (ppE) framework and its variants are widely used by the gravitational-wave community to probe gravity through tests that apply to a large class of theories beyond general relativity. However, the ppE formalism is not truly theory-agnostic as it only captures certain types of deviations from general relativity: those that admit a post-Newtonian series representation. Moreover, each type of deviation in the ppE framework has to be tested separately, making the whole process computationally inefficient and expensive, possibly obscuring the theoretical interpretation of potential deviations that could be detected in the future. In this talk, I will present the neural post-Einsteinian (npE) framework, an extension of the ppE formalism that overcomes the above weaknesses using deep-learning neural networks. I will showcase the application of the new npE framework to future tests of general relativity with the fifth observing run of the LIGO-Virgo-KAGRA collaboration. In particular, I will demonstrate the use of the npE framework to efficiently explore deviations from general relativity beyond what can be mapped by the ppE formalism, including modifications coming from higher-order curvature corrections to the Einstein-Hilbert action and dark-photon interactions in possibly hidden sectors of matter.   

Distinguishing boson stars from black holes using gravitational-wave observations

The spinning compact binaries possess non-vanishing spin-induced multipole moments that leave an imprint on the emitted gravitational wave (GW) signal. These spin-induced multipole moments are uniquely determined for Kerr black holes, given their mass and spin. On the other hand, the spin-induced multipole moments of neutron stars and exotic compact objects depend on their internal composition in addition to their mass and spin. Hence, the GW measurement of the spin-induced multipole moments is useful for probing the true nature of the observed compact object. We demonstrate a novel test to distinguish massive boson stars from binary black holes using GW observations of the spin-induced quadrupole moment. Starting with selected GW events detected during the first three observing runs of ground-based GW detectors, we perform a complete Bayesian analysis with an efficient stochastic sampling algorithm to obtain the posteriors on the boson star model parameters. We employ a GW waveform model for inspiraling binary boson stars with quartic self-interactions, which has contributions from its spin-induced quadrupole moment along with higher modes and precession. This detailed analysis constrains the boson star model parameters, such as the mass of the boson ($\mu_B$) and the self-interaction parameter ($\lambda$), for the first time. We find that, GW190412 gives the best bound in the $\mu_B-\lambda$ plane, constraining $\lambda$ approximately between $1\times 10^5-9\times 10^5$ for bosons with masses of $\mathcal{O}$(1 GeV). In the future, with improved detector sensitivity and an increase in the population of binaries, this method can provide interesting constraints on black hole mimicker candidates.   

Better early than never: Testing GR with GW Polarizations

In addition to the two polarization modes present in general relativity, some modified theories of gravity may contain up to four additional polarizations, which are allowed to travel at different speeds. Detecting these additional modes would be evidence of new physics, but disentangling these modes from those in general relativity requires more than two detectors. In this talk, we discuss a new observational signature of additional, superluminal polarizations that can be searched for with currently available detectors. We here describe this signature and how to search for it in current and future data.   

Rogue echoes from exotic compact objects

Binary systems containing exotic, ultracompact stars may emit repeated bursts of gravitational waves (GWs) following inspiral and merger. The detection of such GW ``echoes'' would be a smoking-gun-signature of new physics, but searches for them have not yielded a convincing detection. In this talk I will show that the delay time between a the initial GW event and its echoes is generically much greater than expectations. I will describe a general argument and provide several specific examples where the time delays can be billions of years, resulting in rogue echoes that are not correlated with GW events and evade tailored searches. However, such echoes may be detectable by unmodeled searches for transient GW events.