arXiv subjects

Hector Villarrubia-Rojo

Publications and source records attributed to Hector Villarrubia-Rojo.

9 recordsLinked to original sources

arXiv2026

Effects of massive spin-2 fields on gravitational wave propagation

Massive spin-2 fields in addition to the standard massless graviton arise naturally in extensions of general relativity, such as massive bigravity or models with extra dimensions. This work explores the observational signatures of these fields on the propagation of gravitational waves. Adopting a phenomenological framework consistent with such theories, we derive an analytical transfer function in the ultrarelativistic limit and establish detectability bounds for inspiral sources. Finally, we provide forecasts for the accessible parameter space using current and future gravitational wave detectors.

gr-qc↗

arXiv2026

Across the Universe: GW231123 as a magnified and diffracted black hole merger

GW231123 appears as the most massive binary black hole (BBH) ever observed by the LIGO interferometers with total mass $190-265 M_\odot$. A high observed mass can be explained by the combination of cosmological redshift and gravitational magnification if the source is aligned with a gravitational lens, such as a galaxy. Small-scale objects such as stars and remnants diffract the signal, distorting the wavefront and providing additional lensing signatures. Here we present an analysis of GW231123 combining for the first time the effects of diffraction by a small-scale lens and gravitational magnification by an external potential, modelled as an embedded point-mass lens (PL), finding an intriguing case for the lensing hypothesis. Lensing is favoured by the data, with a false alarm probability of the observed Bayes factors bounded below $<1\%$, or $\sim 2.6 σ$ confidence level. Including lensing lowers the total source mass of GW231123 to $100-180 M_\odot$, closer to BBHs reported so far, and also removes discrepancies between different waveform approximants and the need for high component spins. We reconstruct all source and lens properties, including the microlens mass $190-850 M_\odot$, its offset, the magnitude of the external gravitational potential and its orientation. The embedded PL analysis leads to a lighter microlens compared to the isolated PL. Within our assumptions, the reconstruction is complete up to an ambiguity between the distance and projected density (mass-sheet degeneracy). Assuming a single galaxy as the macroscopic lens allows us to infer the total amplification of the signal, placing the event at redshift $0.7-2$, and predict the probability $~55\%$ of forming an additional detectable image due to strong lensing by the macrolens. We discuss the implications of our findings on the source and nature of the microlens, including a possible dark matter origin.

astro-ph.GA↗

arXiv2026

Testing General Relativity Through Gravitational Wave Classification: A Convolutional Neural Network Framework

We present a machine learning framework for testing general relativity (GR) with gravitational wave signals from binary black hole mergers. Using the source parameters of 173 BBH events from the GWTC catalog as a realistic astrophysical population, we generate simulated GR waveforms and construct beyond GR (BGR) waveforms by applying controlled phase deformations. We introduce a response function formalism that provides a systematic framework for quantifying how any observable responds to modifications of GR. We train convolutional neural networks (CNNs) on two input representations: whitened waveforms and a response function type observable derived from the waveform mismatch, which isolates the effect of phase deviations from the bulk signal. Using response functions as the CNN input improves the classification sensitivity by a factor of approximately 33 compared to whitened waveforms, demonstrating that the choice of observable representation is as important as the classifier architecture. We study the fundamental limits of this classification through Bayes optimal error analysis, averaging methods that reveal coherent patterns hidden in noise, and a comparison between CNN accuracy and a single feature classifier as a proxy for human performance. At all deformation scales, the CNN outperforms the best single feature approach. We extend the framework to physically motivated theories using the parameterized post Einsteinian (ppE) formalism and apply it to massive gravity, where the classifier detects deviations for graviton masses of order $m_g \sim 10^{-23}\;\mathrm{eV}/c^2$ with aLIGO design sensitivity.

gr-qc↗

arXiv2024

GLoW: novel methods for wave-optics phenomena in gravitational lensing

Wave-optics phenomena in gravitational lensing occur when the signal's wavelength is commensurate to the gravitational radius of the lens. Although potentially detectable in lensed gravitational waves, fast radio bursts and pulsars, accurate numerical predictions are challenging to compute. Here we present novel methods for wave-optics lensing that allow the treatment of general lenses. In addition to a general algorithm, specialized methods optimize symmetric lenses (arbitrary number of images) and generic lenses in the single-image regime. We also develop approximations for simple lenses (point-like and singular isothermal sphere) that drastically outperform known solutions without compromising accuracy. These algorithms are implemented in Gravitational Lensing of Waves (GLoW): an accurate, flexible, and fast code. GLoW efficiently computes the frequency-dependent amplification factor for generic lens models and arbitrary impact parameters in O(1 ms) to O(10 ms) depending on the lens configuration and complexity. GLoW is readily applicable to model lensing diffraction on gravitational-wave signals, offering new means to investigate the distribution of dark-matter and large-scale structure with signals from ground and space detectors.

gr-qc↗

arXiv2024

Signatures of dark and baryonic structures on weakly lensed gravitational waves

Gravitational lensing offers a powerful tool for exploring the matter distribution in the Universe. Thanks to their low frequencies and phase coherence, gravitational waves (GWs) allow for the observation of novel wave-optics features (WOFs) in lensing, inaccessible to electromagnetic signals. Combined with the existing accurate source models, lensed GWs can be used to infer the properties of gravitational lenses. The prospect is particularly compelling for space-borne detectors, where the high signal-to-noise ratio expected from massive black hole binary mergers allows WOFs to be distinguished deep into the weak lensing regime, drastically increasing the detection probability. Here, we investigate in detail the capacity of the LISA mission to detect WOFs caused by dark matter halos, galaxies and the supermassive black holes (SMBHs) within them. We estimate the total optical depth to be $λ_{\rm tot} \sim 6 \times 10 ^{-3}$ for the loudest binaries of total mass $M_{\rm BBH} \sim 10^6 M_{\odot}$, with the dominant contribution coming from SMBHs. We also find that WOFs in low-mass binaries $M_{\rm BBH} \sim 10^4 M_{\odot}$ are more likely due to the central galaxies. Within our model of gravitational lenses, we predict $\mathcal{O}(0.1)-\mathcal{O}(1)$ weakly-lensed events to be detectable during the 5 years of LISA mission, depending on the source population models. We show that WOFs signatures are very sensitive to the properties of dark-matter halos with $M_{\rm vir}\in (10^6-10^8)M_\odot$: increasing the compactness parameter by $\sim 3$ in that range raises the detection rate by $\sim 26$. Additionally, we show that collective effects from the complex inner halo structure can further enhance detectability. This suggests that lensed GWs in LISA will be an excellent probe of dark-matter theories, baryonic and halo sub-structures.

gr-qc↗

arXiv2023

Weakly Lensed Gravitational Waves: Probing Cosmic Structures with Wave-Optics Features

Every signal propagating through the universe is at least weakly lensed by the intervening gravitational field. In some situations, wave-optics phenomena (diffraction, interference) can be observed as frequency-dependent modulations of the waveform of gravitational waves (GWs). We will denote these signatures as Wave-Optics Features (WOFs) and analyze them in detail. Our framework can efficiently and accurately compute WOF in the single-image regime, of which weak lensing is a limit. The phenomenology of WOF is rich and offers valuable information: the dense cusps of individual halos appear as peaks in Green's function for lensing. If resolved, these features probe the number, effective masses, spatial distribution and inner profiles of substructures. High signal-to-noise GW signals reveal WOFs well beyond the Einstein radius, leading to a fair probability of observation by upcoming detectors such as LISA. Potential applications of WOF include reconstruction of the lens' projected density, delensing standard sirens and inferring large-scale structure morphology and the halo mass function. Because WOF are sourced by light halos with negligible baryonic content, their detection (or lack thereof) holds promise to test dark matter scenarios.

gr-qc↗

arXiv2022

Can late-time extensions solve the $H_0$ and $σ_8$ tensions?

We analyze the properties that any late-time modification of the $Λ$CDM expansion history must have in order to consistently solve both the $H_0$ and the $σ_8$ tensions. Taking a model-independent approach, we obtain a set of necessary conditions that can be applied to generic late-time extensions. Our results are fully analytical and merely based on the assumptions that the deviations from the $Λ$CDM background remain small. For the concrete case of a dark energy fluid with equation of state $w(z)$, we derive the following general requirements: (i) Solving the $H_0$ tension demands $w(z)<-1$ at some $z$ (ii) Solving both the $H_0$ and $σ_8$ tensions requires $w(z)$ to cross the phantom divide. Finally, we also allow for small deviations on the effective gravitational constant. In this case, our method is still able to constrain the functional form of these deviations.

astro-ph.CO↗

arXiv2022

Simultaneously solving the $H_0$ and $σ_8$ tensions with late dark energy

In a model independent approach, we derive generic conditions that any late time modification of the $Λ$CDM expansion history must satisfy in order to consistently solve both the $H_0$ and the $σ_8$ tensions. Our results are fully analytical and the method is merely based on the assumption that the late-time deviations from $Λ$CDM remain small. For the concrete case of a dark energy fluid with deviations encoded in the expansion history and the gravitational coupling constant, we present necessary conditions on its equation of state. Solving both the $H_0$ and $σ_8$ tensions requires that $w(z)$ must cross the phantom divide if $G_\text{eff}=G$. On the other hand, for $G_\text{eff}=G+δG(z)$ and $w(z)\leq -1$, it is required that $\displaystyle \frac{δG(z)}{G}<α(z)\frac{δH(z)}{H(z)}<0$ at some redshift $z$.

astro-ph.CO↗

arXiv2020

Proca in the sky

The standard model of cosmology, the $Λ$CDM model, describes the evolution of the Universe since the Big Bang with just a few parameters, six in its basic form. Despite being the simplest model, direct late-time measurements of the Hubble constant compared with the early-universe measurements result in the so-called $H_0$ tension. It is claimed that a late time resolution is predestined to fail when different cosmological probes are combined. In this work, we shake the ground of this belief with a very simple model. We show how, in the context of cubic vector Galileon models, the Hubble tension can naturally be relieved using a combination of CMB, BAO and SNe observations without using any prior on $H_0$ . The tension can be reduced even further by including the local measurement of the Hubble constant.

astro-ph.CO↗