arXiv subjects

Emanuele Berti

Publications and source records attributed to Emanuele Berti.

At least 73 records · Page 4Linked to original sources

arXiv2024

Gravitational Magnus effect from scalar dark matter

In fluid dynamics, the Magnus effect is the force perpendicular to the motion of a spinning object as it moves through a medium. In general relativity, an analogous effect exists for a spinning compact object moving through matter, purely as a result of gravitational interactions. In this work we consider a Kerr black hole moving at relativistic velocities through scalar dark matter that is at rest. We simulate the system numerically and extract the total spin-curvature force on the black hole perpendicular to its motion. We confirm that the force scales linearly with the dimensionless spin parameter $a/M$ of the black hole up to $a/M = 0.99$, and measure its dependence on the speed $v$ of the black hole in the range $0.1 \le v \le 0.55$ for a fixed spin. Compared to previous analytic work applicable at small $v$, higher-order corrections in the velocity are found to be important: the total force is nonzero, and the dependence is not linear in $v$. We find that in all cases the total force is in the opposite direction to the hydrodynamical analogue, although at low speeds it appears to approach the expectation that the Weyl and Magnus components cancel. Spin-curvature effects may leave an imprint on gravitational wave signals from extreme mass-ratio inspirals, where the secondary black hole has a nonnegligible spin and moves in the presence of a dark matter cloud. We hope that our simulations can be used to support and extend the limits of analytic results, which are necessary to better quantify such effects in the relativistic regime.

gr-qc↗

arXiv2024

Black Holes Inside and Out 2024: visions for the future of black hole physics

The gravitational physics landscape is evolving rapidly, driven by our ability to study strong-field regions, in particular black holes. Black Holes Inside and Out gathered world experts to discuss the status of the field and prospects ahead. We hope that the ideas and perspectives are a source of inspiration. Structure: Black Hole Evaporation - 50 Years by William Unruh The Stability Problem for Extremal Black Holes by Mihalis Dafermos The Entropy of Black Holes by Robert M. Wald The Non-linear Regime of Gravity by Luis Lehner Black Holes Galore in D > 4 by Roberto Emparan Same as Ever: Looking for (In)variants in the Black Holes Landscape by Carlos A. R. Herdeiro Black Holes, Cauchy Horizons, and Mass Inflation by Matt Visser The Backreaction Problem for Black Holes in Semiclassical Gravity by Adrian del Rio Black Holes Beyond General Relativity by Enrico Barausse and Jutta Kunz Black Holes as Laboratories: Searching for Ultralight Fields by Richard Brito Primordial Black Holes from Inflation by Misao Sasaki Tests of General Relativity with Future Detectors by Emanuele Berti Black Holes as Laboratories: Tests of General Relativity by Ruth Gregory and Samaya Nissanke Simulating Black Hole Imposters by Frans Pretorius Black Hole Spectroscopy: Status Report by Gregorio Carullo VLBI as a Precision Strong Gravity Instrument by Paul Tiede Testing the nature of compact objects and the black hole paradigm by Mariafelicia De Laurentis and Paolo Pani Some Thoughts about Black Holes in Asymptotic Safety by Alessia Platania Black Hole Evaporation in Loop Quantum Gravity by Abhay Ashtekar How the Black Hole Puzzles are Resolved in String Theory by Samir D. Mathur Quantum Black Holes: From Regularization to Information Paradoxes by Niayesh Afshordi and Stefano Liberati

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arXiv2024

Searching for cosmological stochastic backgrounds by notching out resolvable compact binary foregrounds with next-generation gravitational-wave detectors

Stochastic gravitational-wave backgrounds can be of either cosmological or astrophysical origin. The detection of an astrophysical stochastic gravitational-wave background with ground-based interferometers is expected in the near future. Perhaps even more excitingly, the detection of stochastic backgrounds of cosmological origin by future ground-based interferometers could reveal invaluable information about the early Universe. From this perspective, the astrophysical background is a {\it foreground} that can prevent the extraction of this information from the data. In this paper, we revisit a time-frequency domain notching procedure previously proposed to remove the astrophysical foreground in the context of next-generation ground-based detectors, but we consider the more realistic scenario where we remove individually detectable signals by taking into account the uncertainty in the estimation of their parameters. We find that time-frequency domain masks can still efficiently remove the astrophysical foreground and suppress it to about $5\%$ of its original level. Further removal of the foreground formed by unresolvable events (in particular, unresolvable binary neutron stars), which is about $10$ times larger than the residual foreground from realistic notching, would require detector sensitivity improvements. Therefore, the main limitation in the search for a cosmological background is the unresolvable foreground itself, and not the residual of the notching procedure.

gr-qc↗

arXiv2024

Revealing the elusive companion of the red giant binary 2MASSJ05215658+4359220 from UV HST and Astrosat-UVIT data

Black hole demographics in different environments is critical in view of recent results on massive-stars binarity, and of the multi-messenger detectability of compact objects mergers. But the identification and characterization of non-interacting black holes is elusive, especially in the sparse field stellar population. A candidate non-interactive black hole (BH)+red giant (RG) binary system, 2MASSJ05215658+4359220, was identified by Thompson et al.(2019). We obtained Astrosat/UVIT Far-Ultraviolet (FUV) imaging and Hubble Space Telescope (HST) UV-optical imaging and spectroscopy of the source, to test possible scenarios for the optically-elusive companion. HST/STIS spectra from about 1,600 to 10,230Ang are best fit by the combination of two stellar sources, a red giant with Teff=4250 (uncertainty 150K), logg=2.0, Radius_RG=27.8Rsun (assuming a single-temperature atmosphere), and a subgiant companion with Teff=6,000K, Radius_comp=2.7Rsun, or Teff=5,270K, Radius_comp=4.2Rsun using models with one-tenth or one-third solar metallicity respectively, logg=3.0, extinction E(B-V)=0.50(uncertainty 0.2), adopting the DR3 Gaia distance D=2,463pc (uncertainty 120pc). No FUV data existed prior to our programs. STIS spectra give an upper limit of 10e-17ergs cm-2 s-1 Ang-1 shortward of 2300Ang; an upper limit of >25.7ABmag was obtained in two UVIT FUV broad-bands. The non-detection of FUV flux rules out a compact companion such as a hot WD. The STIS spectrum shows strong MgII lambda2800Ang emission, typical of chromospherically active red giants. The masses inferred by comparison with evolutionary tracks, about 1 Msun for the red giant and between 1.1 and 1.6Msun for the subgiant companion, suggest past mass transfer, although the red giant currently does not fill its Roche lobe. WFC3 imaging in F218W, F275W, F336W, F475W, and F606W shows an unresolved source in all filters.

astro-ph.SR↗

arXiv2024

Dynamical formation of black hole binaries in dense star clusters: Rapid cluster evolution code

Gravitational-wave observations have just started probing the properties of black hole binary merger populations. The observation of binaries with very massive black holes and significantly asymmetric masses motivates the study of dense star clusters as astrophysical environments which can produce such events dynamically. In this paper we present Rapster (for "Rapid cluster evolution"), a new code designed to rapidly model binary black hole population synthesis and the evolution of massive star clusters based on simple, yet realistic prescriptions. We also perform a thorough comparison with the Cluster Monte Carlo code and find generally good agreement. The code can be used to generate large populations of dynamically formed binary black holes.

astro-ph.HE↗

arXiv2024

Spectral instability of black holes: relating the frequency domain to the time domain

Recent work has shown that the quasinormal mode spectrum of black holes is unstable under small perturbations (of order $ε$) of the radial potential, while the early time-domain ringdown waveform is only marginally affected. In this paper we provide further insight into the apparent tension between the frequency-domain and the time-domain descriptions by analyzing the scattering properties of the problem. In the frequency domain, we study analytically the solutions corresponding to the perturbed potential. We show that there are two qualitatively different classes of instabilities, and that both Schwarzschild and Kerr black holes are affected by what we call a "Type II" instability, i.e., an exponential migration of the mode frequencies away from their unperturbed value as the perturbing "bump" moves away from the peak of the unperturbed potential. In the time domain, we elucidate the effect of the spectral instability in terms of the causal structure of the Green's function. By using an equivalent scattering problem we confirm analytically (and show numerically) that the deviation from the unperturbed waveform in the early ringdown stage is proportional to $ε$ when $ε\lesssim10^{-2}$.

gr-qc↗

arXiv2024

Nonlinear quasinormal mode detectability with next-generation gravitational wave detectors

In the aftermath of a binary black hole merger event, the gravitational wave signal emitted by the remnant black hole is modeled as a superposition of damped sinusoids known as quasinormal modes. While the dominant quasinormal modes originating from linear black hole perturbation theory have been studied extensively in this post-merger "ringdown" phase, more accurate models of ringdown radiation include the nonlinear modes arising from higher-order perturbations of the remnant black hole spacetime. We explore the detectability of quadratic quasinormal modes with both ground- and space-based next-generation detectors. We quantify how predictions of the quadratic mode detectability depend on the quasinormal mode starting times. We then calculate the signal-to-noise ratio of quadratic modes for several detectors and binary black hole populations, focusing on the ($220\times220$) mode - i.e., on the quadratic term sourced by the square of the linear $(220)$ mode. For the events with the loudest quadratic mode signal-to-noise ratios, we additionally compute statistical errors on the mode parameters in order to further ascertain the distinguishability of the quadratic mode from the linear quasinormal modes. The astrophysical models used in this paper suggest that while the quadratic mode may be detectable in at most a few events with ground-based detectors, the prospects for detection with the Laser Interferometer Space Antenna (LISA) are more optimistic.

gr-qc↗

arXiv2024

Spin dependence of black hole ringdown nonlinearities

The nonlinear character of general relativity leaves its imprint in the coalescence of two black holes, from the inspiral to the final ringdown stage. To quantify the impact of nonlinearities, we work at second order in black hole perturbation theory and we study the excitation of second-order modes relative to the first-order modes that drive them as we vary the black hole spin and the initial data for the perturbations. The relative amplitude of second-order modes is only mildly dependent on the initial data that we consider, but it strongly decreases for large black hole spins. This implies that the extrapolation of calculations based on the Kerr-CFT correspondence to subextremal Kerr black holes should be viewed with caution.

gr-qc↗

arXiv2024

Tidal Love numbers and approximate universal relations for fermion soliton stars

Fermion soliton stars are a consistent model of exotic compact objects which involve a nonlinear interaction between a real scalar field and fermions through a Yukawa term. This interaction results in an effective fermion mass that depends upon the vacuum structure in the scalar potential. In this work we investigate the tidal deformations of fermion soliton stars and compute the corresponding tidal Love numbers for different model parameters. Furthermore, we discuss the existence of approximate universal relations for the electric and magnetic tidal deformabilities of these stars, and compare them with other solutions of general relativity, such as neutron stars or boson stars. These relations for fermion soliton stars are less universal than for neutron stars, but they are sufficiently different from the ordinary neutron star case that a measurement of the electric and magnetic tidal Love numbers (as potentially achievable by next-generation gravitational wave detectors) can be used to disentangle these families of compact objects. Finally, we discuss the conditions for tidal disruption of fermion soliton stars in a binary system and estimate the detectability of the electromagnetic signal associated with such tidal disruption events.

gr-qc↗

arXiv2024

Supermassive black holes from runaway mergers and accretion in nuclear star clusters

Rapid formation of supermassive black holes occurs in dense nuclear star clusters that are initially gas-dominated. Stellar-mass black hole remnants of the most massive cluster sink into the core, where a massive runaway black hole forms as a consequence of combined effects of repeated mergers and Eddington-limited gas accretion. The associated gravitational wave signals of high-redshift extreme mass-ratio inspirals are a unique signature of the nuclear star cluster scenario.

astro-ph.HE↗

arXiv2024

Systematic bias from waveform modeling for binary black hole populations in next-generation gravitational wave detectors

Next-generation gravitational wave detectors such as the Einstein Telescope and Cosmic Explorer will have increased sensitivity and observing volumes, enabling unprecedented precision in parameter estimation. However, this enhanced precision could also reveal systematic biases arising from waveform modeling, which may impact astrophysical inference. We investigate the extent of these biases over a year-long observing run with $10^5$ simulated binary black hole sources using the linear signal approximation. To establish a conservative estimate, we sample binaries from a smoothed truncated power-law population model and compute systematic parameter biases between the IMRPhenomXAS and IMRPhenomD waveform models. For sources with signal-to-noise ratios above 100, we estimate statistically significant parameter biases in $\sim 3\%-20\%$ of the events, depending on the parameter. We find that the average mismatch between waveform models required to achieve a bias of $\leq 1σ$ for $99\%$ of detections with signal-to-noise ratios $\geq 100$ should be $\mathcal{O}(10^{-5})$, or at least one order of magnitude better than current levels of waveform accuracy.

gr-qc↗

arXiv2024

Probing wave-optics effects and low-mass dark matter halos with lensing of gravitational waves from massive black holes

The Laser Interferometer Space Antenna (LISA) will detect gravitational waves (GWs) emitted by massive black hole binaries (MBHBs) in the low-frequency ($\sim$mHz) band. Low-mass lenses, such as low-mass dark matter halos or subhalos, have sizes comparable to the wavelength of these GWs. Encounters with these lenses produce wave-optics (WO) effects that alter waveform phase and amplitude. Thus, a single event with observable WO effects can be used to probe the lens properties. In this paper, we first compute the probability of observing WO effects in a model-agnostic way. We perform information-matrix analyses over approximately 1000 MBHBs with total mass, mass ratio, and redshift spanning the ranges relevant to LISA. We then calculate lensing rates using three semi-analytical models of MBHB populations. In both cases, we use a waveform model that includes merger, ringdown, and higher-order modes. We use two lens population models: the theory-based Press-Schechter halo mass function and an observation-based model derived from Sloan Digital Sky Survey. We find that the probability of detecting WO effects can be as large as $\sim 3\%$, $\sim1.5\%$, and $\sim 1 \%$ at $1σ$, $3σ$, and $5σ$ confidence levels, respectively. The most optimistic MBHB population model yields $\sim 8$, $\sim 4$, and $\sim 3$ events with detectable WO effects at the same confidence levels, while the rates drop to $\sim 0.01$ in the more pessimistic scenarios. The most likely lens masses probed by LISA are in the range $(10^3, 10^8)\, M_{\odot}$, and the most probable redshifts are in the range $(0.3, 1.7)$. Therefore, LISA observations of WO effects can probe low-mass DM halos, complementing strong lensing and other observations.

astro-ph.CO↗

arXiv2024

Bounds on the mass of superradiantly unstable scalar fields around Kerr black holes

In this work we compute numerical bounds on the mass $μ$ of superradiantly unstable scalar fields in a Kerr black hole background using the continued fraction method. We show that the normalized upper bound on the mass $μ$ increases with the angular momentum number $\ell$ and the azimuthal number $m$, approaching the most stringent analytical bound known to date when $\ell=m \gg 1$. We also provide an analytical fit to the numerically determined mass bound as a function of the dimensionless spin parameter $a/M$ of the black hole with an accuracy of the order $0.1\%$ for the fundamental mode with $\ell=m=1$, and of the order $1\%$ for higher-order modes (up to $\ell=m=20$). We argue that this analytical fit is particularly useful in astrophysical scenarios, since the lowest $\ell=m$ modes are capable of producing the strongest observable imprints of superradiance.

gr-qc↗

arXiv2024

Black holes surrounded by generic matter distributions: polar perturbations and energy flux

We develop a numerical approach to compute polar parity perturbations within fully relativistic models of black hole systems embedded in generic, spherically symmetric, anisotropic fluids. We apply this framework to study gravitational wave generation and propagation from extreme mass-ratio inspirals in the presence of several astrophysically relevant dark matter models, namely the Hernquist, Navarro-Frenk-White, and Einasto profiles. We also study dark matter spike profiles obtained from a fully relativistic calculation of the adiabatic growth of a BH within the Hernquist profile, and provide a closed-form analytic fit of these profiles. Our analysis completes prior numerical work in the axial sector, yielding a fully numerical pipeline to study black hole environmental effects. We study the dependence of the fluxes on the DM halo mass and compactness. We find that, unlike the axial case, polar fluxes are not adequately described by simple gravitational-redshift effects, thus offering an exciting avenue for the study of black hole environments with gravitational waves.

gr-qc↗

arXiv2024

Probing minihalo lenses with diffracted gravitational waves

When gravitational waves pass near a gravitating object, they are deflected, or lensed. If the object is massive, such that the wavelength of the waves is small compared to its gravitational size, lensed gravitational wave events can be identified when multiple signals are detected at different times. However, when the wavelength is long, wave-optics diffraction effects will be important, and a lensed event can be identified by looking for frequency-dependent modulations to the gravitational waveform, without having to associate multiple signals. For current ground-based gravitational wave detectors observing stellar-mass binary compact object mergers, wave-optics effects are important for lenses with masses $\lesssim 1000 M_{\odot}$. Therefore, minihalos below this mass range could potentially be identified by lensing diffraction. The challenge with analyzing these events is that the frequency-dependent lensing modulation, or the amplification factor, is prohibitively expensive to compute for Bayesian parameter inference. In this work, we use a novel time-domain method to construct interpolators of the amplification factor for the Navarro-Frenk-White (NFW), generalized singular isothermal sphere (gSIS) and cored isothermal sphere (CIS) lens models. Using these interpolators, we perform Bayesian inference on gravitational-wave signals lensed by minihalos injected in mock detector noise, assuming current sensitivity of ground-based detectors. We find that we could potentially identify an event when it is lensed by minihalos and extract the values of all lens parameters in addition to the parameters of the GW source. All of the methods are implemented in Glworia, the accompanying open-source Python package, and can be generalized to study lensed signals detected by current and next-generation detectors.

gr-qc↗

arXiv2024

Black hole spectroscopy beyond Kerr: agnostic and theory-based tests with next-generation interferometers

Black hole spectroscopy is a clean and powerful tool to test gravity in the strong-field regime and to probe the nature of compact objects. Next-generation ground-based detectors, such as the Einstein Telescope and Cosmic Explorer, will observe thousands of binary black hole mergers with large signal-to-noise ratios, allowing for accurate measurements of the remnant black hole quasinormal mode frequencies and damping times. In previous work we developed an observable-based parametrization of the quasinormal mode spectrum of spinning black holes beyond general relativity (ParSpec). In this paper we use this parametrization to ask: can next-generation detectors detect or constrain deviations from the Kerr spectrum by stacking multiple observations of binary mergers from astrophysically motivated populations? We focus on two families of tests: (i) agnostic (null) tests, and (ii) theory-based tests, which make use of quasinormal frequency calculations in specific modified theories of gravity. We consider in particular two quadratic gravity theories (Einstein-scalar-Gauss-Bonnet and dynamical Chern-Simons gravity) and various effective field theory-based extensions of general relativity. We find that robust inference of hypothetical corrections to general relativity requires pushing the slow-rotation expansion to high orders. Even when high-order expansions are available, ringdown observations alone may not be sufficient to measure deviations from the Kerr spectrum for theories with dimensionful coupling constants. This is because the constraints are dominated by "light" black hole remnants, and only few of them have sufficiently high signal-to-noise ratio in the ringdown. Black hole spectroscopy with next-generation detectors may be able to set tight constraints on theories with dimensionless coupling, as long as we assume prior knowledge of the mass and spin of the remnant black hole.

gr-qc↗

arXiv2024

Multi-messenger Astrophysics of Black Holes and Neutron Stars as Probed by Ground-based Gravitational Wave Detectors: From Present to Future

The ground-based gravitational wave (GW) detectors LIGO and Virgo have enabled the birth of multi-messenger GW astronomy via the detection of GWs from merging stellar-mass black holes (BHs) and neutron stars (NSs). GW170817, the first binary NS merger detected in GWs and all bands of the electromagnetic spectrum, is an outstanding example of the impact that GW discoveries can have on multi-messenger astronomy. Yet, GW170817 is only one of the many and varied multi-messenger sources that can be unveiled using ground-based GW detectors. In this contribution, we summarize key open questions in the astrophysics of stellar-mass BHs and NSs that can be answered using current and future-generation ground-based GW detectors, and highlight the potential for new multi-messenger discoveries ahead.

astro-ph.HE↗

arXiv2024

Relativistic drag forces on black holes from scalar dark matter clouds of all sizes

We use numerical simulations of scalar field dark matter evolving on a moving black hole background to confirm the regime of validity of (semi-)analytic expressions derived from first principles for both dynamical friction and momentum accretion in the relativistic regime. We cover both small and large clouds (relative to the de Broglie wavelength of the scalars), and light and heavy particle masses (relative to the BH size). In the case of a small dark matter cloud, the effect of accretion is a non-negligible contribution to the total force on the black hole, even for small scalar masses. We confirm that this momentum accretion transitions between two regimes (wave- and particle-like) and we identify the mass of the scalar at which the transition between regimes occurs.

gr-qc↗