Filippo Santoliquido

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 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.

The population of binary black hole (BBH) mergers observed by the LIGO-Virgo-KAGRA (LVK) collaboration offers a window into the cosmic evolution of compact binaries and the physical mechanisms that drive their formation. We employ the semianalytic population-synthesis code - to model BBHs assembled through isolated binary evolution and dynamical interactions in young, globular, and nuclear star clusters. Our framework self-consistently incorporates star formation history, metallicity evolution, and single and binary stellar evolution to quantify their impact on the observable properties of the BBH population and on the relative contribution of distinct formation channels. Our models are characterized by a merger rate, R = 17.5 – 24.1 Gpc − 3 yr − 1 , broadly consistent with LVK constraints. Moreover, the predicted distributions of primary mass, mass ratio, and effective inspiral spin parameter are compatible with those inferred from current LVK observations. Our primary-mass distribution is dominated by isolated binaries at m 1 ≲ 20 M ⊙ , while dynamically assembled first- and higher-generation mergers dominate at larger masses. As a consequence, the subpopulation of mergers with m 1 ≳ 45 M ⊙ exhibits a nearly flat mass-ratio distribution and distinctive spin properties. We leverage our models to explore how: (i) the fraction of stars in isolated binaries and the fraction of stellar mass bound in clusters regulate the overall merger rate; (ii) common-envelope physics shapes the primary-mass distribution and its redshift evolution; (iii) the inclusion of stellar-collision products enhances the formation of higher-generation mergers; and (iv) the natal spin distribution of stellar-mass black holes influences the effective spin of merging BBHs. Using our models to assess possible origins of selected gravitational waves events, we illustrate how the complexity of the underlying astrophysical processes can hinder the possibility to draw definitive conclusions.

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.

Binary black hole (BBH) mergers detected through gravitational waves (GWs) are a promising probe for the cosmic expansion. These sources are standard sirens for which we can directly measure the luminosity distance, but their redshift is degenerate with the determination of their source masses. In analogy to standard candles, the redshift of standard sirens can be obtained using a calibration based on the source mass spectrum, but without the need for a cosmological ladder. The modeling of the BBH mass spectrum is crucial for GW cosmology. In this paper, we propose a new pipeline based on normalizing flows to calibrate population models on synthetic BBH catalogs generated from astrophysical prescriptions. Although these models are less flexible than the current parametric models used in the literature, they have the advantage of directly capturing the correlations between BBH mass and redshift and provide a direct link between different formation channels and cosmological parameter inference. We discuss potential applications and limitations of this method using the B-pop code to generate a mock catalog that contains BBH formed by isolated stellar binaries and in dense stellar environments via dynamical interactions. Using this methodology, we show that B-pop provides a good fit for the mass and redshift distributions of the GW events in GWTC-4.0 data for a flat Λ CDM cosmology with H 0 ∼ 70 km s − 1 Mpc − 1 and Ω m = 0.308 . We also find a correlation between H 0 and the fraction of binaries produced by the dynamical and isolated formation channels arising from the fact that dynamically formed binaries populate the higher side of the mass spectrum.

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.

Among known neutron stars, the Vela pulsar is one of the best targets for gravitational-wave searches. It is also one of the most prolific in terms of glitches, which are sudden frequency changes in a pulsar’s rotation. Such glitches could cause a variety of transient gravitational-wave signals. Here, we search for signals associated with a Vela glitch on 2024 April 29 in data of the two LIGO detectors from the fourth LIGO–Virgo–KAGRA observing run. We search both for seconds-scale burst-like emission, primarily from fundamental ( f -)mode oscillations, and for longer quasi-monochromatic transients up to 4 months in duration, primarily from quasi-static quadrupolar deformations. We find no significant detection candidates, but for the first time we set direct observational upper limits on gravitational strain amplitude that are stricter than what can be indirectly inferred from the overall glitch energy scale. We discuss the short- and long-duration observational constraints in the context of specific emission models. These results demonstrate the potential of gravitational-wave probes of glitching pulsars as detector sensitivity continues to improve.