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Emanuele Berti

Publications and source records attributed to Emanuele Berti.

At least 19 recordsLinked to original sources
arXiv2026

Excitation factors and exceptional points of Kerr-de Sitter quasinormal modes

We compute the excitation factors of Kerr-de Sitter black hole quasinormal modes using two different techniques: a Heun function representation and an expansion of the solution in terms of hypergeometric functions. Exceptional points at which the complex frequencies of different overtones coincide are a generic feature of the Kerr-de Sitter quasinormal mode spectrum. Near exceptional points, the modes exhibit a "hysteresis phenomenon" that was previously found for Kerr black holes perturbed by massive fields and the absolute value of the excitation factors of the individual modes is significantly enhanced, while they acquire nearly opposite phases. We extrapolate the excitation factors to small values of the cosmological constant and find good agreement with previous calculations in the Kerr limit.

gr-qc↗
arXiv2026

All You Need is not $Ω_\mathrm{gw}$: Beyond the Mean of the Cross-Correlation Estimator when Searching for an Astrophysical Gravitational-Wave Background

Searches for the stochastic gravitational-wave background (SGWB) using ground-based detectors rely heavily on the cross-correlation estimator $\bar C_f$, whose statistical properties determine the theoretical foundation for inference frameworks built on it. Past analyses usually assume Gaussianity of $\bar C_f$, but this assumption has not been systematically tested for a background of astrophysical origin like the one produced by compact binary coalescences. In this work, we decompose $\bar C_f$ into three physically motivated components: (1) the mean intensity, (2) the geometrical shot noise, and (3) the polarization leakage, and we compute 90\% credible intervals for its cumulants up to fourth order for the binary black hole, binary neutron star, and neutron star-black hole populations by propagating local merger rate uncertainties. We find that for the upcoming LIGO O5 sensitivity, $\bar C_f$ remains effectively Gaussian for all three populations, validating current Gaussian likelihood frameworks. For Cosmic Explorer (CE), however, the $\bar C_f$ produced by binary black holes becomes non-Gaussian, with skewness and excess kurtosis of $0.31^{+0.04}_{-0.03}$ and $1.3^{+0.4}_{-0.3}$, respectively, in the most sensitive band over a one-year observation period. For binary neutron star and neutron star - black hole mergers, $\bar C_f$ remains largely Gaussian even at CE. These results indicate that the Edgeworth expansion provides an adequate leading-order description of the non-Gaussianity expected in the next-generation era, while also motivating the development of more sophisticated statistical methods for a more accurate treatment.

gr-qc↗
arXiv2026

Black hole spectroscopy: from theory to experiment

The "ringdown" radiation emitted by oscillating black holes has great scientific potential. By carefully predicting the frequencies and amplitudes of black hole quasinormal modes and comparing them with gravitational-wave data from compact binary mergers we can advance our understanding of the two-body problem in general relativity, verify the predictions of the theory in the regime of strong and dynamical gravitational fields, and search for physics beyond the Standard Model or new gravitational degrees of freedom. We summarize the state of the art in our understanding of black hole quasinormal modes in general relativity and modified gravity, their excitation, and the modeling of ringdown waveforms. We also review the status of LIGO-Virgo-KAGRA ringdown observations, data analysis techniques, and the bright prospects of the field in the era of LISA and next-generation ground-based gravitational-wave detectors.

gr-qc↗
arXiv2026

Systematic biases in parameter estimation on LISA binaries. II. The effect of excluding higher harmonics for spin-aligned, high-mass binaries

The Laser Interferometer Space Antenna (LISA) will observe massive black hole binaries (MBHBs) with astoundingly high signal-to-noise ratio, leaving parameter estimation with these signals susceptible to seemingly small waveform errors. Of particular concern for MBHBs are errors due to neglected higher-order modes. We extend Yi et al. [arXiv:2502.12237] to examine errors due to neglected higher-order modes for MBHBs with nonzero (aligned) progenitor spins and total mass up to $10^8\,M_\odot$. For these very massive systems, there can be regions of parameter space in which the $(\ell, |m|)=(2,\,2)$ modes are no longer dominant with respect to higher-order ones. We find that the extent of systematic bias can change significantly when varying the progenitor spins of the binary. We also find that for the heaviest, and therefore shortest, MBHB signals, slight systematic errors can cause severe misinference of the sky localization parameters. We propose an improved likelihood optimization scheme with respect to previous work as a way to predict these effects in a computationally efficient manner.

gr-qc↗
arXiv2026

Modern tidal interaction models for rapid binary population synthesis: II. Binary black hole formation, mergers, and spins

We present predictions for the merger rates and effective spin ($χ_{\rm eff}$) distribution of binary black holes (BBHs) from isolated binary evolution, using a new self-consistent tidal dissipation implementation in the rapid binary population synthesis code COMPAS. Most of the first-born black holes (BHs) in our simulated merging BBHs are formed with zero spins, with the exception of BBHs formed from chemically homogeneous evolution. The spins of the second-born BHs with the new model depend significantly on the efficiency of tidal dissipation and mass transfer history, and crucially, are not always consistent with pre-supernova synchronization. High-$χ_{\rm eff}$ binaries preferentially merge at high redshift due to smaller binary separations at BBH formation and shorter coalescence times, thus rendering them largely inaccessible to current gravitational wave (GW) detectors. We expect the intrinsic spin distribution of merging BBHs formed from isolated evolution to be strongly biased toward low $χ_{\rm eff}$ with current detectors, with a third of systems having $χ_{\rm eff} < 0.05$ and only $\sim 3\%$ with $χ_{\rm eff}>0.5$. However, $χ_{\rm eff}$ will increase as GW detectors become sensitive to higher redshift sources, with up to $\sim 15\%$ of systems having $χ_{\rm eff}>0.5$.

astro-ph.HE↗
arXiv2026

Modern tidal interaction models for rapid binary population synthesis: I. Methods

In this work, we present an updated prescription of contemporary tidal dissipation theory adapted for rapid binary population synthesis. Our simplified expressions encode the dependence of tidal dissipation on stellar structure, stratification, and tidal forcing frequency, while remaining computationally efficient. We implement these prescriptions in the rapid population synthesis code COMPAS, and demonstrate the self-consistent coupling of tides with stellar evolution and binary properties such as orbital periods, spins, and eccentricities for several representative binary systems. When compared with commonly used tidal prescriptions, our equilibrium tidal dissipation efficiencies can be stronger by 1-2 orders of magnitude for low mass main sequence and giant type stars, and dynamical tides can be stronger by 1-7 orders of magnitude due to the explicit dependence on internal stellar structure and the presence of inertial wave dissipation. Despite our simplistic approach, our models agree with detailed stellar simulations to within an order of magnitude across tidal dissipation mechanisms.

astro-ph.SR↗
arXiv2026

Improving low-latency multi-messenger follow-up of neutron star-black hole mergers with mode-by-mode filtering

Rapid parameter estimation for neutron star-black hole (NSBH) mergers is essential for deciding whether, where, and how electromagnetic facilities should follow up gravitational-wave alerts. Current low-latency analyses typically use only the dominant quadrupole harmonic, leaving strong degeneracies among luminosity distance, inclination, and intrinsic binary parameters. We show that mode-by-mode filtering of the $(2,2)$, $(3,3)$, and $(4,4)$ signal-to-noise-ratio (SNR) time series enables low-latency marginalization over higher-order-mode information at a computational cost comparable to quadrupole-only analyses. Applied to simulated NSBH detections in a LIGO-Virgo network at design sensitivity, our method improves constraints on luminosity distance, viewing angle, localization volume, and source-frame secondary mass, thereby sharpening crucial estimates of electromagnetic detectability and host-galaxy association. We also validate the approach on public data for previously detected NSBH events, finding the largest improvement for the asymmetric, higher-SNR event GW190814.

gr-qc↗
arXiv2026

Primordial Black-Hole-Based Pathways to Little Red Dots

The James Webb Space Telescope has uncovered a population of compact, high-redshift sources, the Little Red Dots (LRDs), which may host supermassive black holes (BHs) significantly heavier than their stellar content compared with local scaling relations. These objects challenge standard models of early galaxy formation and may represent an extreme class of early BH hosts. In this Letter, we investigate whether these BHs could have a primordial origin. We first show that the direct formation of these BH masses in the early Universe is excluded by stringent cosmic microwave background $μ$-distortion limits. We then investigate the assembly of massive BHs from lighter, observationally allowed primordial black holes (PBHs) via hierarchical mergers, finding that, although this channel can operate depending on the merger history, it faces challenges in explaining the observations due to the rarity of the required high-redshift dark matter halos. Finally, we estimate gas accretion onto intermediate-mass PBHs, while jointly tracking metallicity evolution, and identify regions of parameter space in which such growth could reproduce the observed properties of LRDs. As a special case, we focus on the strongly lensed source QSO1, whose extremely low metallicity and large mass provide a stringent test of these formation channels.

astro-ph.CO↗
arXiv2026

Investigating the formation channel of GW231123: Population III stars or hierarchical mergers?

The gravitational wave event GW231123, with component black hole masses lying within or above the pair-instability mass gap, poses a significant challenge to current stellar evolution models. In this work, we describe how we investigated its origin by coupling the galaxy formation model GAMESH with the cluster population synthesis code RAPSTER and using two distinct binary population synthesis codes, SEVN and BSEEMP. This framework allowed us, for the first time, to reconstruct the life cycle of GW231123-like candidates within the same cosmological simulation, enabling a self-consistent comparison between different formation channels. Although both population synthesis codes can in principle produce black holes compatible with GW231123, we find that isolated binary evolution fails to reproduce the inferred merger redshift. In SEVN, massive black hole binaries form with semi-major axes > 10^3 Rsun , preventing coalescences within a Hubble time. In BSEEMP, candidates arise only at extremely low metallicities (Z = 10^{-10}), which contribute negligibly to the star formation rate density in our overdense simulated volume. Our results therefore strongly support a dynamical hierarchical origin. The observed black hole masses are naturally reproduced through successive mergers in dense globular clusters. The high dimensionless spins reported by the LIGO-Virgo-KAGRA Collaboration are consistent with this hierarchical population. We found a local merger rate density of 0.78 Gpc^{-3} yr^{-1}, with a peak at z = 4 - 6, tracing the maximum formation rate of globular clusters in metal-poor environments (Z = 0.006). Overall, GW231123 may represent a benchmark event for a robust population of hierarchical black holes formed in the early Universe.

astro-ph.GA↗
arXiv2026

Predicting intermediate-mass black hole formation in star clusters with machine learning

Whether intermediate-mass black holes reside in nearby star clusters has remained contested for decades. We address this question by training neural network and random forest regressors on synthetic catalogs generated with the {\sc Rapster} cluster evolution code, mapping observable cluster properties such as total mass and half-mass radius onto the mass of the heaviest black hole built up through repeated mergers. Applying these models to nearby globular and nuclear star clusters, we forecast the intermediate-mass black hole population that each system may host. Globular clusters are unlikely to contain black holes more massive than $\sim 100\,M_\odot$, with an occupation fraction near 0.02, although they can produce remnants within the upper mass gap with masses approaching $100\,M_\odot$. Among nuclear star clusters, a handful of cases, including NGC 5102 and NGC 5206, yield predicted central black hole masses above $100\,M_\odot$, which we contrast with kinematically inferred estimates. Where the observationally claimed masses exceed our predictions, the implication is that the assembly history involved processes beyond hierarchical mergers, most plausibly accretion of gas and stars. Finally, we employ a normalizing flow to quantify, for individual globular clusters, the likelihood that their initial conditions were favorable to a collisional runaway during the first few million years after formation.

astro-ph.GA↗
arXiv2026

Modeling the frequency-domain ringdown amplitude of comparable-mass mergers with greybody factors

It was recently shown that, in a binary coalescence, the greybody factor of the remnant black hole modulates the post-merger ringdown signal. In this work, we demonstrate that a simple four-parameter model based on the greybody factor accurately reproduces the frequency-domain amplitude of a large set of comparable-mass, aligned-spin numerical relativity waveforms from the SXS catalog, achieving mismatches of order ${\cal O}(10^{-5})$ and improving existing models by roughly two orders of magnitude. We also identify the optimal initial frequency for applying the model in the frequency domain and provide analytical fits of the model parameters in terms of the progenitor masses and aligned spins. Our results pave the way for new consistency tests of the ringdown phase, complementary to traditional black hole spectroscopy.

gr-qc↗
arXiv2026

Gravitational-wave parameter estimation to the Moon and back: massive binaries and the case of GW231123

We study the prospects of the Lunar Gravitational-Wave Antenna (LGWA), a proposed deci-Hz GW detector, to observe binary black holes (BBHs) and enable multiband science with ground-based detectors. We assess the detectability of the events observed by current instruments up to the GWTC-4.0 data release, and of simulated populations consistent with the latest reconstruction by the LIGO-Virgo-KAGRA Collaboration. We find that LGWA alone would have been able to observe more than one third of the events detected so far, and that it could detect $\sim\!90$ events merging in the ground-based band per year out to redshifts $z\sim3-5$. Current detectors at design sensitivity and 100% duty cycle could detect thousands of BBHs per year, with one to a few hundred multiband counterparts in LGWA. Third-generation (3G) detectors can observe most of the BBHs detected by LGWA merging in their frequency band in the simulated mass range $7\,{\rm M}_\odot\lesssim M_{\rm tot}\lesssim 600\,{\rm M}_\odot$, enabling systematic joint analyses of hundreds of events. The short time to merger from the deci-Hz band to the Hz-kHz band (typically months to a year) allows for early warning, targeted follow-up, and archival searches. Multiband observations of intermediate-mass BBHs in the deci-Hz band are particularly promising. We perform an injection study for a GW231123-like system (the most massive BBH detection to date, which accumulates $\sim 10^5$ inspiral cycles in LGWA) and show that deci-Hz observations can measure the chirp mass even better than 3G instruments and yield good sky localization and inclination measurement, even with a single observatory. Opening the deci-Hz band would substantially improve the prospects of GW astronomy for intermediate-mass BBHs.

gr-qc↗
arXiv2026

Quasinormal modes of rotating black holes beyond general relativity in the WKB approximation

Exploring gravitational theories beyond general relativity (GR) with black hole (BH) spectroscopy requires accurate and flexible methods for computing their quasinormal mode (QNM) spectrum. A popular method of choice is the higher-order Wentzel-Kramers-Brillouin (WKB) approximation, mostly applied to nonrotating BHs. While previous studies demonstrated that the higher-order WKB method can also be used for Kerr BHs in GR, there has been little work on rotating BHs in modified theories of gravity. In this work, we revive the idea by extending WKB calculations of the Kerr QNM spectrum to higher order and assessing its accuracy against continued-fraction tabulated data. We then apply the WKB approximation beyond GR, comparing it against both linearized and continued fraction calculations in the parametrized beyond-Teukolsky formalism and in higher-derivative gravity (HDG) theories. We find that the frequencies computed by the WKB method in theories beyond GR have better accuracy than the measurement errors for GW250114, the event with the highest ringdown signal-to-noise ratio observed to date.

gr-qc↗
arXiv2026

Gravitational electric-magnetic duality at the light ring and quasinormal mode isospectrality in effective field theories

Black hole perturbations are characterized by a superposition of damped exponentials known as quasinormal modes. In general relativity, the spectra of parity-even and parity-odd quasinormal modes coincide -- a property known as isospectrality, which is typically broken by corrections beyond general relativity. Recently, certain higher-derivative operators were shown to preserve isospectrality in the high-frequency (eikonal) regime. Motivated by the relation between the light ring Penrose limit and the eikonal limit, we study isospectrality in a class of plane-wave spacetimes. In general relativity, we show that dynamical metric fluctuations on these backgrounds admit a gravitational analog of electric-magnetic duality, which enforces isospectrality. Requiring this duality to persist in the presence of higher-derivative corrections constrains the couplings so that isospectrality is preserved. We conclude that gravitational electric-magnetic duality at the light ring is the organizing principle behind isospectrality in the eikonal limit, and we conjecture that this remains true for other duality-invariant corrections to general relativity.

gr-qc↗
arXiv2026

Waveform Modelling for the Laser Interferometer Space Antenna

LISA, the Laser Interferometer Space Antenna, will usher in a new era in gravitational-wave astronomy. As the first anticipated space-based gravitational-wave detector, it will expand our view to the millihertz gravitational-wave sky, where a spectacular variety of interesting new sources abound: from millions of ultra-compact binaries in our Galaxy, to mergers of massive black holes at cosmological distances; from the beginnings of inspirals that will venture into the ground-based detectors' view to the death spiral of compact objects into massive black holes, and many sources in between. Central to realising LISA's discovery potential are waveform models, the theoretical and phenomenological predictions of the pattern of gravitational waves that these sources emit. This white paper is presented on behalf of the Waveform Working Group for the LISA Consortium. It provides a review of the current state of waveform models for LISA sources, and describes the significant challenges that must yet be overcome.

gr-qc↗
arXiv2026

Novel ringdown tests of general relativity with black hole greybody factors

We present GreyRing, a new model for the post-merger signal in black-hole binary coalescences based on the greybody factor of the remnant. The model accurately reproduces the full frequency-domain ringdown signal of a large set of comparable-mass, aligned-spin numerical relativity waveforms, achieving mismatches of order ${\cal O}(10^{-6})$ for the dominant $(\ell,m)=(2,2)$ mode, and typically outperforming state-of-the-art time-domain models. Building on this model, we introduce a novel consistency test of strong gravity based on the greybody factor: the remnant mass and spin inferred from GreyRing can be compared with those obtained through standard black hole spectroscopy. This agnostic test relies exclusively on the post-merger signal and does not require the inclusion of overtones or the choice of very early ringdown starting times, combining the advantages of inspiral-merger-ringdown consistency tests and traditional black hole spectroscopy. We apply the test to GW250114 and find that the remnant mass and spin inferred from GreyRing are consistent with those measured from the full signal. Remarkably, the inferred parameters can be measured with a precision comparable to, or slightly better than, that achieved with standard black-hole spectroscopy. Our greybody-factor waveform model allows for new precision tests of strong gravity using the ringdown signal.

gr-qc↗
arXiv2026

Not too close! Evaluating the impact of the baseline on the localization of binary black holes by next-generation gravitational-wave detectors

Next-generation (XG) gravitational-wave detectors, such as Cosmic Explorer (CE) and the Einstein Telescope (ET), will observe compact binary coalescences at unprecedented rates and signal-to-noise ratios (SNRs). Accurate sky localization of these sources is crucial for several aspects of the science case of CE and ET. The localization of most binary black hole (BBH) signals, which will spend at most a few minutes within the XG detector's effective sensitivity band, will continue to rely primarily on timing triangulation across a network of detectors. A key design choice for triangulation is the baseline between instruments. We investigate how the baseline affects the localization capabilities of a two-detector CE network, analyzing both fixed-parameter injections and a realistic BBH population consistent with the latest GWTC-4 results. For detector-frame total masses up to $\sim\!100\,{\rm M}_\odot$, we find that baselines corresponding to light travel times of $8-11$ ms ($\sim\!2300-3300$ km) offer a reasonable compromise, producing predominantly unimodal or bimodal sky localizations suitable for electromagnetic follow-up and statistical host galaxy identification and galaxy cross-correlation studies. Shorter baselines significantly degrade localization, particularly for high SNR events. Crucially, we find that adding a third detector to the network eliminates localization multimodality for a substantial fraction of sources. A network with two CEs and LIGO-India provides unimodal posteriors for a good fraction of events, whereas two CEs plus ET would provide unimodal posteriors for essentially all of them. These considerations should be useful to inform the development of the XG detector network.

gr-qc↗
arXiv2026

Milky Way Globular Clusters: Nurseries for Dynamically-Formed Binary Black Holes

We present a novel self-consistent theoretical framework to characterize the formation, evolution, and merger sites of dynamically-formed black hole binaries, with a focus on explaining the most massive events observed by the LIGO-Virgo-KAGRA Collaboration. Our approach couples the galaxy formation model GAMESH with cluster population synthesis codes to trace the cosmic evolution of globular clusters simultaneously with mergers of massive black holes. Our reference model, which includes prescriptions for both cluster formation and disruption depending on properties of specific galaxies, accurately reproduces the observed age-mass distribution of the Milky Way globular clusters. We find that approximately 30% of the globular clusters observed in our galaxy's halo may have originated from satellite galaxies of the Milky Way. We confirm that hierarchical black hole mergers provide a significant contribution to the formation of black holes in and above the pair-instability mass gap. However, quantifying their contribution is challenging, as different population synthesis codes yield divergent results in terms of black hole mass function and merger rates. Furthermore, we characterize the host galaxies where massive black holes form in terms of their dark matter, stellar mass, and metallicity. Ultimately, we demonstrate that the merger and birth rate densities of binary black holes increase with redshift till z = 5. This cosmic evolution is a crucial signature with significant implications for future detectors like the LISA, the Einstein Telescope and Cosmic Explorer, which will be capable to probe the high-redshift Universe.

astro-ph.GA↗