Session Z16: Beyond Einstein Gravity: Theories, Modeling, and Tests III

A morphology-independent test of the mixed polarization content of transient gravitational wave signals

The purely tensorial polarization content of gravitational waves is one of the fundamental predictions of General Relativity. Recent detections of compact binary mergers from ground-based detectors can be used to test this prediction. In this talk I will describe a morphology-independent test of the polarization content of the signals. The test does not rely on knowledge of the intrinsic signal morphology, but depends only on the response of a detector network. I will also discuss results from signals GW190521 and GW170817.    

Testing Gravitational Theories with Broken Lorentz Symmetry by Gravitational Wave Observations

In this talk, I shall present our recent studies on testing Einstein-aether theory by gravitational wave (GW) observations. In particular, I shall show that the GW observations and stability of black holes already reduce the four coupling constants of the theory to two, and future observations of GWs and quasi-normal modes of black holes shall further reduce the parameter space of the theory.  

Einstein-aether theory is a particular type of the vector-tensor theory, in which the vector field is timelike and unity, so it defines a preferred direction. As a result, it locally breaks the Lorentz symmetry. Yet, it is self-consistent (free of ghosts,  instabilities, and so on), and is consistent with all the observations and experiments. Its Cauchy problem is also well posed. In addition, due  to the presence of the aether, three different species of gravitons, spin-0, spin-1, and spin-2, exist. This will bring dramatical differences in emissions of 

gravitational waves.   

Improved bounds on higher-order curvature theories of gravity through gravitational wave catalogs

While Einstein's theory of general relativity (GR) has been extremely successful at describing our observations for the past 100 years, there are a variety of alternative theories of gravity that can help solve some open problems in physics. In this search for physics beyond GR, gravitational wave astronomy is proving to be an effective tool at constraining these other theories. In this talk, I will discuss a recent work we have published in which we combined information from multiple GW sources to place tight, robust constraints on a few specific theories of gravity that involve higher order corrections to the Einstein-Hilbert action, Einstein-dilaton-Gauss-Bonnet (EdGB) and dynamical Chern Simons (dCS) gravity. To begin, I will first outline the methods used to analyze the data from each source. From there, I will discuss the conclusions these analyses led us to, including a tighter constraint on EdGB. Finally, I will present how we attempted to fully understand the sources of error, like uncertainties in the waveform model both within and outside GR, and how these factors do not significantly impact our results.   

Constraints on Einstein-dilation-Gauss-Bonnet gravity from Black Hole-Neutron Star Gravitational Wave Events

Recent gravitational wave observations allow us to probe gravity in the strong and dynamical field regime. In this talk, we focus on testing Einstein-dilation Gauss-Bonnet gravity motivated by string theory. In particular, we use two new neutron star black hole binaries (GW200105 and GW200115). We also consider GW190814 which is consistent with both a binary black hole and a neutron star black hole binary. Adopting the leading post-Newtonian correction and carrying out a Bayesian Markov-chain Monte Carlo simulations, we derive the 90% credible upper bound on the coupling constant of the theory as  √α_GB ≤ 1.33km, whose consistency is checked with an independent Fisher analysis. This bound is stronger than the bound obtained in previous literature by combining selected binary black hole events in GWTC-1 and GWTC-2 catalogs. We also derive a combined bound of √α_GB ≤ 1.18km, by stacking GW200105, GW200115, GW190814, and selected binary black hole events. In order to check the validity of the effect of higher post-Newtonian terms, we derive corrections to the waveform phase up to second post Newtonian order by mapping results in scalar-tensor theories to Einstein-dilation Gauss-Bonnet gravity. We find that such higher-order terms improve the bounds by14.5% for GW200105 and 6.9% for GW200115 respectively.   

Tests of General Relativity with GWTC-3

Gravitational-wave observations deliver a unique way to perform tests of the general relativity (GR) in the dynamical strong-field regime. The detections of compact binary coalescences by the advanced LIGO and advanced Virgo detectors during the past observing runs have permitted us to perform several tests of general relativity (GR). We detail the various testing GR analyses performed on these events (GWTC-3), focusing on the second part of the third observing run (O3b) of the LIGO and Virgo detectors.  We report results from individual events and the combined results, including the GWTC-2 events wherever appropriate. O3b consists of 15 new events satisfying the selection criteria. From the consistency tests, we find the residual power after subtracting the best-fit template being consistent with the instrumental noise. Also, the final mass and spin estimates obtained separately from the low and high-frequency parts of the gravitational-wave signal were found to agree. Measurements of parametric deviations of post-Newtonian coefficients agreed to GR predictions, with an improvement by a factor of ∼ 2 in the −1PN parameter. The spin-induced quadrupole moment estimates provide consistent results with a binary black hole system. We find no evidence of modified dispersion, extra polarization modes, and post-merger echoes. The tests performed for merger remnant properties provide consistent results. Furthermore, the graviton bound is updated to $1.23\times 10^{-23}eV/c^{2}$ within the 90\% credibility.    

Tests of Gravitational-Wave Birefringence with the Open Gravitational-Wave Catalog

The routine detection of gravitational-wave events from compact binary coalescences has allowed precise tests of gravity in a strong field, dynamical field, and high energy regimes. In this work, we report the results of testing gravitational-wave birefringence using compact binary coalescence events from the 3rd Open Gravitational-wave Catalog (3-OGC). Birefringence, an effect where the left- and right-handed polarizations of gravitational waves follow different equations of motion, occurs when the parity symmetry of gravity is broken. This arises naturally in the effective field theory extension of general relativity. Using Bayesian inference and state-of-the-art waveform modeling, we use all events in 3-OGC to constrain the lower limit of the energy scale at which parity violation effects emerge. Overall we do not find evidence for a violation of general relativity, and thus we constrain the parity-violating energy scale to 0.11 GeV at 90% confidence level, which is an improvement over previous results by one order of magnitude. Intriguingly, we find an outlier, GW190521, that supports the existence of birefringence over general relativity with a higher match-filtering signal-to-noise ratio and Bayes factor. The inclusion of birefringence for GW190521 gives consistent sky location estimation with a possible electromagnetic counterpart reported Zwicky Transient Facility (ZTF). However, the inferred MPVMPV​ from GW190521 is in tension with the combined constraints from other events, we thus hypothesize that the non-zero deviation may be caused by the limitations of the existing waveform approximants. More gravitational wave detections in the future from compact binary coalescence will further improve the constraints on birefringence and shed light on possible systematics for GW190521-type events.   

Splitting the third hair: constraints on Kerr-Newman black holes from merger-ringdown gravitational-wave observations

Gravitational waves are providing increasingly stringent constraints on the properties of black holes. Dynamical processes involving such compact objects also represent engines of discovery for new particles and modifications of the Einstein-Hilbert action. In this regard, we discuss a search for a U(1) charge in the post-merger signal of binary black hole coalescences observed by LIGO and Virgo. Besides a “standard” electric charge, this "third hair” can be also identified with a hidden electromagnetic charge in models of minicharged dark matter, the vector charge in theories mediated by a gravitational vector field, or a topologically induced charge. We model the remnant object sourcing the post-merger signal as a Kerr-Newman black hole, and calculate the corresponding dominant quasi-normal mode frequencies for arbitrary values of charge and angular momentum. Then, we construct a template to describe the ringdown of a charged remnant following a binary black hole coalescence. By applying this model to LIGO-Virgo detections, we find no evidence for or against the Kerr-Newman hypothesis, and obtain upper bounds on the maximum amount of charge present in these mergers. Finally, we outline the prospects to detect black hole charge with LIGO and Virgo at their design sensitivity.   

GW190521: Quantifying the Evidence for Near-Horizon Quantum effects

The exceptionally loud ringdown of GW190521 makes it an ideal candidate to search for gravitational wave echoes, a proposed smoking gun for the quantum structure of black hole horizons. We perform an unprecedented multi-pronged search for echoes via two well-established and independent pipelines: a template-based search for stimulated emission of Hawking radiation (or Boltzmann echoes), and the model-agnostic coherent WaveBurst (cWB) search.  Given GW190521 parameters, stimulated Hawking radiation from the merger is expected to lead to post-merger echoes at horizon mode frequency of ~ 50 Hz, repeating at intervals of ~ 1 second, due to partial reflection off Planckian quantum structure of the horizon. In this talk, I will be presenting the results found when performing two independent searches and discussing their results. I will argue how both methods agree on the existence of evidence in favor of echoes and also their agreement in the echo parameters. The statistical significance of these findings will also be discussed.   

Testing the Weak Equivalence Principle near black holes

Today we have quite stringent constraints on possible violations of the Weak Equivalence Principle from the comparison of the acceleration of test-bodies of different composition in Earth's gravitational field. In this talk, I show how we can test the Weak Equivalence Principle in the strong gravitational field of black holes by studying the reflection features of their accretion disks. Constraints on a possible violation of the Weak Equivalence Principle by massive particles and X-ray photons are inferred from a NuSTAR observation of the black hole binary EXO 1846-031.