arXiv subjects

Giorgio Mentasti

Publications and source records attributed to Giorgio Mentasti.

15 recordsLinked to original sources

arXiv2026

An explicit and differentiable Wilson-Daubechies-Meyer transform for gravitational-wave data analysis

The Wilson-Daubechies-Meyer (WDM) time-frequency transform has been widely used in gravitational-wave astronomy, yet a self-contained, mathematically explicit reference for practitioners remains lacking. This is especially true for those wishing to adopt the transform in modern Python and JAX inference workflows. We present wdm_transform, an open-source Python package implementing the WDM wavelet-packet time-frequency transform, and document its mathematical foundations, statistical properties, and practical implementation for gravitational-wave data analysis. The package supplies NumPy and JAX backends, both transforms (forward and inverse) validated to floating-point precision, with the JAX backend enabling GPU-accelerated transforms of million-point data streams in tens of milliseconds. As a worked example, we verify that the WDM-domain likelihood reproduces frequency-domain posteriors for a resolved LISA galactic binary under a shared stationary noise model, confirming numerical equivalence of the two representations in that controlled setting. This work paves the way for systematic optimisation of WDM tilings, a particularly promising direction for the non-stationary noise, stochastic backgrounds, and data gaps anticipated in future detectors, and for direct comparisons with alternative time-frequency representations needed to meet the challenges of future gravitational-wave data analysis.

gr-qc↗

arXiv2026

Sachs-Wolfe effect as a smoking gun for cosmological gravitational wave backgrounds

The Sachs-Wolfe (SW) effect, arising from large-scale structures in the universe, modifies the frequencies of gravitational waves (GWs) sourced by a cosmological background. We show that for backgrounds with $Ω_{\rm GW}\gtrsim 10^{-10}$, this effect imprints anisotropies and spectral distortions that can be detectable with a network of space-based interferometers (such as LISA + Taiji) and, if not taken into account, may bias the estimate of the theoretical model of the GW background. The effect is particularly enhanced in the high-frequency end of the spectrum. The SW-induced anisotropies and spectral distortions present in a GW background sourced at primordial times will correlate with the SW signature present in the CMB. Any detection of a cross-correlation between the GW anisotropies and the CMB at large scales is therefore a smoking gun for confirming the primordial nature of the background.

gr-qc↗

arXiv2026

The Doppler effect of the Milky Way rotation on LISA

The galactic background of gravitational waves (GWs) is expected to be anisotropic due to the spatial distribution and kinematics of sources in the Milky Way. In this work, we model the stellar density and velocity profiles of the Galaxy and compute the resulting GW spectrum as a function of direction. We account for the Doppler shift induced by the peculiar velocities of stars and the observer's motion. Using a Fisher matrix formalism, we forecast the ability of future detectors (e.g., LISA) to distinguish between models that include or neglect these kinematic effects. We find that if one does not take into account the rotation of the galaxy, the inference of the parameters describing the galactic background can suffer observable biases.

astro-ph.GA↗

arXiv2025

Cross-correlating Astrometric and Timing Residuals to Constrain Stochastic Gravitational-Wave Backgrounds

We investigate the cross-correlation between astrometric and timing-residual observables for distant sources, such as pulsars and galaxies, and equivalent observables for nearby solar system bodies. Using the unified spin-weighted formalism introduced in [1], we derive the angular correlation functions-generalised Hellings-Downs curves-that describe the response of these mixed observables to a stochastic, unpolarised gravitational-wave background (SGWB). We compute the expected signal-to-noise ratio (SNR) and sensitivity for such measurements, focusing on cross-correlations between pulsar timing array (PTA) redshift signals and astrometric or distortion (shimmering) effects induced in solar system objects such as asteroids. Although the current astrometric precision of asteroid tracking does not yet provide competitive constraints relative to PTA-only surveys, the method offers a complementary probe with enhanced sensitivity at higher frequencies. Future wide-field surveys capable of sub-milliarcsecond precision could make this approach a viable tool for detecting or constraining the SGWB. A key advantage of the technique is its reduced susceptibility to correlated systematics across different measurement domains, providing an independent cross-check of PTA detections and a potential observational bridge between PTA and LISA frequency bands.

gr-qc↗

arXiv2025

Searching for Gravitational Waves with Gaia and its Cross-Correlation with PTA: Absolute vs Relative Astrometry

Astrometric missions like Gaia provide exceptionally precise measurements of stellar positions and proper motions. Gravitational waves traveling between the observer and distant stars can induce small, correlated shifts in these apparent positions, a phenomenon known as astrometric deflection. The precision and scale of astrometric datasets make them well-suited for searching for a stochastic gravitational wave background, whose signature appears in the two-point correlation function of the deflection field across the sky. Although Gaia achieves high accuracy in measuring angular separations in its focal plane, systematic uncertainties in the satellite's absolute orientation limit the precision of absolute position measurements. These orientation errors can be mitigated by focusing on relative angles between star pairs, which effectively cancel out common-mode orientation noise. In this work, we compute the astrometric response and the overlap reduction functions for this relative astrometry approach, correcting previous expressions presented in the literature. We use a Fisher matrix analysis to compare the sensitivity of relative astrometry to that of conventional absolute astrometry. Our analysis shows that while the relative method is theoretically sound, its sensitivity is limited for closely spaced star pairs within a single Gaia field of view. Pairs with large angular separations could provide competitive sensitivity, but are practically inaccessible due to Gaia's scanning law. Finally, we demonstrate that combining astrometric data with observations from pulsar timing arrays leads to slight improvements in sensitivity at frequencies greater than approximately 10^-7 Hz.

gr-qc↗

arXiv2025

Strong scale-dependence does not enhance the kinematic boosting of gravitational wave backgrounds

Existing expressions in the literature appear to indicate that Doppler boosting, due to our proper motion with respect to the isotropic frame of the universe, can amplify stochastic gravitational wave backgrounds whose energy spectra exhibit strong scale dependence, for example, those generated by large scalar perturbations in models of primordial black holes or by astrophysical populations with broken power-law behaviour. It has been suggested that this enhancement could increase the signal-to-noise ratio of such backgrounds in pulsar timing measurements, as well as in ground- and space-based observatories. We show that the reported enhancement is an artefact of a Taylor expansion of the boosted signal, typically performed in the literature under the assumption of a small boosting parameter. This approximation fails to reproduce the correct result for signals with strong scale dependence. When Doppler boosting is treated exactly, the apparent amplification disappears. Using representative spectra, we demonstrate that Doppler motion induces only blue- and red-shifting by the expected amount; it does not lead to additional amplification or introduce new spectral features. The exact expression for the kinematic boost can and should be easily applied in analysing such backgrounds.

astro-ph.CO↗

arXiv2025

A unified spin-harmonic framework for correlating pulsar timing, astrometric deflection, and shimmering gravitational wave observations

We present a unified spin-weighted harmonic framework that delivers analytic, diagonal expressions for the overlap (correlation) functions of three low frequency gravitational wave observables-pulsar timing redshifts, astrometric deflections, and time-dependent image distortions (``shimmering''). Writing each response in spin-$s$ spherical harmonics and rotating to a basis in which the wave tensor has definite helicity, we obtain compact closed-form series for every auto- and cross-correlation, recovering the Hellings-Downs curve as the $s=0$ limit and deriving its astrometric ($s=\pm 1$) and shimmering ($s=\pm 2$) analogues. The formalism naturally extends to non-standard scalar-breathing, longitudinal, and vector polarisation modes, clarifying when higher-spin observables are (and are not) sourced and providing a complete set of harmonic spectra $C_\ell$ ready for parameter estimation pipelines. These results supply the common theoretical language needed to combine upcoming pulsar timing, Gaia-class astrometric, and high resolution imaging data sets, enabling coherent, multi probe searches for stochastic gravitational wave backgrounds, tests of general relativity and its alternatives across the nano- to micro-hertz gravitational wave band.

astro-ph.CO↗

arXiv2025

Covariant diffusion and drift of the stochastic GW background with LISA

We study the covariant diffusion and drift of massless particles on the light cone within the context of quantum gravity phenomenology. Unlike modified dispersion relations that violate Lorentz invariance and grow with frequency, this model introduces a stochastic correction to the massless geodesic equation while preserving Lorentz invariance, and is dominant at lower frequencies due to the larger spacetime support of long-wavelength modes. The effect is phenomenologically described by just two diffusion and drift parameters, $κ_1$ and $κ_2$, whose values are already constrained by measurements of the CMB blackbody spectrum. We show that a direct measurement and characterization of a gravitational wave (GW) background frequency spectrum can improve bounds on these diffusion and drift parameters by over 12 orders of magnitude compared to those from the CMB. In particular, we find that detecting a GW background sourced by realistic models of first-order phase transitions or primordial black holes (PBH) with LISA can constrain the parameters down to a value of $κ_1,\,κ_2\lesssim 10^{-56}\,\text{kg}\,\text{m}^2\text{s}^{-3}$.

gr-qc↗

arXiv2025

Prospects for detecting new dark physics with the next generation of atomic clocks

Wide classes of new fundamental physics theories cause apparent variations in particle mass ratios in space and time. In theories that violate the weak equivalence principle (EP), those variations are not uniform across all particles and may be detected with atomic and molecular clock frequency comparisons. In this work we explore the potential to detect those variations with near-future clock comparisons. We begin by searching published clock data for variations in the electron-proton mass ratio. We then undertake a statistical analysis to model the noise in a variety of clock pairs that can be built in the near future according to the current state of the art, determining their sensitivity to various fundamental physics signals. Those signals are then connected to constraints on fundamental physics theories that lead directly or indirectly to an effective EP-violating, including those motivated by dark matter, dark energy, the vacuum energy problem, unification or other open questions of fundamental physics. This work results in projections for tight new bounds on fundamental physics that could be achieved with atomic and molecular clocks within the next few years. Our code for this work is packaged into a forecast tool that translates clock characteristics into bounds on fundamental physics.

hep-ph↗

arXiv2024

Cosmic Shimmering: the Gravitational Wave Signal of Time-Resolved Cosmic Shear Observations

We introduce a novel approach for detecting gravitational waves through their influence on the shape of resolved astronomical objects. This method, complementary to pulsar timing arrays and astrometric techniques, explores the time-dependent distortions caused by gravitational waves on the shapes of celestial bodies, such as galaxies or any resolved extended object. By developing a formalism based on that adopted in the analysis of weak lensing effects, we derive the response functions for gravitational wave-induced distortions and compute their angular correlation functions. Our results highlight the sensitivity of these distortions to the lowest frequencies of the gravitational wave spectrum and demonstrate how they produce distinct angular correlation signatures, including null and polarisation-sensitive correlations. These findings pave the way for future high-resolution surveys to exploit this novel observable, potentially offering new insights into the stochastic gravitational wave background and cosmological models.

astro-ph.CO↗

arXiv2024

Observing gravitational waves with solar system astrometry

The subtle influence of gravitational waves on the apparent positioning of celestial bodies offers novel observational windows. We calculate the expected astrometric signal induced by an isotropic Stochastic Gravitational Wave Background (SGWB) in the short distance limit. Our focus is on the resultant proper motion of Solar System objects, a signal on the same time scales addressed by Pulsar Timing Arrays (PTA). We derive the corresponding astrometric deflection patterns, finding that they manifest as distinctive dipole and quadrupole correlations, or in some cases, may not be present. Our analysis encompasses both Einsteinian and non-Einsteinian polarisations. We estimate the upper limits for the amplitude of a scale-invariant SGWB that could be obtained by tracking the proper motions of large numbers of solar system objects such as asteroids. With the Gaia satellite and the Vera C. Rubin Observatory poised to track an extensive sample of asteroids-ranging from $O(10^5)$ to $O(10^6)$, we highlight the significant future potential for similar surveys to contribute to our understanding of the SGWB.

gr-qc↗

arXiv2024

Probing the galactic and extragalactic gravitational wave backgrounds with space-based interferometers

We employ the formalism developed in \cite{Mentasti:2023gmg} and \cite{Bartolo_2022} to study the prospect of detecting an anisotropic Stochastic Gravitational Wave Background (SGWB) with the Laser Interferometer Space Antenna (LISA) alone, and combined with the proposed space-based interferometer Taiji. Previous analyses have been performed in the frequency domain only. Here, we study the detectability of the individual coefficients of the expansion of the SGWB in spherical harmonics, by taking into account the specific motion of the satellites. This requires the use of time-dependent response functions, which we include in our analysis to obtain an optimal estimate of the anisotropic signal. We focus on two applications. Firstly, the reconstruction of the anisotropic galactic signal without assuming any prior knowledge of its spatial distribution. We find that both LISA and LISA with Taiji cannot put tight constraints on the harmonic coefficients for realistic models of the galactic SGWB. We then focus on the discrimination between a galactic signal of known morphology but unknown overall amplitude and an isotropic extragalactic SGWB component of astrophysical origin. In this case, we find that the two surveys can confirm, at a confidence level $\gtrsim 3σ$, the existence of both the galactic and extragalactic background if both have amplitudes as predicted in standard models. We also find that, in the LISA-only case, the analysis in the frequency domain (under the assumption of a time average of data taken homogeneously across the year) provides a nearly identical determination of the two amplitudes as compared to the optimal analysis.

gr-qc↗

arXiv2023

Prospects for detecting anisotropies and polarization of the stochastic gravitational wave background with ground-based detectors

We build an analytical framework to study the observability of anisotropies and a net chiral polarization of the Stochastic Gravitational Wave Background (SGWB) with a generic network of ground-based detectors. We apply this formalism to perform a Fisher forecast of the performance of a network consisting of the current interferometers (LIGO, Virgo and KAGRA) and planned third-generation ones, such as the Einstein Telescope and Cosmic Explorer. Our results yield limits on the observability of anisotropic modes, spanning across noise- and signal-dominated regimes. We find that if the isotropic component of the SGWB has an amplitude close to the current limit, third-generation interferometers with an observation time of $10$ years can measure multipoles (in a spherical harmonic expansion) up to $\ell = 8$ with ${\cal O }\left( 10^{-3} - 10^{-2} \right)$ accuracy relative to the isotropic component, and an ${\cal O }\left( 10^{-3} \right)$ amount of net polarization. For weaker signals, the accuracy worsens as roughly the inverse of the SGWB amplitude.

gr-qc↗

arXiv2023

Intrinsic limits on the detection of the anisotropies of the Stochastic Gravitational Wave Background

For any given network of detectors, and for any given integration time, even in the idealized limit of negligible instrumental noise, the intrinsic time variation of the isotropic component of the Stochastic Gravitational Wave Background (SGWB) induces a limit on how accurately the anisotropies in the SGWB can be measured. We show here how this sample limit can be calculated and apply this to three separate configurations of ground-based detectors placed at existing and planned sites. Our results show that in the idealized, best-case scenario individual multipoles of the anisotropies at $\ell \leq 8$ can only be measured to $\sim 10^{-5} - 10^{-4}$ level over 5 years of observation as a fraction of the isotropic component. As the sensitivity improves as the square root of the observation time, this poses a very serious challenge for the measurement of the anisotropies of SGWB of cosmological origin, even in the case of idealised detectors with arbitrarily low instrumental noise.

gr-qc↗

arXiv2021

ET sensitivity to the anisotropic Stochastic Gravitational Wave Background

We study the sensitivity of a pair of Einstein Telescopes (ET) (hypothetically located at the two sites currently under consideration for ET) to the anisotropies of the Stochastic Gravitational Wave Background (SGWB). We focus on the $\ell =0,2,4$ multipoles of an expansion of the SGWB in spherical harmonics, since the sensitivity to other multipoles is suppressed due to the fact that this pair of detector operates in a regime for which the product between the observed frequency and the distance between the two sites is much smaller than one. In this regime, the interferometer overlap functions for the anisotropic signal acquire very simple analytic expressions. These expressions can also be applied to any other pairs of interferometers (each one of arbitrary opening angle between its two arms) operating in this regime. Once the measurements at the vertices of the two sites are optimally combined, the sensitivity to the multipoles of the SGWB depends only on the latitude of the two sites, on the difference of their longitude, but not on the orientation of their arms.

astro-ph.CO↗