arXiv subjects

Angelo Ricciardone

Publications and source records attributed to Angelo Ricciardone.

At least 37 records · Page 2Linked to original sources

arXiv2025

Non-linear effects on the Cosmological Gravitational Wave Background anisotropies

The Cosmological Gravitational Wave Background (CGWB) anisotropies contain valuable information about the physics of the early universe. Given that General Relativity is intrinsically nonlinear, it is important to look beyond first-order contributions in cosmological perturbations. In this work, we present a non-perturbative approach for the computation of CGWB anisotropies at large scales, providing the extension of the initial conditions and the Sachs-Wolfe effect for the CGWB, which encodes the full non-linearity of the scalar metric perturbations. We also derive the non-perturbative expression for three-point correlation of the gravitational wave energy density perturbation in the case of an inflationary CGWB with a scale-invariant power spectrum and negligible primordial non-Gaussianity. We show that, under such conditions, the gravitational wave energy density perturbations are lognormally distributed, leading to an interesting effect such as intermittency.

astro-ph.CO↗

arXiv2025

\texttt{GWBird}: a toolkit for the characterization of the Stochastic Gravitational Wave Background for Ground, Space, and Pulsar Timing Array detectors

The detection of the Stochastic Gravitational Wave Background (SGWB) is one of the most challenging tasks for both current and next-generation detectors. Successfully distinguishing the SGWB from instrumental noise and environmental effects requires accurate and flexible analysis tools capable of detecting the signal and determining its origin. In this paper, we introduce a unified framework and a user-friendly tool for SGWB characterization: \texttt{GWBird} (Gravitational Wave Background Inventory of Response functions for Detectors). This code enables the computation of overlap reduction functions (ORFs), power-law integrated sensitivity curves (PLS), angular response functions, and angular PLS (APLS). It supports the full range of gravitational wave polarization modes (tensor, scalar, and vector), allowing for the characterization of both isotropic and anisotropic SGWB components for all the polarizations. Additionally, the code includes functions for circular polarization characterization, which is particularly relevant for probing parity-violating signals. The framework integrates analyses for ground-based, space-based, and Pulsar Timing Array (PTA) detectors, offering a versatile framework for SGWB analysis. The \texttt{GWBird} code is publicly available at:~\github{https://github.com/ilariacaporali/GWBird}

astro-ph.IM↗

arXiv2025

Scalar-Induced Gravitational Waves in Modified Gravity

Scalar-Induced Gravitational Waves (SIGWs), second-order tensor modes sourced by first-order scalar fluctuations in General Relativity (GR), are expected to contribute to the Stochastic Gravitational Wave Background (SGWB) potentially detectable by current and future gravitational wave interferometers. In the framework of GR, this SGWB represents an unavoidable contribution to the gravitational wave spectrum. In this paper, we go beyond GR and we investigate the behavior of SIGWs in f(R) gravity. We explore how the SIGW spectrum is influenced across a broad range of frequencies, from the nano-Hz regime, where the Pulsar Timing Array (PTA) operates, through the milli-Hz band probed by the space-based LISA detector, up to the kilo-Hz frequency range, where the ground-based LIGO/Virgo/KAGRA network is currently operational. Our results indicate that the beyond-GR correction leaves an observational imprint, mainly in the low-frequency part of the spectrum, giving the possibility to use SIGW to constrain GR on scales on which we have limited information.

gr-qc↗

arXiv2025

Assessing the Impact of Unequal Noises and Foreground Modeling on SGWB Reconstruction with LISA

In the search for stochastic gravitational wave backgrounds (SGWB) of cosmological origin with LISA, it is crucial to account for realistic complications in the noise and astrophysical foreground modeling that may impact the signal reconstruction. To address these challenges, we updated the $\texttt{SGWBinner}$ code to incorporate both variable noise levels across LISA arms and more complex foreground spectral shapes. Our findings suggest that, while moderate variations of the noise amplitudes have a minimal impact, poor foreground modeling (i.e., templates requiring many free parameters) significantly degrades the reconstruction of cosmological signals. This underlines the importance of accurate modeling and subtraction of the astrophysical foregrounds to characterize possible cosmological components. To perform this more challenging analysis, we have integrated the $\texttt{JAX}$ framework, which significantly improves the computational efficiency of the code, in the $\texttt{SGWBinner}$ code, enabling faster Bayesian likelihood sampling and more effective exploration of complex SGWB signals.

gr-qc↗

arXiv2025

Likelihoods for Stochastic Gravitational Wave Background Data Analysis

We present a systematic study of likelihood functions used for Stochastic Gravitational Wave Background (SGWB) searches. By dividing the data into many short segments, one customarily takes advantage of the Central Limit Theorem to justify a Gaussian crosscorrelation likelihood. We show, with a hierarchy of ever more realistic examples, beginning with a single frequency bin and one detector, and then moving to two and three detectors with white and colored signal and noise, that approximating the exact Whittle likelihood by various Gaussian alternatives can induce systematic biases in the estimation of the SGWB parameters. We derive several approximations for the full likelihood and identify regimes where Gaussianity breaks down. We also discuss the possibility of conditioning the full likelihood on fiducial noise estimates to produce unbiased SGWB parameter estimation. We show that for some segment durations and bandwidths, particularly in space-based and pulsar-timing arrays, the bias can exceed the statistical uncertainty. Our results provide practical guidance for segment choice, likelihood selection, and data-compression strategies to ensure robust SGWB inference in current and next-generation gravitational wave detectors.

gr-qc↗

arXiv2025

Reconstructing Primordial Curvature Perturbations via Scalar-Induced Gravitational Waves with LISA

Many early universe scenarios predict an enhancement of scalar perturbations at scales currently unconstrained by cosmological probes. These perturbations source gravitational waves (GWs) at second order in perturbation theory, leading to a scalar-induced gravitational wave (SIGW) background. The LISA detector, sensitive to mHz GWs, will be able to constrain curvature perturbations in a new window corresponding to scales $k \in [10^{10}, 10^{14}] \,{\rm Mpc}^{-1}$, difficult to probe otherwise. In this work, we forecast the capabilities of LISA to constrain the source of SIGWs using different approaches: i) agnostic, where the spectrum of curvature perturbations is binned in frequency space; ii) template-based, modeling the curvature power spectrum based on motivated classes of models; iii) ab initio, starting from first-principles model of inflation featuring an ultra-slow roll phase. We compare the strengths and weaknesses of each approach. We also discuss the impact on the SIGW spectrum of non-standard thermal histories affecting the kernels of SIGW emission and non-Gaussianity in the statistics of the curvature perturbations. Finally, we propose simple tests to assess whether the signal is compatible with the SIGW hypothesis. The pipeline used is built into the SIGWAY code.

astro-ph.CO↗

arXiv2025

Impact of correlated noise on the reconstruction of the stochastic gravitational wave background with Einstein Telescope

Einstein Telescope (ET) is a proposed next-generation Gravitational Wave (GW) interferometer designed to detect a large number of astrophysical and cosmological sources with unprecedented sensitivity. A key target for ET is the detection of a stochastic gravitational-wave background (SGWB), a faint signal from unresolved GW sources. In its proposed triangular configuration, correlated Newtonian noise of seismic origin poses some challenges for the SGWB detection. We study the impact of correlated noise on the SGWB detection and relative parameter estimation for ET in the triangular configuration, comparing it to a 2L configuration with two separated L-shaped detectors. We perform a Bayesian analysis on simulated data, which shows that accurate reconstruction of the SGWB parameters and instrumental noise is achievable if the noise is properly modeled. We illustrate that neglecting correlated noise leads to significant biases in the parameter reconstruction. Our results show that while the 2L configuration provides slightly better parameter estimation precision, mainly due to its longer arm length, the triangular configuration remains competitive when accurate noise modeling is provided.

gr-qc↗

arXiv2024

Inflation without an Inflaton

We propose a novel scenario in which scalar perturbations, that seed the large scale structure of the Universe, are generated without relying on a scalar field (the inflaton). In this framework, inflation is driven by a de Sitter space-time (dS), where tensor metric fluctuations (i.e., gravitational waves) naturally arise from quantum vacuum oscillations, and scalar fluctuations are generated via second-order tensor effects. We compute the power spectrum of such scalar fluctuations and show it to be consistent with near scale-invariance. We derive the necessary conditions under which scalar perturbations become significant and much larger than the tensor modes, and we identify a natural mechanism to end inflation via a transition to a radiation-dominated phase. Our proposed mechanism could remove the need for a model-dependent scenario: the choice of a scalar field, as the inflaton, to drive inflation.

astro-ph.CO↗

arXiv2024

Gravitational waves from inflation in LISA: reconstruction pipeline and physics interpretation

Various scenarios of cosmic inflation enhance the amplitude of the stochastic gravitational wave background (SGWB) at frequencies detectable by the LISA detector. We develop tools for a template-based analysis of the SGWB and introduce a template databank to describe well-motivated signals from inflation, prototype their template-based searches, and forecast their reconstruction with LISA. Specifically, we classify seven templates based on their signal frequency shape, and we identify representative fundamental physics models leading to them. By running a template-based analysis, we forecast the accuracy with which LISA can reconstruct the template parameters of representative benchmark signals, with and without galactic and extragalactic foregrounds. We identify the parameter regions that can be probed by LISA within each template. Finally, we investigate how our signal reconstructions shed light on fundamental physics models of inflation: we discuss their impact for measurements of \emph{e.g.,} ~the couplings of inflationary axions to gauge fields; the graviton mass during inflation; the fluctuation seeds of primordial black holes; the consequences of excited states during inflation, and the presence of small-scale spectral features.

astro-ph.CO↗

arXiv2024

Enhancing gravitational-wave host localization with SKYFAST: rapid volume and inclination angle reconstruction

The scientific impact of GW170817 strongly supports the need for an efficient electromagnetic follow-up campaign to gravitational-wave event candidates. The success of such campaigns depends critically on a fast and accurate localization of the source. In this paper, we present SKYFAST, a new pipeline for rapid localization of gravitational-wave event hosts. SKYFAST runs alongside a full parameter estimation (PE) algorithm, from which posterior samples are taken. It uses these samples to reconstruct an analytical posterior for the sky position, luminosity distance, and inclination angle using a Dirichlet Process Gaussian Mixture Model, a Bayesian non-parametric method. This approach allows us to provide an accurate localization of the event using only a fraction of the total samples produced by the full PE analysis. Depending on the PE algorithm employed, this can lead to significant time savings, which is crucial for identifying the electromagnetic counterpart. Additionally, in a few minutes, SKYFAST generates a ranked list of the most probable galaxy hosts from a galaxy catalog of choice. This list includes information on the inclination angle posterior conditioned to the position of each candidate host, which is useful for assessing the detectability of gamma-ray burst structured jet emissions.

astro-ph.HE↗

arXiv2024

Inflationary Initial Conditions for the Cosmological Gravitational Wave Background

The initial conditions on the anisotropies of the stochastic gravitational-wave background of cosmological origin (CGWB) largely depend on the mechanism that generates the gravitational waves. Since the CGWB is expected to be non-thermal, the computation of the initial conditions could be more challenging w.r.t. the Cosmic Microwave Background (CMB), whose interactions with other particles in the early Universe lead to a blackbody spectrum. In this paper, we show that the initial conditions for the cosmological background generated by quantum fluctuations of the metric during inflation deviate from adiabaticity. These primordial gravitational waves are indeed generated by quantum fluctuations of two independent degrees of freedom (the two polarization states of the gravitons). Furthermore, the CGWB plays a negligible role in the Einstein's equations, because its energy density is subdominant w.r.t. ordinary matter. Therefore, the only possible way to compute the initial conditions for inflationary gravitons is to perturb the energy-momentum tensor of the gravitational field defined in terms of the small-scale tensor perturbation of the metric. This new and self-consistent approach shows that a large, non-adiabatic initial condition is present even during the single-field inflation. Such a contribution enhances the total angular power spectrum of the CGWB compared to the standard adiabatic case, increasing also the sensitivity of the anisotropies to the presence of relativistic and decoupled particles in the early Universe. In this work we have also proved that our findings are quite general and apply to both single-field inflation and other scenarios in which the CGWB is generated by the quantum fluctuations of the metric, like the curvaton.

astro-ph.CO↗

arXiv2024

Proper time path integrals for gravitational waves: an improved wave optics framework

When gravitational waves travel from their source to an observer, they interact with matter structures along their path, causing distinct deformations in their waveforms. In this study we introduce a novel theoretical framework for wave optics effects in gravitational lensing, addressing the limitations of existing approaches. We achieve this by incorporating the proper time technique, typically used in field theory studies, into gravitational lensing. This approach allows us to extend the standard formalism beyond the eikonal and paraxial approximations, which are traditionally assumed, and to account for polarization effects, which are typically neglected in the literature. We demonstrate that our method provides a robust generalization of conventional approaches, including them as special cases. Our findings enhance our understanding of gravitational wave propagation, which is crucial for accurately interpreting gravitational wave observations and extracting unbiased information about the lenses from the gravitational wave waveforms.

astro-ph.CO↗

arXiv2024

Investigating the impact of galaxies' compact binary hosting probability for gravitational-wave cosmology

With the advent of future-generation interferometers a huge number of Gravitational Wave (GW) signals is expected to be measured without an electromagnetic counterpart. Although these signals do not allow a simultaneous measurement of the redshift and the luminosity distance, it is still possible to infer cosmological parameters. In this paper, we focus on the systematic biases that could arise from mismodeling the GW host probability when inferring the Hubble constant ($H_0$) with GW dark sirens jointly with galaxy catalogues. We discuss the case in which the GW host probability is a function of galaxies' luminosity and redshift as it has been predicted by state-of-the-art compact binary coalescences (CBCs) synthetic catalogues. We show that, in the limiting case in which the analysis is done with a complete galaxy catalog covering a footprint of $\sim 10~\rm {deg}^2$, mismatching the host probability in terms of galaxy's luminosity will introduce a bias on $H_0$. In this case, the magnitude of the bias will depend on the distribution of the Large-Scale Structure over the line-of-sight. Instead, in the limit of a complete wide-field of view galaxy catalog and GW events localized at O$({\rm Gpc})$ distance, mismatching the redshift dependence of the GW hosting probability is more likely to introduce a systematic bias.

astro-ph.CO↗

arXiv2024

Cosmological Background Interpretation of Pulsar Timing Array Data

We discuss the interpretation of the detected signal by Pulsar Timing Array (PTA) observations as a gravitational wave background (GWB) of cosmological origin. We combine NANOGrav 15-years and EPTA-DR2new data sets and confront them against backgrounds from supermassive black hole binaries (SMBHBs), and cosmological signals from inflation, cosmic (super)strings, first-order phase transitions, Gaussian and non-Gaussian large scalar fluctuations, and audible axions. We find that scalar-induced, and to a lesser extent audible axion and cosmic superstring signals, provide a better fit than SMBHBs. These results depend, however, on modeling assumptions, so further data and analysis are needed to reach robust conclusions. Independently of the signal origin, the data strongly constrain the parameter space of cosmological signals, for example, setting an upper bound on primordial non-Gaussianity at PTA scales as $|f_{nl}| \lesssim 2.34$ at 95% CL.

astro-ph.CO↗

arXiv2024

Non-Gaussianity from the Cross-correlation of the Astrophysical Gravitational Wave Background and the Cosmic Microwave Background

Since the first LIGO/Virgo detection, Gravitational Waves (GWs) have been very promising as a new complementary probe to understand our Universe. One of the next challenges of GW search is the detection and characterization of the stochastic gravitational wave background (SGWB), that is expected to open a window on the very early Universe (cosmological background) and to provide us new information on astrophysical source populations (astrophysical background). One way to characterize the SGWB and to extract information about its origin is through the cross-correlation with other cosmological probes. To this aim, in this paper, we explore the cross-correlation between the astrophysical background anisotropies and the Cosmic Microwave Background (CMB) ones. Such a signal is sensitive to primordial non-Gaussianity (nG) through the GW bias. Thus, we study the capability of next generation space-based interferometers to detect such a cross-correlation signal and to constrain primordial nG.

astro-ph.CO↗

arXiv2024

Fully non-Gaussian Scalar-Induced Gravitational Waves

Scalar-induced Gravitational Waves (SIGWs) represent a particular class of primordial signals which are sourced at second-order in perturbation theory whenever a scalar fluctuation of the metric is present. They form a guaranteed Stochastic Gravitational Wave Background (SGWB) that, depending on the amplification of primordial scalar fluctuations, can be detected by GW detectors. The amplitude and the frequency shape of the scalar-induced SGWB can be influenced by the statistical properties of the scalar density perturbations. In this work we study the intuitive physics behind SIGWs and we analyze the imprints of local non-Gaussianity of the primordial curvature perturbation on the GW spectrum. We consider all the relevant non-Gaussian contributions up to fifth-order in the scalar seeds without any hierarchy, and we derive the related GW energy density $Ω_{\rm GW}(f)$. We perform a Fisher matrix analysis to understand to which accuracy non-Gaussianity can be constrained with the LISA detector, which will be sensitive in the milli-Hertz frequency band. We find that LISA, neglecting the impact of astrophysical foregrounds, will be able to measure the amplitude, the width and the peak of the spectrum with an accuracy up to $\mathcal{O}(10^{-4})$, while non-Gaussianity can be measured up to $\mathcal{O}(10^{-3})$. Finally, we discuss the implications of our non-Gaussianity expansion on the fraction of Primordial Black Holes.

astro-ph.CO↗

arXiv2024

LISA Definition Study Report

The Laser Interferometer Space Antenna (LISA) is the first scientific endeavour to detect and study gravitational waves from space. LISA will survey the sky for Gravitational Waves in the 0.1 mHz to 1 Hz frequency band which will enable the study of a vast number of objects ranging from Galactic binaries and stellar mass black holes in the Milky Way, to distant massive black-hole mergers and the expansion of the Universe. This definition study report, or Red Book, presents a summary of the very large body of work that has been undertaken on the LISA mission over the LISA definition phase.

astro-ph.CO↗

arXiv2023

Unraveling the CMB lack-of-correlation anomaly with the cosmological gravitational wave background

Since the very first observations, the Cosmic Microwave Background (CMB) has revealed on large-scales unexpected features known as anomalies, which challenge the standard $Λ$ cold dark matter ($Λ$CDM) cosmological model. One such anomaly is the "lack-of-correlation", where the measured two-point angular correlation function of CMB temperature anisotropies is compatible with zero, differently from the predictions of the standard model. This anomaly could indicate a deviation from the standard model, unknown systematics, or simply a rare realization of the model itself. In this study, we explore the possibility that the lack-of-correlation anomaly is a consequence of living in a rare realization of the standard model, by leveraging the potential information provided by the cosmological gravitational wave background (CGWB) detectable by future gravitational wave (GW) interferometers. We analyze both constrained and unconstrained realizations of the CGWB to investigate the extent of information that GWs can offer. To quantify the impact of the CGWB on the lack-of-correlation anomaly, we employ established estimators and introduce a new estimator that addresses the "look-elsewhere" effect. Additionally, we consider three different maximum multipoles, denoted as $\ell_{\rm max}$, to account for the anticipated capabilities of future GW detectors ($\ell_{\rm max} = 4, 6, 10$). Summarizing our findings for the case of $\ell_{\rm max} = 4$, we identify the angular range $[63^\circ - 180^\circ]$ as the region where future observations of the CGWB maximize the probability of rejecting the standard model. Furthermore, we calculate the expected significance of this observation, demonstrating that 98.81% (81.67%) of the GW realizations enhance the current significance of the anomaly when considering the full-sky (masked) Planck SMICA map as our CMB sky.

astro-ph.CO↗