Gregory Ashton

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.

Ground-based gravitational wave detectors are now routinely surveying the dark Universe, finding hundreds of collisions between compact objects. However, terrestrial non-Gaussian noise artefacts, commonly known as glitches, reduce the sensitivity to signals and can overlap signals, producing biased astrophysical inferences. We introduce a hierarchical Bayesian model to measure the glitch rate, which improves upon existing trigger-counting methods in its capacity to measure the rate down into the low signal-to-noise regime without contamination from the Gaussian noise background, provided the population is accurately modelled. The framework accommodates any glitch model, and measures the rate with respect to the model chosen: here we use the antiglitch model, so the rate inferred is that of short-duration glitches rather than of all glitches. The methodology builds on standard hierarchical inference, but includes several novel features: hierarchical inference with quantile compression (HIQC), a generic approximation for the recycled hyperlikelihood, and a time-domain rate estimated by fitting basis functions. We validate the methodology using simulated data with injected glitches and then apply it to data from the fourth LIGO-Virgo-KAGRA observing run, demonstrating time-resolved inferences of the glitch rate over a 24 h period. The inferred glitch rate is consistent with estimates from trigger counts, but requires no arbitrary threshold and provides a more fine-grained view of the temporal behaviour. Finally, we demonstrate how our individual-detector rate estimates can be transformed into a coincident glitch probability and utilise this to provide evidence that the retracted gravitational-wave candidate GW230630_070659 is likely a pair of coincident glitches.

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.

In their fourth observing run, the LIGO-Virgo-KAGRA gravitational-wave observatories have found hundreds of new signals, but many are contaminated by non-Gaussian transient noise artifacts known as glitches. Left unaddressed, glitches can bias parameter inference and lead to misleading astrophysical conclusions. We present a series of case studies using ilby, a parameter estimation tool that employs a time-domain likelihood jointly modeling the astrophysical signal with a physical waveform and non-Gaussian noise with a Gaussian process. We first show that when the detector noise is Gaussian, ilby produces results consistent with those obtained with the standard Gaussian-noise likelihood, and then consider events affected by non-Gaussian features. For GW231123, the highest-mass binary black hole candidate observed to date, analyses using reveal coherent residual structure that leads to measurable shifts in inferred source parameters. In contrast, analyses employing show no significant excess residual power and remain consistent across likelihood choices. This demonstrates that waveform systematics and flexible noise modeling are intrinsically coupled, as the Gaussian process terms can partially absorb coherent waveform mismatches. For GW191109, we find that evidence for spin misalignment remains robust despite glitches in both LIGO detectors. For GW230630_070659, excluded from GWTC-4.0 owing to poor data quality, we find the data to be consistent with a binary black hole waveform model, with no additional residual power identified by the Gaussian process component. Overall, these results highlight how ilby can be used to perform glitch-robust inference and as a tool to understand waveform modeling systematics.

Bayesian inference is commonly employed in the analysis of gravitational-wave signals not only to estimate the source parameters, but also for model selection. The latter provides insight into the physics of the source and has the potential to inform the direction for future model development. Although model comparison is usually performed by analyzing the data separately with different models and comparing the obtained Bayesian evidences, an alternative approach consists in sampling directly over the model itself. Here, we present , a reversible jump Markov chain Monte Carlo sampler capable of performing transdimensional inference on gravitational-wave signals from compact binary coalescences. Employing , a single analysis provides simultaneously the model odds ratio and the parameter posteriors for the favored models, hence yielding a potentially substantial computational advantage, particularly when comparing many models or analyzing highly informative data. is built on the sampler and is specifically designed to compare models describing different kinds of sources, i.e., binary black hole, binary neutron star, or neutron star-black hole systems, as well as multiple models for the same source class. We validate the sampler on a set of injections, finding agreement with the results obtained with the nested sampler . We then use to analyze the real events GW190425 and GW230529, for which the system’s parameters alone do not provide conclusive evidence of the presence of a neutron star component. can be adapted to any model-comparison scenario, thus providing a valuable tool in particular for next-generation detectors, where analyzing data separately with competing models becomes computationally even more demanding.

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.

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.