Francesco Di Renzo

We present constraints on ultralight bosons using binary black hole mergers observed in the second and third parts of the fourth LIGO-Virgo-KAGRA observing run. Directed searches are conducted for long-transient gravitational waves from ultralight vector boson clouds around merger remnants, using a hidden-Markov-model (HMM) tracking scheme. We target the remnant black holes formed in the binary coalescences that produced GW250114 and GW250207. We find no evidence for such signals from either target. Estimating our search sensitivity at a threshold corresponding to a 1% false alarm probability, we thus disfavor vector boson masses in the range of [ 2.80 , 3.95 ] × 10 − 13 eV with greater than 90% confidence. In addition, we derive constraints on ultralight scalar and vector bosons from the inferred high spins of the constituent black holes in three binaries, using events GW240515, GW241113, and GW241225_08. The excluded mass ranges in this approach depend on the assumed black-hole ages. At 10 5 years , corresponding to typical dynamically formed binaries, we exclude scalar and vector bosons in the ranges [ 1.39 , 6.94 ] × 10 − 13 eV and [ 0.32 , 14.4 ] × 10 − 13 eV at 90% confidence, respectively.

Gravitational waves can be gravitationally lensed by massive objects along their path. Depending on the lens mass and the lens–source geometry, this can lead to the observation of a single distorted signal, multiple repeated events, or a mixture of both. We present the results for gravitational-wave lensing searches on the data from the first part of the fourth LIGO–Virgo–KAGRA observing run (O4a). We search for strongly lensed events in the newly acquired data by (1) searching for an overall phase shift present in an image formed at a saddle point of the lens potential, (2) looking for pairs of detected candidates with consistent frequency evolution, and (3) identifying subthreshold counterpart candidates to the detected signals. Beyond strong lensing, we also look for lensing-induced distortions in all detected signals using an isolated point-mass model. We do not find evidence for strongly lensed gravitational-wave signals and use this result to constrain the rate of detectable strongly lensed events and the merger rate density of binary black holes at high redshift. In the search for single distorted lensed signals, we find one outlier: GW231123_135430, for which we report more detailed investigations. While this event is interesting, the associated waveform uncertainties make its interpretation complicated, and future observations of the populations of binary black holes and of gravitational lenses will help determine the probability that this event could be lensed.

We present the data quality (DQ) report builder toolkit, DQRbuild , a suite of DQ tools that have been developed to vet gravitational-wave (GW) events in preparation for the fourth LIGO-Virgo-KAGRA observing run. We explain the main functionality and the many scientific tests that we support. To validate the performance of the tools included in the toolkit, we run a series of tests on all significant candidates shared as public alerts in the third observing run to compare against what was manually reported using human intervention. We find that these automated tools can now identify 96% of the problems identified by humans during this previous observing run, with a 24% false alarm rate. We conclude with a commentary on the prospects and potential challenges for fully automating the process of vetting the DQ for GW events identified in future observing runs.

We search for gravitational-wave background signals produced by various early Universe processes in the Advanced LIGO O4a dataset, combined with the data from the earlier O1, O2, and O3 (LIGO-Virgo) runs. The absence of detectable signals enables powerful constraints on fundamental physics. We derive gravitational-wave background energy density upper limits from the O1–O4a data to constrain parameters associated with various possible processes in the early Universe: first-order phase transitions, cosmic strings, domain walls, stiff equation of state, axion inflation, second-order scalar perturbations, primordial black hole binaries, and parity violation. In our analyses, the presence of an astrophysical background produced by compact (black hole and neutron star) binary coalescences throughout the Universe is also considered. We address the implications for various cosmological and high-energy physics models based on the obtained parameter constraints. We conclude that LIGO-Virgo data already yield significant constraints on numerous early Universe scenarios.

We present the results of a search for continuous gravitational waves (GWs) from the low-mass X-ray binary Scorpius X-1 using LIGO data from the first part of the fourth LIGO–Virgo–KAGRA observing run. By applying the resampling version of the cross-correlation pipeline to search for signal frequencies f ₀ between 25 and 200 Hz (corresponding to neutron star spin frequencies of 12.5–100 Hz for GWs due to triaxiality, or ∼15–20 to ∼120–150 Hz for GWs due to r -modes), we are able to achieve longer coherence times than before, and for the first time set upper limits below the standard torque balance level, independent of neutron star spin inclination, for 50 Hz ≲ f ₀ ≲ 200 Hz. The most sensitive upper limits on the gravitational wave amplitude h ₀ , at the upper end of the frequency band searched, approach 5 × 10 ⁻²⁶ marginalized over inclination angle and 2 × 10 ⁻²⁶ assuming the most favorable inclination. The marginalized upper limits correspond to a sensitivity depth of 70–75 Hz ^−1/2 , improving sensitivity considerably over previous searches. Expressed as constraints on the triaxial deformation of the neutron star, the limits correspond to an ellipticity of 3 × 10 ⁻⁵ if the GW frequency f ₀ is 75 Hz and 3 × 10 ⁻⁶ if f ₀ = 200 Hz, approaching deformations that could be supported by ordinary nuclear matter. Outliers from the search were ruled out as potential signals by a combination of hierarchical follow-up and analysis of additional data from later in the observing run.

We present a search of data from the first part of the fourth observing run of LIGO-Virgo-KAGRA for three kinds of dark matter—dilatons (spin-0), dark photons (spin-1), and tensor bosons (spin-2)—using three independent methods, each extended to search for all three candidates. Each dark matter candidate could interact with different standard-model particles in the instruments, causing unique differential strains on the interferometers. While we do not find any evidence for a signal, we place the most stringent upper limits to date on the couplings of each dark matter model to the interferometers. For scalars with masses between [ 4 × 10 − 14 , 1.5 × 10 − 13 ] eV that couple to photons or electrons, our constraints on the scalar-to-electron or photon coupling improve upon those from the third observing run by 1 order of magnitude, with the tightest limit of ∼ 10 − 20 GeV − 1 at a mass of ∼ 2 × 10 − 13 eV . For vectors with masses between [ 7 × 10 − 13 , 8.47 × 10 − 12 ] eV that couple to baryons, our constraints on the dark-photon or baryon coupling supersede those from MICROSCOPE and Eöt-Wash by 1 to 2 orders of magnitude, reaching a minimum of ∼ 5 × 10 − 24 at a mass of ∼ 10 − 12 eV . For tensors with masses of [ 4 × 10 − 14 , 8.47 × 10 − 12 ] eV (the full mass range analyzed) that couple via a Yukawa interaction, our constraints, which are the first obtained from gravitational-wave interferometers, surpass those from fifth-force experiments by 4 to 5 orders of magnitude, achieving a limit as low as ∼ 8 × 10 − 9 at ∼ 2 × 10 − 13 eV . Our results show that each method has substantially different sensitivity to each dark matter candidate, highlighting the importance of a multimethod approach to search for new physics. Moreover, they demonstrate that gravitational-wave interferometers can simultaneously probe multiple ultralight dark matter models and place unprecedented constraints on tensor fields arising in a broad range of modified-gravity theories.

Context . Low-latency analyses of gravitational-wave (GW) data from LIGO, Virgo, and KAGRA enable rapid detection of compact binary coalescences (CBCs) and prompt sky localization, essential for electromagnetic follow-up in multi-messenger astronomy. Aims . We evaluate the performance and limitations of low-latency sky localization using the BAYESTAR algorithm, and investigate the impact of low-significance Virgo triggers. Methods . We injected simulated CBC signals into Gaussian-stationary noise and into Virgo data from the second part of the third LIGO-Virgo observing run (O3b), then reconstructed skymaps across multiple detector network configurations. Localization accuracy was assessed using percentile-percentile plots, the Jaccard index, and the Kullback-Leibler divergence. Results . Skymap posteriors for binary neutron star mergers are statistically self-consistent, with deviations below 3 σ , particularly when Virgo is included in the network, whereas skymaps for neutron star–black hole and binary black hole mergers tend to be overconfident. Adding a third detector generally improves accuracy, but the searched area can degrade when Virgo’s signal-to-noise ratio (S/N) is low (≤5). For high-S/N events, relying on two detectors can mislocalize the source. Excluding Virgo can therefore cause the HL skymap to miss the true location when Virgo has a strong antenna response; in such cases a three-detector configuration is required to recover the correct position and avoid misleading multi-messenger follow-up. Conclusions . We introduce diagnostics to flag problematic skymaps and apply them to O3 public alerts, recovering simulation-predicted trends and flagging a few anomalous morphologies. The results are relevant for improving rapid vetting of GW alerts and guiding observational strategies in multi-messenger astronomy.

Glitches frequently contaminate data in gravitational wave detectors, complicating the observation and analysis of astrophysical signals. This work introduces VIGILant, an automatic pipeline for classification and visualization of glitches in the Virgo detector. Using a curated dataset of Virgo O3b glitches, two machine learning approaches are evaluated: tree-based models (decision tree, random forest, and extreme gradient boosting) using structured Omicron parameters, and convolutional neural networks (ResNet) trained on spectrogram images. While tree-based models offer higher interpretability and fast training, the ResNet34 model achieved superior performance, reaching a F1 score of 0.9772 and accuracy of 0.9833 in the testing set, with inference times of tens of milliseconds per glitch. The pipeline has been deployed for daily operation at the Virgo site since observing run O4c, providing the Virgo collaboration with an interactive dashboard to monitor glitch populations and detector behavior. This allows to identify low-confidence predictions, highlighting glitches requiring further attention.

We analyze data from 142 of the 218 gravitational-wave (GW) sources in the fourth LIGO–Virgo–KAGRA Collaboration (LVK) Gravitational-Wave Transient Catalog (GWTC-4.0) to estimate the Hubble constant H ₀ jointly with the population properties of merging compact binaries. We measure the luminosity distance and redshifted masses of GW sources directly; in contrast, we infer GW source redshifts statistically through (i) location of features in the compact object mass spectrum and merger rate evolution, and (ii) identifying potential host galaxies in the GW localization volume. Probing the relationship between source luminosity distances and redshifts obtained in this way yields constraints on cosmological parameters. We also constrain parameterized deviations from general relativity that affect GW propagation, specifically those modifying the dependence of a GW signal on the source luminosity distance. Assuming our fiducial model for the source-frame mass distribution and using GW candidates detected up to the end of the fourth observing run (O4a), together with the GLADE+ all-sky galaxy catalog, we estimate H 0 = 75 . 4 − 9.1 + 12.8 ( 75 . 4 − 13.3 + 24.5 ) km s − 1 Mpc − 1 . This value is reported as a median with 68.3% (90%) symmetric credible interval, and includes combination with the H ₀ measurement from GW170817 and its electromagnetic counterpart. Using a parameterization of modified GW propagation in terms of the magnitude parameter Ξ ₀ , we estimate Ξ 0 = 1 . 4 − 0.4 + 1.0 ( 1 . 4 − 0.6 + 2.8 ) , where Ξ ₀ = 1 recovers the behavior of general relativity.

Rotating nonaxisymmetric neutron stars (NSs) are promising sources for continuous gravitational waves (CWs). CWs may, if detected, inform us about the internal structure and equation of state of NSs. Here, we present a narrowband search for CWs from known pulsars, for which a matched-filter search can be applied. Narrowband searches are robust to mismatches between electromagnetic (EM) and gravitational emissions, in contrast to fully targeted searches where they are assumed to be phase-locked. In this work, we search for the CW counterparts emitted by 34 pulsars using data from the first and second parts of the fourth LIGO–Virgo–KAGRA observing run. We use the 5n-vector narrowband pipeline, which applies frequency-domain matched filtering. In previous searches, it covered a narrow range in the frequency—frequency time derivative ( f — f ̇ ) space. Here, we also explore a range in the second time derivative of the frequency f ̈ around the EM observations. Additionally, for the first time, we target sources in a binary system with this kind of search. We find no evidence for CWs and therefore set upper limits on the strain amplitude emitted by each pulsar. For 20 analyses, we report an upper limit below the theoretical spin-down limit. The tightest constraint is for pulsar PSR J0534+2200 (the Crab pulsar), for which our strain upper limit on the CW amplitude corresponds to ≤0.04% of the spin-down power being radiated in the CW channel.