arXiv subjects

Nicola Bellomo

Publications and source records attributed to Nicola Bellomo.

At least 19 recordsLinked to original sources

arXiv2026

Breaking binary formation mechanism degeneracies with gravitational wave clustering

Thanks to the almost 400 gravitational wave events detected, we are currently able to grasp the fundamental features of black hole mass, spin, and distance distributions. However, such a fast increase in the precision of the measurements does not necessarily correspond to a better theoretical understanding of gravitational wave sources, especially in current scenarios where the number of free parameters is significantly larger than the number of inferred properties of the black hole population. In this work, we showcase how the landscape of theoretical models can be chipped away by complementary data-analysis strategies, in particular by studying the statistical properties of gravitational wave anisotropic distribution. Specifically, we show how gravitational wave clustering is sensitive to two unique features of each binary formation mechanism: the time-delay distribution and the properties of the binary hosts. First, we consider a model-agnostic scenario and show the impact that different time-delay distributions have on the gravitational wave bias. Then, we consider a realistic scenario where gravitational wave events are sourced either by isolated binary evolution or dynamical processes in globular clusters, and study how the gravitational wave bias is unique sensitive to the specific properties of the environment. In both scenarios, we show how the cross-correlation between galaxy and gravitational wave catalogs is able to distinguish between models with different time delays or with different binary sub-populations originated in specific formation channels.

gr-qc↗

arXiv2026

Cross-Correlating the Universe: The Gravitational Wave Background and Large-Scale Structure

The nature of the gravitational wave background (GWB) is a key question in modern astrophysics and cosmology, with significant implications for understanding the structure and evolution of the Universe. We demonstrate how cross-correlating large-scale structure (LSS) tracers with the GWB spatial anisotropies can extract a clear astrophysical imprint from the GWB signal. Focusing on the unresolved population of supermassive black hole binaries (SMBHBs) as the primary source for the GWB at nHz frequencies, we construct full-sky maps of galaxy distributions and characteristic strain of the GWB to explore the relationship between GWB anisotropies and the LSS. We find that at current pulsar timing array (PTA) sensitivities, few loud SMBHBs act as Poisson-like noise. This results in anisotropies dominated by a small number of sources, making GWB maps where SMBHBs trace the LSS indistinguishable from a GWB from a uniform distribution of SMBHBs. In contrast, we find that the bulk of the unresolved SMBHBs produce anisotropies which mirror the spatial distribution of galaxies, and thus trace the LSS. Importantly, we show that cross-correlations are required to retrieve a clear LSS imprint in the GWB. Specifically, we forecast the distinguishability of this LSS signature at a $3σ$ level in near-future PTA experiments that probe angular scales of $\ell_{\text{max}} \geq 42$, and $5σ$ for $\ell_{\text{max}} \geq 72$ in optimistic settings. These values assume that GWB anisotropy maps can be reconstructed at these angular resolutions and that loud sources above a resolvability threshold can be identified and removed. Our approach opens new avenues to employ the GWB as an LSS tracer, providing unique insights into SMBHB population models and the nature of the GWB itself. Our results motivate further exploration of synergies between next-generation PTAs and LSS tracers.

astro-ph.CO↗

arXiv2026

Bias from small-scale leakage in Pulsar Timing Array maps

Pulsar Timing Array experiments are rapidly approaching the era of gravitational wave background anisotropy detection. The timing residuals of each pulsar are an integrated measure of the gravitational-wave power across all angular scales. However, due to the limited number of monitored pulsars, current analyses are only able to reconstruct the angular structure of the background at large scales. We show analytically that this mismatch between the integrated all-sky signal and the truncated reconstruction introduces a previously unaccounted source of systematic bias in the anisotropic background angular power spectrum. The source of this systematic error, that we call ``small-scale leakage'', is the intrinsic presence of unaccounted gravitational wave power at scales smaller than the reconstructed scales. This unmodeled power leaks into large-scale modes, artificially increasing the recovered value of the inferred angular power spectrum by at least one order of magnitude in a wide range of scales. Importantly, this effect is fundamentally independent of the geometry of the pulsar configuration, the anisotropy reconstruction method, the use of different regularization schemes, and the presence of pulsar noise. As the quality of pulsar timing array experiments improves, a robust understanding of small-scale leakage will become paramount for reliable detection and characterization of the gravitational wave background. Thus, the theoretical formalism developed here will be essential to estimate the magnitude of this systematic uncertainty in anisotropy searches.

astro-ph.IM↗

arXiv2026

Mitigating galaxy systematics with gravitational wave clustering

Although currently poorly constrained, cosmological ultra-large scales are expected to provide formidable tests not only of General Relativity, but also of the content of $Λ$CDM and the Early Universe. However, in this regime, cosmic variance plays a major role in limiting sensitivity, and controlling systematic errors becomes a crucial aspect in preserving the limited information content of current and future observations. Multi-tracer analyses represent a useful technique that simultaneously allows to limit the impact of cosmic variance and mitigate the presence of systematics. In this sense, gravitational waves might represent the perfect alternative tracer of the large-scale structure, since their detection suffers from a set of uncertainties completely different from that of traditional large-scale structure surveys. In this work, we provide a concrete example of how gravitational wave clustering mitigates the presence of systematics, and facilitate the discovery of New Physics signatures. Specifically, we focus on systematics that degrade the constraining power on local Primordial non-Gaussianity for future galaxy surveys; and show how catalogs of gravitational wave events detected by third-generation observatories reduce the impact of systematics while being able to maintain flexibility in the statistical analysis. Additionally, we release a new version of the Multi_CLASS code, which now provides an enhanced level of customization of the different tracers and is compatible with the latest releases of CLASS.

astro-ph.CO↗

arXiv2026

Advancing Astrophysics with the SKA II

Advancing Astrophysics with the SKA II (AASKAII), written by our science community, outlines the transformative scientific advances that will be enabled by the SKA telescopes. In the decade since the publication of the previous edition, telescope designs have matured, construction has commenced, and the SKA Organisation has evolved into the SKA Observatory (SKAO). At the same time, observations from SKA precursor and pathfinder telescopes have provided new insights into longstanding scientific challenges while revealing entirely new phenomena. Published in advance of the first science verification campaign for the SKA Observatory, this volume looks ahead to the coming decades of discovery and innovation in radio astronomy. AASKAII spans the broad range of scientific research enabled by the SKA telescopes, SKA-Mid and SKA-Low. The contributions are organised into six thematic categories according to their scientific focus. The opening section presents overview chapters from the SKA Science Working Groups, around which our community is organised. Each overview provides the broader context that connects the contributions in this volume to the key scientific questions being pursued by their respective communities.

astro-ph.IM↗

arXiv2026

Exploring Gravitational Wave Science Frontiers with the SKAO

The Square Kilometre Array Observatory (SKAO) will be an important component of the global gravitational wave network. This article provides an overview of chapter eight of the Advancing Astrophysics with the SKA II (AASKAII) book, in which gravitational waves are a new addition, since the previous edition preceded the announcement of the first detection of gravitational waves in 2016. The chapter investigates the impact that this new observatory will have on numerous gravitational wave science cases. From testing General Relativity, to measuring the properties of the nanohertz gravitational wave background and exploiting new synergies with other upcoming experiments, the SKAO will play a key role in the next decades of gravitational wave science.

gr-qc↗

arXiv2026

Using SKAO to Understand the Clustering of Gravitational Wave Sources

Coalescing Binary Black Holes (BBHs) trace the Large-Scale Structure (LSS) of the Universe, and their clustering properties can be extracted from Gravitational Wave (GW) data. Next-generation detectors, such as the Einstein Telescope and Cosmic Explorer, will enable statistical studies of GW sources thanks to the massive number of detected events. However, such events will still suffer from significant instrumental and theoretical uncertainties. Cross-correlating GW maps with other LSS surveys provides a promising strategy to mitigate these limitations. The SKA-Mid intensity mapping and radio continuum surveys offer ideal datasets for cross-correlation studies with GWs (SKAO$\times$ET2CE). Their wide sky coverage and deep redshift sensitivity will allow precise probing of the epochs and environments where stellar BBHs form most efficiently. In this chapter, we forecast the potential of cross-correlation angular power spectra to extract information on the distribution and clustering properties of GW events. First, we model the number density and bias of three independent tracers: GW sources, neutral hydrogen intensity maps, and radio galaxies. We estimate the constraining power of SKA-Mid$\times$ET2CE on the GW clustering bias, which carries information on the origin of GW progenitors, e.g., whether they formed through stellar evolution or are primordial black holes. Finally, we develop a semi-analytic model for GW events hosted by SKAO galaxies as a function of the time-delay distribution between the binary formation and merger, which is still largely uncertain to date. We forecast the signal-to-noise ratio of their cross-correlation with SKA-Mid, and demonstrate that SKA-Mid$\times$ET2CE will foster our understanding of the time-delay distribution.

astro-ph.CO↗

arXiv2026

Synergies Between Pulsar Timing Array and Astrometry

The presence of a gravitational wave background can be established not only via exquisitely precise pulsar timing array (PTA) measurements, but also via astrometric observations. In fact, the very same background responsible for the delay in the arrival time of pulse is also responsible of an apparent displacement of galactic objects as stars and asteroids. In this chapter we explore the natural synergy between the SKA Observatory, and current/future astrometric probes of the position of Milky Way objects. On top of presenting the potential of SKAO alone in terms of detecting a gravitational wave background, we also demonstrate the increased sensitivity that is actually achievable when SKAO measurements are used in combination with astrometric ones. In particular, we observe an approximate improvement ranging from~$10\%$ up to~$50\%$ in terms of forecast sensitivity for a PTA-astrometry joint-analysis.

astro-ph.IM↗

arXiv2026

CMB Limits on the Absorption of Light Vector and Axial-Vector Dark Matter

Leptophilic sub-MeV spin-1 dark matter (DM) can be converted into a photon via inelastic scattering with a free electron or absorption by a neutral hydrogen atom in the primordial plasma. We study for the first time the impact of the energy injection resulting from such processes on cosmic microwave background (CMB) anisotropies. We obtain upper limits on the vector and axial-vector DM-electron couplings using Planck 2018 temperature, polarization, and lensing data for DM masses between 100 eV and 100 keV. We find that, due to the suppression of the hydrogen atomic form factor at high energies, inelastic scattering provides the dominant constraint for DM masses above the keV scale. At lower masses, hydrogen ionization through DM absorption is the leading channel, driven by the higher efficiency of post-recombination energy injection in modifying the free-electron fraction. Although the bounds we derive are considerably weaker than existing laboratory and astrophysical limits, they provide a robust and independent cosmological probe of leptophilic DM interactions.

astro-ph.CO↗

arXiv2025

From the Solar System to cosmological distances: a complete formalism for gravitational wave astrometry

The presence of a gravitational wave background (GWB) can be established not only via exquisitely precise pulsar timing array (PTA) measurements, but also via astrometric observations. Indeed, the very same background responsible for the delay in the arrival time of pulse causes an apparent displacement of galactic objects as stars and asteroids. In this work we provide a framework that allows to derive the displacement of sources overcoming the usually adopted ``infinite distance'' approximation. We also present how this formalism can be used to study the displacements of objects at distances comparable to the GW wavelength, as asteroids, and of objects with a non-trivial three-dimensional distribution, as stars in the Milky Way. Thus, it can be used to probe frequencies beyond PTA experiments, reaching the mHz GWs, also detectable by LISA. We forecast the capability of observing the astrometric deflection induced by a GWB evaluating the harmonic signal-to-noise ratio including correlations between different probes. We find an SNR greater than one for the relevant cases considered and as a consequence a promising Fisher forecast, suggesting a constraining power up to the percent level for a flat background.

astro-ph.CO↗

arXiv2025

CMB Constraints on Loop-Induced Decays of Leptophilic Dark Matter

Leptophilic sub-MeV spin-zero dark matter (DM) decays into photons via one-loop processes, a scenario that has been in part overlooked in current literature. In this work, we provide updated and comprehensive upper limits on scalar, pseudo-scalar, and axion-like DM-electron couplings based on the latest NPIPE cosmic microwave background data from Planck. Our bounds on the couplings are not only competitive with astrophysical and terrestrial experiments, but outperform them in certain regions of parameter space. Notably, we present the most stringent limits to date on scalar DM with masses around a few keV and pseudo-scalar DM with masses between 100 eV and a few keV. Additionally, we explore, for the first time, the impact of implementing a cosmology-consistent treatment of energy deposition into the cosmic medium.

hep-ph↗

arXiv2025

Testing the growth of cosmic structures during the Dark Ages

Hydrogen 21-cm Line Intensity Mapping offers the unique opportunity to access the Dark Ages and trace the formation and evolution of the large scale structure of the Universe prior to star and galaxy formation. In this work we investigate the potential of future Earth- and Moon-based 21-cm surveys to constrain the growth of structures during the currently unexplored redshift range $30 < z < 200$. On the one hand we show how foreground contamination could limit the capabilities of future instruments in achieving precision below the $10\%$ level. On the other hand, observations from the far side of the Moon have the potential to reach percent or even sub-percent precision in terms of reconstructing the growth of cosmic structures, if foregrounds are robustly accounted for. Such exquisite precision will provide tight constraints on models that induce deviations from $Λ$CDM, not only during the Dark Ages, but also during recombination or that manifest mostly in the low-redshift Universe, like Early Dark Energy and nDGP models, respectively. Thus, because of their insensitivity to non-linearities or astrophysical processes, line intensity mapping surveys will provide a formidable consistency check to potential claims of discoveries of new physics that affect the growth of structures.

astro-ph.CO↗

arXiv2025

The Science of the Einstein Telescope

Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.

gr-qc↗

arXiv2025

New Trends in Kinetic Theory Towards the Complexity of Living Systems

The development of a mathematics for living systems is one of the most challenging prospects of this century. The search began with the pioneering contribution of Ilia Prigogine, who developed methods from statistical physics to describe the dynamics of vehicular traffic. This visionary seminal research contribution has given rise to a great deal of research activity, which began at the end of the last century and has been further developed in this century by several authors who have developed mathematical methods, generally focused on applications. These methods are somewhat inspired by the classical kinetic theory, but significant differences have led to the concept of active particles and to a kinetic theory that is ultimately very different from the classical theory. Different approaches have been developed, each of which is in some way an alternative to the others. This paper develops a critical analysis of the scientific activity after Prigogine with the aim of developing a unified mathematical theory, taking into account the conceivable interactions that a mathematical theory of living systems can have with studies of artificial intelligence.

math.AP↗

arXiv2024

Gravitational wave background from primordial black holes in globular clusters

Primordial black holes still represent a viable candidate for a significant fraction, if not for the totality, of dark matter. If these compact objects have masses of order tens of solar masses, their coalescence can be observed by current and future ground-based gravitational wave detectors. Therefore, finding new gravitational wave signatures associated with this dark matter candidate can either lead to their detection or help constraining their abundance. In this work we consider the phenomenology of primordial black holes in dense environments, in particular globular clusters. We model the internal structure of globular clusters in a semi-analytical fashion, and we derive the expected merger rate. We show that, if primordial black holes are present in globular clusters, their contribution to the GW background can be comparable to other well-known channels, such as early- and late-time binaries, thus enhancing the detectability prospects of primordial black holes and demonstrating that this contribution needs to be taken into account.

astro-ph.CO↗

arXiv2024

The accretion, emission, mass and spin evolution of primordial black holes

Throughout the cosmic history, primordial black holes may experience an efficient phase of baryonic mass accretion from the surrounding medium. While the realm of accretion physics is marked by numerous uncertainties, and a comprehensive understanding remains elusive, recent investigations have delved into this area, exploring its implications for the cosmological evolution of these compact objects. Notably, primordial black holes could experience characteristic growths of their masses and spins, accompanied by the emission of radiation, ultimately responsible for feedback effects that could weaken the efficiency of the process. In this chapter we review the basic formalism to describe the accretion rate, luminosity function and feedback effects, in order to provide distinctive predictions for the evolution of the primordial black hole mass and spin parameters.

astro-ph.CO↗

arXiv2023

Constraining extended cosmologies with GW$\times$LSS cross-correlations

The rapid development of gravitational wave astronomy provides the unique opportunity of exploring the dynamics of the Universe using clustering properties of coalescing binary black hole mergers. Gravitational wave data, along with information coming from future galaxy surveys, have the potential of shedding light about many open questions in Cosmology, including those regarding the nature of dark matter and dark energy. In this work we explore which combination of gravitational wave and galaxy survey datasets are able to provide the best constraints both on modified gravity theories and on the nature of the very same binary black hole events. In particular, by using the public Boltzmann code \texttt{Multi\_CLASS}, we compare cosmological constraints on popular $Λ$CDM extensions coming from gravitational waves alone and in conjunction with either deep and localized or wide and shallow galaxy surveys. We show that constraints on extensions of General Relativity will be at the same level of existing limits from gravitational waves alone or one order of magnitude better when galaxy surveys are included. Furthermore, cross-correlating both kind of galaxy survey with gravitational waves datasets will allow to confidently rule in or out primordial black holes as dark matter candidate in the majority of the allowed parameter space.

astro-ph.CO↗

arXiv2023

Impact of freeze-in on dark matter isocurvature

Dark matter freeze-in is a compelling cosmological production mechanism in which all or some of the observed abundance of dark matter is generated through feeble interactions it has with the Standard Model. In this work we present the first analysis of freeze-in dark matter fluctuations and consider two benchmark models: freeze-in through the direct decay of a heavy vector boson and freeze-in through pair annihilation of Standard Model particles in the thermal bath. We provide a theoretical framework for determining the impact of freeze-in on curvature and dark matter isocurvature perturbations. We determine freeze-in dark matter fluid properties from first principles, tracking its evolution from its relativistic production to its final cold state, and calculate the evolution of the dark matter isocurvature perturbation. We find that in the absence of initial isocurvature, the freeze-in production of dark matter does not source isocurvature. However, for an initial isocurvature perturbation seeded by inflation, the nonthermal freeze-in process may allow for a fraction of the isocurvature to persist, in contrast to the exponential suppression it receives in the case of thermal dark matter. In either case, the evolution of the curvature mode is unaffected by the freeze-in process. We show sensitivity projections of future cosmic microwave background experiments to the amplitude of uncorrelated, totally anticorrelated, and totally correlated dark matter isocurvature perturbations. From these projections, we infer the sensitivity to the abundance of freeze-in dark matter that sustains some fraction of the primordial isocurvature.

astro-ph.CO↗