A particularly compelling aspect of the GW190521 event detected by the LIGO-Virgo-KAGRA (LVK) collaboration is that it has an extremely short duration, and lacks a clearly identifiable inspiral phase usually observed in the binary black holes (BBHs) coalescence. In this work, we hypothesize that GW190521 might represent a single, isolated gravitational wave (GW) echo pulse from the wormhole, which is the postmerger remnant of BBHs in another universe and connected to our universe through a throat. The ringdown signal after BBHs merged in another universe can pass through the throat of wormhole and be detected in our universe as a short-duration echo pulse. Our analysis results indicate that our model yields a network signal-to-noise ratio comparable to that of the standard BBHs merger model reported by the LVK collaboration. For GW190521, Bayesian model selection yields ln ℬEcho BBH ≃ -2.9, indicating that the data favor the BBH hypothesis over our echo-for-wormhole model.

Gravitational wave (GW) observations of binary black hole (BBH) coalescences provide a unique opportunity to test general relativity (GR) in the strong-field regime. To ensure the reliability of these tests, it is essential to identify and address potential sources of error, particularly those arising from missing physics in the waveform models used in GW data analysis. This paper investigates potential biases in these tests arising from strong gravitational lensing, an effect not currently incorporated into the standard framework for GR tests. In the geometric optics approximation, strong lensing produces three types of images: Type I, Type II, and Type III. While Type I and Type III images do not distort the signal, Type II images introduce a characteristic phase shift that can mimic GR deviations for signals with higher-order modes, precession, or eccentricity. We assess the response of four standard GR tests on simulated Type II lensed BBH signals, including the two parameterized tests (TIGER and FTI), the modified dispersion relation test and the inspiral-merger-ringdown consistency test. We focus on precessing waveforms for binaries with total masses of $20M_{\odot}$ and $80M_{\odot}$, and dimensionless spins of 0.5 and 0.95, considering a fixed signal-to-noise ratio of 25 using the design A+ sensitivity of the LIGO-Virgo network. Our findings indicate that more mass-asymmetric and higher-spin binaries show larger false deviations from GR in the TIGER and modified dispersion relation tests when applying GR tests to Type II lensed signals. These results highlight the risk of false GR violations as detector sensitivity improves in future observational runs. Therefore, it is crucial to consider the possibility of strong lensing before drawing conclusions about deviations from GR in GW signals.

Binary black hole coalescence phenomenology from numerical relativity

Gravitational wave (GW) data from observed binary black hole coalescences (BBHC) have been demonstrated in recent analyses to validate the Hawking Area Theorem (HAT) for black hole horizons. The result of such analyses is imposed here as a criterion of {\it absolute} consistency on the logarithmic (in horizon area) corrections to the Bekenstein-Hawking Area Formula (BHAF) for the black hole entropy, when these corrections are computed both from non-perturbative quantum fluctuations of spacetime in matter-free quantum general relativity, as well as arising due to perturbative quantum matter field fluctuations around a stationary classical black hole background spacetime. This criterion of absolute consistency is seen to be obeyed provided certain restrictions ensue on the spin-parity and number of species of the spectrum of quantum matter fluctuations. Such constraints appear to restrict the Beyond-Standard-Model (BSM) part of the matter fluctuation spectrum. Some species of the constrained, yet-unobserved BSM particle spectrum are currently under active consideration in particle cosmology as candidates for dark matter.

In this paper we construct the first phenomenological waveform model, which contains the ``complete'' $\ensuremath{\ell}=2$ spherical harmonic mode content for gravitational wave signals emitted by the coalescence of binary black holes with spin precession: The model contains the dominant part of the gravitational wave displacement memory, which manifests in the $(\ensuremath{\ell}=2,m=0)$ spherical harmonic in a co-precessing frame, as well as the oscillatory component of this mode. The model is constructed by twisting up the oscillatory contribution of the mode, as it was previously done for the rest of spherical harmonic modes in IMRPhenomTPHM and the Phenom family of waveform models. Regarding the displacement memory contribution present in the aligned spin (2,0) mode, we discuss a procedure to analytically compute the ``precessing memory'' in all the $\ensuremath{\ell}=2$ modes using the integration derived from the Bondi-Metzner-Sachs balance laws. The final waveform of the (2,0) mode is then obtained by summing together both contributions. We implement this as an extension of the computationally efficient IMRPhenomTPHM waveform model, and we test its accuracy by comparing against a set of numerical relativity simulations. Finally, we employ the model to perform a Bayesian parameter estimation injection analysis.

Abstract We report the observation of gravitational waves from two binary black hole coalescences during the fourth observing run of the LIGO–Virgo–KAGRA detector network, GW241011 and GW241110. The sources of these two signals are characterized by rapid and precisely measured primary spins, nonnegligible spin–orbit misalignment, and unequal mass ratios between their constituent black holes. These properties are characteristic of binaries in which the more massive object was itself formed from a previous binary black hole merger and suggest that the sources of GW241011 and GW241110 may have formed in dense stellar environments in which repeated mergers can take place. As the third-loudest gravitational-wave event published to date, with a median network signal-to-noise ratio of 36.0, GW241011 furthermore yields stringent constraints on the Kerr nature of black holes, the multipolar structure of gravitational-wave generation, and the existence of ultralight bosons within the mass range 10 −13 –10 −12 eV.

Introduction:General Relativity (GR) is always nonlinear; the Einstein field equations (EFE) are themselves a representation of the nonlinear coupling interactions between spacetime curvature and matter-energy. This nonlinearity becomes more important in high-curvature areas, such as black hole event horizons, and binary black hole coalescences, where linear models are insufficient. These nonlinear effects are important to model gravitational phenomena and particularly important in view of the recent detection of gravitational waves. Objectives:This work will consist of the following: The three research areas are: (i) The exploration of the effects of strong field nonlinearities in regions of high spacetime curvature, (ii) The analysis of black hole perturbation and mode coupling, (iii) The analysis of nonlinear effects in systems of coalescing black holes. By elucidating these aspects, we seek to enhance the understanding of stability, waveforms, and fundamental properties of spacetime in harsh environments. Methods:To achieve these goals, we employed the higher order of perturbation theory and accurate numerical modeling. First-order and second-order perturbative expansions were used to investigate stability and mode coupling in Schwarzschild and Kerr black hole environments. AMR was applied to capture the nonlinear evolutions of the binary black hole mergers, and constraint preserving boundary conditions were applied to simulate the black hole boundaries. Results:The analytic and perturbative analysis revealed that there is nonlinear behavior near the horizon of black holes and second order mode coupling may cause instability in high curvature regime. In analyzing the numerical solutions of the binary black hole mergers, it was discovered that nonlinear effects govern the horizon growth and modify the gravitational wave signal in both amplitude and phase. AMR techniques were useful in preserving the numerical stability and accuracy in these areas that are highly nonlinear. Conclusions:This work is centered on the role of non-linearities in selecting high-curvature spacetimes, black hole stability, waveform predictions and the cosmic censorship conjecture. According to our findings, nonlinear modeling is essential in astrophysics, and our study provides a foundation for future research on gravitational phenomena and black hole information theory.

Using gravitational waves to probe the geometry of the ringing remnant black hole formed in a binary black hole coalescence is a well-established way to test Einstein's theory of general relativity. However, doing so requires knowledge of when the predictions of black hole perturbation theory, i.e., quasinormal modes (QNMs), are a valid description of the emitted gravitational wave as well as what the amplitudes of these excitations are. In this work, we develop an algorithm to systematically extract QNMs from the ringdown of black hole merger simulations. Our algorithm improves upon previous ones in three ways: it fits over the two-sphere, enabling a complete model of the strain; it performs a reverse search in time for QNMs using a more robust nonlinear least squares routine called varpro; and it checks the variance of QNM amplitudes, which we refer to as ``stability,'' over an interval matching the natural timescale of each QNM. Using this algorithm, we not only demonstrate the stability of a multitude of QNMs and their overtones across the parameter space of quasicircular, nonprecessing binary black holes, but we also identify new quadratic QNMs that may be detectable in the near future using ground-based interferometers. Furthermore, we provide evidence which suggests that the source of remnant black hole perturbations is roughly independent of the overtone index in a given angular harmonic across binary parameter space, at least for overtones with $n\ensuremath{\lesssim}2$. This finding may hint at the spatiotemporal structure of ringdown perturbations in black hole coalescences, as well as the regime of validity of perturbation theory in the ringdown of these events.

Gravitational-wave (GW) observations of binary black-hole (BBH) coalescences are expected to address outstanding questions in astrophysics, cosmology, and fundamental physics. Inference of BBH parameters relies on waveform models, and realizing the full discovery potential of upcoming LIGO-Virgo-KAGRA observing runs and new ground-based facilities (such as the Einstein Telescope and Cosmic Explorer) hinges on the accuracy of these waveform models. Using linear-signal approximation methods and Bayesian analysis, we start to assess our readiness for what lies ahead using two state-of-the-art quasicircular, spin-precessing models: v5 and henom. We find that systematic biases increase with the spin of the BH, with parameter biases being approximately 6 to 8 times likelier, if the primary-spin magnitude exceeds 0.5 compared to when it is less than 0.5. Additionally, we ascertain that current waveforms can accurately recover the distribution of masses in the LVK astrophysical population but not spins. Upon exploring the broader parameter space of BHs, we find that systematic biases increase with detector-frame total mass, binary asymmetry, and spin precession, with a majority of such binaries incurring parameter biases, extending up to redshifts around 3 in future detectors. Furthermore, we examine three “golden” events characterized by mass ratios of approximately 6 to 10, significant spin magnitudes (0.6−0.9), and high precession, evaluating how systematic biases may affect their scientific outcomes. Our findings reveal that current waveforms fail to enable the unbiased measurement of the Hubble-Lemaître parameter and sky localization from loud signals, even for current detectors. Moreover, highly asymmetric systems within the lower BH mass gap exhibit biased measurements of the secondary-companion mass, which impacts the physics of both neutron stars and formation channels. Similarly, we deduce that the primary mass of massive binaries (>60M⊙) will also be biased, affecting supernova physics. Future progress in analytical calculations and numerical-relativity simulations, crucial for calibrating the models, must target regions of the parameter space with significant biases to develop more accurate models. Only then can precision GW astronomy fulfill the promise it holds.

Abstract In the framework of general relativity (GR), gravitational waves (GWs) travel at the speed of light across all frequencies. However, massive gravity and weak equivalence principle (WEP) violation may lead to frequency-dependent variations in the propagation speed of GWs, which can be examined by comparing the theoretical and observed discrepancies in the arrival times of GW signals at various frequencies. This provides us with an opportunity to test these theories. For massive gravity, we consider that gravitons may have a nonzero rest mass. For WEP violations, we hypothesize that different massless particles exposed to the same gravitational source should exhibit varying gravitational time delays. The gravitational time delay induced by massive gravitational sources is proportional to γ + 1, where the parameter γ = 1 in GR. Therefore, we can quantify these two deviations using phenomenological parameters m g and ∣Δγ∣, respectively. In this study, we use selected GW data from binary black hole coalescences in the LIGO-Virgo catalogs GWTC-2.1 and GWTC-3 to place constraints on the parameters m g and ∣Δγ∣. We also compute Bayes factors for models that assume the existence of graviton mass and WEP violation compared to the standard GW model, respectively. The absolute value of the natural logarithm of the Bayes factor is generally less than two. Our analysis reveals no significant preference for either model. Additionally, the Bayes factors between these two models do not provide obvious evidence in favor of either one.

Black holes offer a unique laboratory for fundamental physics and are crucial for understanding theories beyond Einstein's theory of general relativity. In this paper, we focus on 4D effective field theories and string-theory inspired models that include scalar fields. We focus on one such model, axi-dilaton gravity, a quadratic gravity theory with two kinetically coupled scalar fields, an axion and a dilaton. To study the evolution and structure of these fields around black holes, we introduce canuda--axidil, the first open-source, parametrized numerical relativity code for quadratic and biscalar gravity. Using this code, we perform single black hole simulations to show the dynamical formation of axion and dilaton hairs and quantify the effect of higher-order terms in coupling and spin. Through these simulations, we measure the impact of black hole spin and curvature coupling strength on the profiles of the axion and dilaton and show that including kinetic coupling between the fields increases the observed deviations from general relativity. Furthermore, we simulate the axion and dilaton fields around a binary black hole coalescence demonstrating the growth of axion hair during the inspiral and the production of radiative modes for both fields.

The coalescence of binary black holes (BBHs) provides a unique arena to test general relativity (GR) in the dynamical, strong-field regime. To this end, we present , a parametrized, multipolar, spin-precessing waveform model for BBHs in quasicircular orbits, built within the effective-one-body formalism. Compared to its predecessor, , our model introduces parametrized deviations from GR not only in the plunge-merger-ringdown stages, but also in the inspiral phase through modifications to the conservative dynamics. Additionally, it incorporates, for the first time, spin-precession effects. The free deviation parameters can be used to perform null tests of GR using current and future gravitational-wave observations. We validate through Bayesian parameter estimation, focusing on the quasinormal-mode frequency and damping time of the (ℓ,m,n)=(2,2,0) mode. Our analysis of synthetic signals from numerical-relativity (NR) simulations of highly precessing BH mergers shows that, while correctly recovers consistency with GR, neglecting spin precession can lead to false detections of deviations from GR even at current detector sensitivity. Conversely, when analyzing a synthetic signal from a NR simulation of a binary boson-star merger, the model successfully identifies a deviation from a GR BBH signal. Finally, we reanalyze 12 events from the third Gravitational Wave Transient Catalog. Using a hierarchical combination of these events, we constrain fractional deviations in the frequency and damping time of the (2,2,0) quasinormal-mode to δf220=0.00−0.06+0.06 and δτ220=0.15−0.24+0.26 at 90% credibility. These results are consistent with those from the LIGO-Virgo-KAGRA Collaboration, which did not account for spin-precession effects.

Abstract We present the numerical relativity module within AthenaK, an open-source performance-portable astrophysics code designed for exascale computing applications. This module employs the Z4c formulation to solve the Einstein equations. We demonstrate its accuracy through a series of standard numerical relativity tests, including convergence of the gravitational waveform from binary black hole coalescence. Furthermore, we conduct scaling tests on OLCF Frontier, NERSC Perlmutter, and ALCF Aurora, where AthenaK exhibits excellent weak-scaling efficiency of 80% on up to 65,536 AMD MI250X GPUs on Frontier (relative to four GPUs) and 67% on Aurora up to 24,576 Intel Data Center Max Series GPUs (relative to 12 GPUs) and strong-scaling efficiencies of 84% and 77% on AMD MI250X and NVIDIA A100 GPUs on Frontier and Perlmutter, respectively. Additionally, we observe a significant performance boost, with 2 orders of magnitude speedup (≳200×) on a GPU compared to a single CPU core, affirming that AthenaK is well suited for exascale computing, and thereby expanding the potential for breakthroughs in numerical relativity research.

Abstract A recently developed tool allows for a description of spacetime as a manifold with a Lorentz-invariant (lower) limit length built-in. This is accomplished in terms of geometric quantities depending on two spacetime events (bitensors) and looking at the 2-point function of fields on it, all this being well suited to embody nonlocality at the small scale. What one gets is a metric bitensor with components singular in the coincidence limit of the two events, capable to provide a finite distance in the same limit. We discuss here how this metric structure encompasses also the case of null separated events, and describe some results one obtains with the null qmetric which do have immediate thermodynamic/statistical interpretation for horizons. One of them is that the area transverse to null geodesics converging to a base point goes to a finite value in the coincidence limit (instead of shrinking to 0). We comment on the discreteness this seems to imply for the area of black hole horizons as well as on possible ensuing effects in gravitational waves from binary black hole coalescences.

Quadratic quasinormal modes encode fundamental properties of black hole spacetimes. They are also one of the key ingredients of nonlinearities of general relativity in the ringdown stage of binary black hole coalescence. In this Letter, we classify all possible quadratic coupling channels of quasinormal modes for a generic Kerr black hole and use a frequency-domain pseudospectral code with hyperboloidal slicing to calculate these couplings. After accounting for all the channels in systems with reflection symmetry, our results become consistent with those extracted from numerical simulations and time-domain fits. This agreement provides a compelling example demonstrating the success of black hole second-order perturbation theory. We also explore potential applications of our calculations in future ringdown data analysis by carrying out a detectability survey for various quadratic modes. We find that a few of them are observationally relevant for third-generation ground-based detectors like Cosmic Explorer, as well as the spaceborne Laser Interferometer Space Antenna.

Context. Dynamical interactions in star clusters are an efficient mechanism to produce the coalescing binary black holes (BBHs) that have been detected with gravitational waves (GWs). Aims. We want to understand how BBH coalescence can occur during – or after – binary-single interactions with different mass ratios. Methods. We perform gravitational scattering experiments of binary-single interactions using different mass ratios of the binary components (q2 ≡ m2/m1 ≤ 1) and the incoming single (q3 ≡ m3/m1). We extract cross-sections and rates for (i) GW capture during resonant interactions; (ii) GW inspiral in between resonant interactions and apply the results to different globular cluster conditions. Results. We find that GW capture during resonant interactions is most efficient if q2 ≃ q3 and that the mass-ratio distribution of BBH coalescence due to inspirals is ∝ m1−1q2.9﹢α, where α is the exponent of the BH mass function. The total rate of GW captures and inspirals depends mostly on m1 and is relatively insensitive to q2 and q3. We show that eccentricity increase in direct (that is, non-resonant) encounters approximately doubles the rate of BBH inspirals in between resonant encounters. For a given GC mass and radius, the BBH merger rate in metal-rich GCs is approximately double that of metal-poor GCs, because of their (on average) lower BH masses (m1) and steeper BH mass function, yielding binaries with lower q. Conclusions. Our results enable the mass-ratio distribution of dynamically formed BBH mergers to be translated to the underlying BH mass function. The additional mechanism that leads to a doubling of the inspirals provides an explanation for the reported high fraction of in-cluster inspirals in N-body models of clusters.

The ringdown phase following a binary black hole coalescence is a powerful tool for measuring properties of the remnant black hole. Future gravitational wave detectors will increase the precision of these measurements and may be sensitive to the environment surrounding the black hole. This work examines how environments affect the ringdown from a binary coalescence. Our analysis shows that for astrophysical parameters and sensitivity of planned detectors, the ringdown signal is indistinguishable from its vacuum counterpart, suggesting that ringdown-only analyses can reliably extract the (redshifted) mass and spin of the remnant black hole. These conclusions include models with spectral instabilities, suggesting that these are not relevant from an observational viewpoint. Deviations from inspiral-only estimates could then enhance the characterisation of environmental effects present during the coalescence.

During the binary black hole coalescence, gravitational waves emitted at the ringdown stage can be well described by black hole perturbation theory, where the quasinormal modes (QNMs) become the important ingredient in modeling the pattern waveform. In general relativity (GR), the QNMs can be obtained from solving the Regge–Wheeler (RW) equation of a non-rotating black hole. While in Horndeski gravity, the isospectrality between the odd and even parity perturbations is broken due to the scalar field, the odd perturbation equation can be simplified into a modified RW equation from the perturbed action. In this paper, we propose a new auxiliary field and tortoise coordinate to refine the modified RW equation in Horndeski gravity, and calculate the QNM frequencies of the odd perturbation of a specific hairy black hole. We find that this proposal not only cures the superluminal propagation addressed in the previous literature, but also holds the original QNM spectrum of the odd perturbation. Moreover, our results indicate that such a Horndeski hairy black hole is stable under the odd perturbation, which is also verified by the time evolution of the perturbation. In particular, in contrast to GR, the modes with ℓ=2 can decay faster than modes with ℓ>22$$\\end{document}]]> for a certain range of the Horndeski hair, and the link between the null geodesics and QNM for the odd perturbation in the current theory is violated. We then use the ringdown QNMs to preliminarily investigate the signal-to-noise ratio (SNR) rescaled measurement error of the Horndeski hair. We obtain significant effects of the angular momentum and overtone on the error bound of the hair parameter. We hope that our findings will inspire further theoretical and phenomenological work on the testing of the no-hair theorem of black holes using gravitational wave physics.

ABSTRACT Gravitational waves (GWs) have rapidly become important cosmological probes since their first detection in 2015. As the number of detected events continues to rise, upcoming instruments like Einstein Telescope (ET) and Cosmic Explorer (CE) will observe millions of compact binary mergers. These detections, coupled with galaxy surveys by instruments such as the Dark Spectroscopic Energy Instrument (DESI), Euclid, and the Vera Rubin Observatory, will provide unique information on the large-scale structure of the universe by cross-correlating GWs with the distribution of galaxies hosting them. In this paper, we focus on how cross-correlations constrain the clustering bias of GWs emitted by the coalescence of binary black holes (BBHs). This parameter links BBHs to the underlying dark matter distribution, hence informing us how they populate galaxies. Using a multitracer approach, we forecast the precision of these measurements under different survey combinations. Our results indicate that current GW detectors will have limited precision, with measurement errors as high as $\displaystyle \sim 50~{{\ \rm per\ cent}}$. However, third-generation detectors like ET, when cross-correlated with Legacy Survey of Space and Time (LSST) data, can improve clustering bias measurements to within 2.5 per cent. Furthermore, we demonstrate that these cross-correlations can enable a per cent-level measurement of the magnification lensing effect on GWs. Despite this, there is a degeneracy between magnification and evolution biases, which hinders the precision of both. This degeneracy is most effectively addressed by assuming knowledge of one bias or targeting an optimal redshift range of $\displaystyle 1 \lt z \lt 2.5$. Our analysis opens new avenues for studying the distribution of BBHs and testing the nature of gravity through large-scale structure.

The gravitational field around an astrophysical black hole (BH) is thought to be described by the Kerr spacetime, which is a solution of the Einstein equation. Signatures of binary black hole (BBH) coalescence in gravitational waves (GW) follow the Kerr spacetime as the theoretical foundation. Hence, any possible deviations from the Kerr spacetime around BHs serve as a test of the nature of gravity in the strong-field regime and of the predictions of General Relativity. In our study, we perform a theory-agnostic test of the Kerr hypothesis using BBH inspirals from the third Gravitational-wave Transient Catalog (GWTC-3). Considering the Johannsen metric, we compute the leading-order deviation to the emitted GW in the frequency domain. Our results provide constraints on two deformation parameters (α13 and ϵ3) and demonstrate the degeneracy between these two non-Kerr parameters.