arXiv subjects

Juan Calderón Bustillo

Publications and source records attributed to Juan Calderón Bustillo.

At least 19 recordsLinked to original sources

arXiv2026

Testing General Relativity with GWTC-4.0 through mixture models

Gravitational-wave observations of compact binary mergers have enabled precision tests of gravity in the strong-field dynamical regime. Current approaches combine single-event results that assume deviations from General Relativity (GR) are uniformly distributed across events, limiting their flexibility and potentially biasing the inferred evidence. We introduce a simple mixture-model framework in which a fraction $ζ$ of events is consistent with GR, while a fraction $1 - ζ$ deviates from it, without imposing constraints on the population distribution of the deviation parameters. We apply this method to publicly available results from the LIGO-Virgo-KAGRA (LVK) collaboration on O1-O4a compact binary mergers obtained through FTI, pSEOBNR, and KerrPostMerger tests. We find that the data are consistent with all events satisfying GR. However, we obtain respective Bayes factors $B^{ζ=1}_{ζ\neq 1} \simeq 20 $, $10$ and $15$, which are much smaller than those inferred from existing LVK analyses, indicating that the data require greater flexibility in modelling possible deviations than standard approaches permit. In light of our results, we recommend using flexible mixture models to test GR across compact-merger catalogues, unless there are obvious physical motivations to impose more restrictive models, as in the case of graviton-mass estimates.

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arXiv2026

Testing bosonic dark matter through white dwarf mass measurements

Mass estimates of white dwarfs via electromagnetic methods, often differ from those obtained through gravitational redshift measurements, in some cases with discrepancies ranging in $5-15\%$ across independent datasets. Although many of the discrepancies reported in large spectroscopic surveys and confirmed by high-precision techniques such as astrometric microlensing and wide-binary analyses may be attributable to thermal effects, model uncertainties or measurement errors prevent a complete description of some of the observations. Here, we explore an alternative explanation based on the presence of a gravitationally coupled bosonic scalar field that contributes to the stellar mass while remaining electromagnetically invisible. We construct stationary, static mixed configurations consisting of a white dwarf that presents a bosonic scalar field (dark matter) component, forming a composite white dwarf-boson star system. We explore families of solutions showing that a scalar field fraction $f_{\rm DM} \sim 5-15\%$ to the mass contribution can account for the observed redshift excess. Our models provide a physically motivated explanation for the mass bias, might offer new observational signatures, and allow us to place preliminary constraints on the mass and compactness of the scalar field configuration. Finally, using our theoretical framework in combination with Bayesian model selection we provide plausible bounds for the mass of the constituent (ultralight) bosonic particle.

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arXiv2026

Intrinsic handedness in O1-O4a black-hole mergers: probing orbital precession, remnant retention in dense environments and cosmological mirror asymmetry

Precessing binary black-holes generically produce an imbalance of right- and left- handed gravitational waves, reflecting the breaking of mirror symmetry by the merger dynamics. We study this phenomenon using the observer-independent quantity $V_{\rm GW}$, a gravitational analogue of the optical Stokes parameter that quantifies the intrinsic handedness of the emitted radiation. Using 91 LIGO-Virgo-KAGRA black-hole mergers from the O1-O4a observing runs, we find that $92\%$ of the analyzed events favour non-vanishing $V_{\rm GW}$, indicating a predominance of precessing dynamics across the events. Through a recently established relation between $V_{\rm GW}$ and the remnant black hole recoil, we further constrain the retention of merger remnants in dense stellar environments, finding that at most $8\%$ could remain gravitationally bound to globular or nuclear star clusters and subsequently participate in hierarchical merger channels. We finally investigate the cosmological distribution of black-hole merger handedness. The observed $V_{\rm GW}$ distribution is consistent with symmetry under $V_{\rm GW}\rightarrow -V_{\rm GW}$, and yields an average value $\langle V_{\rm GW}\rangle=-1.9^{+6.1}_{-6.6}\times10^{-3}$ ($90\%$ credibility), consistent with the absence of a preferred handedness and with expectations from large-scale statistical isotropy. In particular, the inclusion of O4a events reduces uncertainties in $\langle V_{\rm GW} \rangle$ by $\sim 40\%$ with respect to O1-O3 events. These results establish black-hole merger handedness as a unified probe of orbital precession, remnant recoil, hierarchical formation, and cosmological mirror symmetry.

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arXiv2026

A complete measurement of a black-hole recoil through higher-order gravitational-wave modes

General relativity predicts that gravitational waves (GWs) carry linear momentum. Consequently, the remnant black hole of a black-hole merger can inherit a recoil velocity or ``kick'' of crucial implications in, e.g., black-hole formation scenarios. While the kick magnitude is determined by the mass ratio and spins of the source, estimating its direction requires a measurement of the \textit{two orientation angles} of the source. While the orbital inclination angle is commonly reported in GW observations, the scientific potential of the azimuthal one has not been exploited to date. We show how the presence of more than one GW emission mode allows one to constrain this angle and, consequently, the kick direction of a real GW event. We analyse the GW190412 signal, which contains higher-order modes, with a numerical-relativity surrogate waveform model for black-hole mergers. We rule out kick magnitudes below the typical escape velocity of dense globular clusters $v_{\text{esc}}\approx 50$\,km/s with a Bayes Factor of $\simeq 21$ (or $\simeq 95\%$ probability). The kick forms angles $θ_{KL}^{-100M}=32^{+35}_{-14}\,°$ with the orbital angular momentum defined at a reference time $t_{\rm ref}=-100\,M$ before merger (with $M$ denoting the system mass in geometric units), $θ_{KN}=44^{+19}_{-17}\,°$ with the line-of-sight. The projections of the kick and line-of-sight onto the orbital plane form an angle $ϕ_{KN}^{-100M}=69^{+33}_{-38}\,°$. All quantities are quoted at a $90\%$ credible level. Finally, by analyzing numerically simulated signals, we show that recoils can be estimated in an unbiased way using the NRSur7dq4 waveform model. We briefly discuss the potential application of this type of measurement for multi-messenger observations of black-hole mergers occurring in Active Galactic Nuclei.

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arXiv2025

Kick & spin: new probes for multi-messenger black-hole mergers in AGNs

Recoiling remnants of black-hole mergers in dense environments can produce bright electromagnetic (EM) counterparts to the gravitational-wave (GW) emission. Significance assessments of such GW-EM candidates are restricted to time and sky-localisation consistency, omitting the physics governing the EM emission process. Different emission mechanisms, however, impose different observability constraints on the remnant black-hole recoil and spin, which are gravitational-wave observables. We present a statistical framework that includes such parameters. We assess the consistency of the GW190521-ZTF19abanrhr pair with two types of emission processes: a Blandford-Znajek jet closely aligned with the final spin axis and a diffusive isotropic flare. Assuming the sky-location of ZTF19abanrhr, we find these mechanisms to be respectively strongly and moderately disfavoured with log-evidences $\log_{10}{\cal I}_{\rm jet} = -1.65$ and $\log_{10}{\cal I}_{\rm diff} = -0.075$. Combining these with odds for a common sky-location $Ω$ we obtain respective combined odds $\log_{10} {\cal O}_{Ω,{\rm jet}}= -1.17$ and $\log_{10} {\cal O}_{Ω,{\rm diff}}= +0.39$ for a true GW-EM coincidence as opposed to a random one. Our method leverages a previously unexplored evidence axis to assess GW-EM associations and constrain both the physics powering flare mechanisms and the properties of AGNs.

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arXiv2025

Identifying Proca-star mergers via consistent ultralight-boson mass estimates across gravitational-wave events

While black-hole and neutron-star mergers are the most plausible sources of current gravitational-wave observations, mergers of exotic compact objects may mimic these signals. Proca stars -- Bose-Einstein condensates of complex vector ultralight bosons -- have gained significant attention for their potential to replicate certain gravitational-wave events while yielding consistent estimates of the boson mass $μ_{B}$ forming the stars. Using a mixture model within a Bayesian framework, we demonstrate that consistent boson-mass estimates across events can be exploited to obtain conclusive evidence for the existence \cor{of} a number $n$ of Proca-star families characterized by respective boson masses $μ_{B}^{i}$, even if no individual event can be conclusively identified as such. Our method provides posterior distributions for $n$ and $μ_{B}^{i}$. Applying this framework to the high-mass events GW190521, GW190426\_190642 \cor{and} GW200220\_061928, we obtain a Bayes Factor ${\cal{B}}^{n=0}_{n=1}=2$ against the Proca-star hypothesis, primarily rooted in the limitation of current Proca-star merger \cor{signal models} to intrinsically weak head-on cases. We show that conclusive evidence $\log{\cal{B}}^{n=1}_{n=0} \geq 5$ could be achieved after 5 to 9 observations of similar event sets, at the $90\%$ credible level. Our framework provides a new way to detect exotic compact objects, somewhat using gravitational-wave detectors as particle detectors.

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arXiv2025

Gravitational-wave signatures of mirror (a)symmetry in binary black hole mergers: measurability and correlation to gravitational-wave recoil

Precessing binary black-hole mergers can produce a net flux of circularly-polarized gravitational waves. This imbalance between left- and right-handed circularly polarized waves, quantified via the Stokes pseudo-scalar $V_{\rm GW}$, originated from mirror asymmetries in the binary. We scan the parameter space of black-hole mergers to investigate correlations between $V_{\rm GW}$ and chiral magnitudes constructed out of the intrinsic parameters of the binary. To this end, we use both numerical-relativity simulations for (quasi-circular) and eccentric precessing mergers from both the SXS and RIT catalogues, as well as the state-of-the-art surrogate model for quasi-circular precessing mergers NRSur7dq4. We find that, despite being computed by manifestly different formulas, $V_{\rm GW}$ is linearly correlated to the helicity of the final black hole, defined as the projection of its recoil velocity onto its spin. Next, we test our ability to perform accurate measurements of $V_{\rm GW}$ in gravitational-wave observations through the injection and recovery of numerically simulated signals. We show that $V_{\rm GW}$ can be estimated unbiasedly using the surrogate waveform model NRSur7dq4 even for signal-to-noise ratios of nearly 50, way beyond current gravitational-wave observations.

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arXiv2025

Black-hole ringdown analysis with inspiral-merger informed templates and limitations of classical spectroscopy

The ringdown of a perturbed black hole consists of a superposition of quasi-normal modes (QNMs), with complex frequencies determined by the black hole's mass and spin, while phases and amplitudes depend on binary parameters. Traditional semi-agnostic ringdown analyses ignore the remnant's binary-merger origin, assuming instead generic perturbations. This can lead to an unphysical number of free parameters and usage of priors inconsistent with those imposed by full inspiral-merger-ringdown (IMR) analysis. Here we revisit an alternative approach, first proposed in Calderón Bustillo et al. 2021, in which ringdowns are modeled using the post-merger portion of full IMR waveform models. This naturally includes all QNMs without adding extra degrees of freedom and ties priors to binary parameters. We analyze the signal GW150914 with the post-merger portion of the IMR surrogate model NRSur7dq4, finding decisive evidence for the no-hair theorem with a Bayes factor $>650:1$ or $>99.8$% probability, compared to only $3:1$ from semi-agnostic spectroscopy, consistent with previous work. We also find modest evidence ($\simeq 5:1$) for the $(\ell,m,n)=(2,2,1)$ overtone and none for higher modes. Using simulated signals, we validate our formalism by showing accurate recovery of remnant properties. Next, by analysing simulated signals with post-merger signal-to-noise ratios up to 100, we show that classical spectroscopy based on overtone detection alone cannot provide strong no-hair tests. Finally, we demonstrate that the non-orthogonality of QNMs makes the inferred mode content strongly model-dependent, leading to multiple inconsistent interpretations that are equally well-supported by the data. We briefly discuss potential implications for recent events, such as GW231123 and GW250114.

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arXiv2025

Self-consistent treatment of thermal effects in neutron-star post-mergers: observational implications for third-generation gravitational-wave detectors

We assess the impact of accurate, self-consistent modelling of thermal effects in neutron-star merger remnants in the context of third-generation gravitational-wave detectors. This is done through the usage, in Bayesian model selection experiments, of numerical-relativity simulations of binary neutron star (BNS) mergers modelled through: a) nuclear, finite-temperature (or ``tabulated'') equations of state (EoSs), and b) their simplifed piecewise (or ``hybrid'') representation. These cover four different EoSs, namely SLy4, DD2, HShen and LS220. Our analyses make direct use of the Newman-Penrose scalar $ψ_4$ outputted by numerical simulations. Considering a detector network formed by three Cosmic Explorers, we show that differences in the gravitational-wave emission predicted by the two models are detectable with a natural logarithmic Bayes Factor $\log{\cal{B}}\geq 5$ at average distances of $d_L \simeq 50$Mpc, reaching $d_L \simeq 100$Mpc for source inclinations $ι\leq 0.8$, regardless of the EoS. This impact is most pronounced for the HShen EoS. For low inclinations, only the DD2 EoS prevents the detectability of such modelling differences at $d_L \simeq 150$Mpc. Our results suggest that the usage a self-consistent treatment of thermal effects is crucial for third-generation gravitational wave detectors.

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arXiv2024

Testing mirror symmetry in the Universe with LIGO-Virgo black-hole mergers

Certain precessing black-hole mergers produce gravitational waves with net circular polarization, understood as an imbalance between right- and left-handed amplitudes. According to the Cosmological Principle, such emission must average to zero across all binary mergers in our Universe to preserve mirror-reflection symmetry at very large scales. We present a new independent gravitational-wave test of this hypothesis. Using a novel observable based on the Chern-Pontryagin pseudo-scalar, we measure the emission of net circular polarization across 47 black-hole mergers recently analyzed by Islam et. al. with a state-of-the art model for precessing black-hole mergers in General Relativity. The average value obtained is consistent with zero. Remarkably, however, we find that at least $82\%$ of the analysed sources must have produced net circular polarization. Of these, GW200129 shows strong evidence for mirror asymmetry, with a Bayes Factor of 12.6 or, equivalently, $93.1\%$ probability. We obtain consistent (although stronger) results of $97.5\%$ and $94.3\%$ respectively using public results on this event from Hannam et. al. and performing our own parameter inference. This finding further implies evidence of astrophysical sources that can spontaneously emit circularly polarized photons by quantum effects. Forthcoming black-hole merger detections will enable stronger constraints on large-scale mirror asymmetry and the Cosmological Principle.

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arXiv2024

Kicking time back in black-hole mergers: Ancestral masses, spins, birth recoils and hierarchical-formation viability of GW190521

Pair-instability supernova (PISN) prevents black-hole formation from stellar collapse within the approximate mass range $M\in [65,130]M_\odot$. However, such black holes may form hierarchically through merging ancestral black holes, whose properties determine those of the ``child'' one: mass, spin, and recoil velocity. Crucially, the child will leave its host environment if its ``birth recoil'' exceeds the corresponding escape velocity, preventing further mergers. We exploit relations between the final recoil and spin of quasi-circular black-hole mergers to obtain posterior probability distributions for the hypothetical ancestral masses, spins and birth recoils of the component black holes of GW190521. To this, we present a Bayesian framework applicable to existing estimates for the components of black-hole merger observations. We consider both the quasi-circular (generically spinning) analysis performed by the LIGO-Virgo-KAGRA collaboration and the eccentric (aligned-spin) one performed by Romero-Shaw et. al. We evaluate the probability $p_{2g}$ that the GW190521 components inferred by these analyses formed from the merger of stellar-origin black holes and were retained by their environment. For the primary component, which populates the PISN gap, such scenario is strongly suppressed if GW190521 happened in a Globular Cluster with $p_{2g} \sim 10^{-3}$ unless it was quasi-circular and its ancestors had aligned-spins, uncharacteristic of hierarchical formation channels, or small spins, which yields $p_{2g} \simeq 10^{-2}$. If GW190521 was eccentric, we obtain $p_{2g} \simeq 0.1$ for any host other than an AGN, and zero for a Globular Cluster. If GW190521 was quasi-circular, a Nuclear-Star Cluster origin is possible with $p_{2g} \in (\sim 0.4 \sim ,0.8)$

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arXiv2024

Rotating Fermion-Boson Stars

Rotating fermion-boson stars are hypothetical celestial objects that consist of both fermionic and bosonic matter interacting exclusively through gravity. Bosonic fields are believed to arise in certain models of particle physics describing dark matter and could accumulate within neutron stars, modifying some of their properties and gravitational wave emission. Fermion-boson stars have been extensively studied in the static non-rotating case, exploring their combined stability and their gravitational radiation in binary mergers. However, stationary rotating configurations were yet to be found and investigated. The presence of a bosonic component could impact the development of the bar-mode instability in differentially rotating neutron stars. Therefore, the study of rotating fermion-boson stars has important implications for astrophysics, as they could provide a new avenue for the detection of gravitational waves. In addition, these objects may shed light on the behavior of matter under extreme conditions, such as those found in the cores of neutron stars, and explain any tension in the determination of the dense-matter equation of state from multi-messenger observations. In this work we study a new consistent method of constructing uniformly rotating fermion-boson stars and we analyse some of their main properties. These objects might offer alternative explanations for current observations populating the lower black-hole mass gap, as the $2.6 M_\odot$ compact object involved in GW190814.

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arXiv2024

A numerical-relativity surrogate model for hyperbolic encounters of black holes: challenges in parameter estimation

We present a surrogate numerical-relativity model for close hyperbolic black-hole encounters with equal masses and spins aligned with the orbital momentum. Our model, generated in terms of the Newman-Penrose scalar $ψ_4$, spans impact parameters $b/M\in [11, 15]$ and spin components $χ_{i} \in [-0.5,0.5]$, modeling the $(\ell,m)=(2,0)$, $(2, \pm 2)$, $(3,\pm 2)$ and $(4,\pm 4)$ emission multipoles. The model is faithful to numerical relativity simulations, yielding mismatches lower than $10^{-3}$. We test the ability of our model to recover the parameters of numerically simulated signals. We find that, despite the high accuracy of the model, parameter inference struggles to correctly capture the parameters of the source even for SNRs as large as 50 due to the strong degeneracies present in the parameter space. This indicates that correctly identifying these systems will require of extremely large signal loudness, only typical of third generation detectors. Nevertheless, we also find that, if one attempts to infer certain combinations of such degenerated parameters, there might be a chance to prove the existence of this type of events, even with the current ground-based detectors, as long as these combinations make sense astrophysically and cosmologically.

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arXiv2024

Numerical relativity surrogate models for exotic compact objects: the case of head-on mergers of equal-mass Proca stars

We present several high-accuracy surrogate models for gravitational-wave signals from equal-mass head-on mergers of Proca stars, computed through the Newman-Penrose scalar $ψ_4$. We also discuss the current state of the model extensions to mergers of Proca stars with different masses, and the particular challenges that these present. The models are divided in two main categories: two-stage and monolithic. In the two-stage models, a dimensional reduction algorithm is applied to embed the data in a reduced feature space, which is then interpolated in terms of the physical parameters. For the monolithic models, a single neural network is trained to predict the waveform from the input physical parameter. Our model displays mismatches below $10^{-3}$ with respect to the original numerical waveforms. Finally, we demonstrate the usage of our model in full Bayesian parameter inference through the accurate recovery of numerical relativity signals injected in zero-noise, together with the analysis of GW190521. For the latter, we observe excellent agreement with existing results that make use of full numerical relativity.

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arXiv2024

Impact of Bayesian Priors on the Inferred Masses of Quasi-Circular Intermediate-Mass Black Hole Binaries

Observation of gravitational waves from inspiralling binary black holes has offered a unique opportunity to study the physical parameters of the component black holes. To infer these parameters, Bayesian methods are employed in conjunction with general relativistic waveform models that describe the source's inspiral, merger, and ringdown. The results depend not only on the accuracy of the waveform models but also on the underlying fiducial prior distribution used for the analysis. In particular, when the pre-merger phase of the signal is barely observable within the detectors' bandwidth, as is currently the case with intermediate-mass black hole binary signals in ground-based gravitational wave detectors, different prior assumptions can lead to different interpretations. In this study, we utilise the gravitational-wave inference library, $\texttt{Parallel Bilby}$, to evaluate the impact of mass prior choices on the parameter estimation of intermediate-mass black hole binary signals. While previous studies focused primarily on analysing event data, we offer a broader, more controlled study by using simulations. Our findings suggest that the posteriors in total mass, mass ratio and luminosity distance are contingent on the assumed mass prior distribution used during the inference process. This is especially true when the signal lacks sufficient pre-merger information and/or has inadequate power in the higher-order radiation multipoles. In conclusion, our study underscores the importance of thoroughly investigating similarly heavy events in current detector sensitivity using a diverse choice of priors. Absent such an approach, adopting a flat prior on the binary's redshifted total mass and mass ratio emerges as a reasonable choice, preventing biases in the detector-frame mass posteriors.

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arXiv2024

Solving the Teukolsky equation with physics-informed neural networks

We use physics-informed neural networks (PINNs) to compute the first quasi-normal modes of the Kerr geometry via the Teukolsky equation. This technique allows us to extract the complex frequencies and separation constants of the equation without the need for sophisticated numerical techniques, and with an almost immediate implementation under the \texttt{PyTorch} framework. We are able to compute the oscillation frequencies and damping times for arbitrary black hole spins and masses, with accuracy typically below the percentual level as compared to the accepted values in the literature. We find that PINN-computed quasi-normal modes are indistinguishable from those obtained through existing methods at signal-to-noise ratios (SNRs) larger than 100, making the former reliable for gravitational-wave data analysis in the mid term, before the arrival of third-generation detectors like LISA or the Einstein Telescope, where SNRs of ${\cal O}(1000)$ might be achieved.

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arXiv2023

Impact of ringdown higher-order modes on black-hole mergers in dense environments: the scalar field case, detectability and parameter biases

Dense environments hosting compact binary mergers can leave an imprint on the gravitational-wave emission which, in turn, can be used to identify the characteristics of the environment. To demonstrate such scenario, we consider a simple setup of binary black holes with an environment consisting of a scalar-field bubble. We use this as a proxy for more realistic environments and as an example of the simplest physics beyond the standard model. We perform Bayesian inference on the numerical relativity waveforms using state-of-the-art waveform templates for black-hole mergers. In particular, we perform parameter estimation and model selection on signals from black-hole mergers with different mass ratios, total mass and loudness, and hosted by scalar-field bubbles of varying field amplitude. We find that sub-dominant gravitational wave modes emitted during the coalescence and ringdown are key to identifying environmental effects. In particular, we find that for face-on signals dominated by the quadrupole mode, the environment is only detectable if both the ringdown and the late inspiral/early merger fall in the detector band, so that inconsistencies can be found between the inferred binary parameters and those of the final black hole. For edge-on mergers we find that the environment can be detected even if only the ringdown is in band, thanks to the information encoded in the quasi-normal mode structure of the final black-hole.

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arXiv2023

Comparison of neural network architectures for feature extraction from binary black hole merger waveforms

We evaluate several neural-network architectures, both convolutional and recurrent, for gravitational-wave time-series feature extraction by performing point parameter estimation on noisy waveforms from binary-black-hole mergers. We build datasets of 100,000 elements for each of four different waveform models (or approximants) in order to test how approximant choice affects feature extraction. Our choices include \texttt{SEOBNRv4P} and \texttt{IMRPhenomPv3}, which contain only the dominant quadrupole emission mode, alongside \texttt{IMRPhenomPv3HM} and \texttt{NRHybSur3dq8}, which also account for high-order modes. Each dataset element is injected into detector noise corresponding to the third observing run of the LIGO-Virgo-KAGRA (LVK) collaboration. We identify the Temporal Convolutional Network (TCN) architecture as the overall best performer in terms of training and validation losses and absence of overfitting to data. Comparison of results between datasets shows that the choice of waveform approximant for the creation of a dataset conditions the feature extraction ability of a trained network. Hence, care should be taken when building a dataset for the training of neural networks, as certain approximants may result in better network convergence of evaluation metrics. However, this performance does not necessarily translate to data which is more faithful to numerical relativity simulations. We also apply this network on actual signals from LVK runs, finding that its feature-extracting performance can be effective on real data.

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