arXiv subjects

Orlando Luongo

Publications and source records attributed to Orlando Luongo.

At least 37 records · Page 2Linked to original sources

arXiv2026

Gravitational radiation reaction around a static black hole surrounded by a Dehnen type dark matter halo

We consider the motion of a particle in the geometry of a Schwarzschild-like black hole embedded in a dark matter (DM) halo with Dehnen type density profile and calculate the orbital periods along with the evolution of the semi-latus rectum and eccentricity for extreme mass ratio inspirals (EMRIs). Such a system emits gravitational waves (GWs), and the particle's orbit evolves under radiation reaction. We also consider the effects of dynamical friction and accretion of DM on the orbital parameters. We find that the eccentricity and semi-latus rectum decrease faster with respect to the case in which EMRI is in empty spacetime.

gr-qc↗

arXiv2026

Gravitational waves from two scalar fields unifying the dark sector with inflation

We investigate the gravitational-wave background predicted by a two-scalar-field cosmological model that aims to unify primordial inflation with the dark sector, namely late-time dark energy and dark matter, in a single and self-consistent theoretical framework. The model is constructed from an action inspired by several extensions of general relativity and string-inspired scenarios and features a non-minimal interaction between the two scalar fields, while both remain minimally coupled to gravity. In this context, we derive the gravitational-wave energy spectrum over wavelengths ranging from today's Hubble horizon to those at the end of inflation. We employ the continuous Bogoliubov coefficient formalism, originally introduced to describe particle creation in an expanding Universe, in analogy to the well-established mechanism of gravitational particle production and, in particular, generalized to gravitons. Using this method, which enables an accurate description of graviton creation across all cosmological epochs, we find that inflation provides the dominant gravitational-wave contribution, while subdominant features arise at the inflation-radiation, radiation-matter, and matter-dark energy transitions, i.e., epochs naturally encoded inside our scalar field picture. The resulting energy density spectrum is thus compared with the sensitivity curves of the planned next-generation ground- and space-based gravitational-wave observatories. The comparison identifies frequency bands where the predicted signal could be probed, providing those windows associated with potentially detectable signals, bounded by our analyses. Consequences of our recipe are thus compared with numerical outcomes and the corresponding physical properties discussed in detail.

gr-qc↗

arXiv2026

Probing dynamical embeddings in a five-dimensional spacetime in light of DESI BAO

We here investigate the observational viability of Nash gravity as an alternative to the standard $Λ$CDM cosmology. Based on Nash's embedding theorem, the model introduces orthogonal perturbations via variations in the extrinsic curvature, generating scalar-type metric perturbations directly from geometry, without the need to introduce additional fields. We confront the model with current observational data, including Cosmic Microwave Background (CMB) measurements from Planck, Baryon Acoustic Oscillations (BAO) from DESI DR2, and recent Type Ia supernova (SN Ia) compilations. Our analysis shows that Nash gravity provides a good fit to the data, yielding a slightly higher value for the Hubble constant, $H_0 = 69.32 \pm 0.72$ km/s/Mpc, compared to the $Λ$CDM model, thus offering a potential alleviation of the $H_0$ tension. Furthermore, the model naturally predicts a suppressed growth of structure, with $S_8 \approx 0.76$ across various joint analyses, potentially alleviating the so-called $S_8$ tension, assuming that this discrepancy is not solely due to systematic effects in other independent measurements. In some cases, Nash gravity achieves a better fit to the data than the $Λ$CDM paradigm at the $2σ$ level.

gr-qc↗

arXiv2025

The macroscopic precession model: describing quasi-periodic oscillations including internal structures of test bodies

The relativistic precession model (RPM) is widely-considered as a benchmark framework to interpret quasi-periodic oscillations (QPOs), albeit several observational inconsistencies suggest that the model remains incomplete. The RPM ensures \emph{structureless test particles} and attributes precession to geodesic motion alone. Here, we refine the RPM by incorporating the internal structure of rotating test bodies, while preserving the test particle approximation (TPA), and propose a \emph{macroscopic precession model} (MPM) by means of the Mathisson-Papapetrou-Dixon (MPD) equations, applied to a Schwarzschild background, which introduces 1) a shift in the Keplerian frequency and 2) an \emph{effective spin correction} to the radial epicyclic frequency that, once the spin tensor is modeled, reproduces a quasi-Schwarzschild-de Sitter (SdS) correction. We apply the MPM to eight neutron star low mass X-ray binaries (NS-LMXBs), performing Markov chain Monte Carlo (MCMC) fits to twin kHz QPOs and find observational and statistical evidence in favor of precise power law spin reconstructions. Further, our model accurately predicts the $3:2$ frequency clustering, the disk boundaries and the NS masses. From the MPM model, we thus conclude that complexity of QPOs can be fully-described including the test particle internal structure.

gr-qc↗

arXiv2025

Constraints on Chiral-Quintom dark energy after DESI DR2 and impact on unifying dark energy with inflation

The recent data release DR2 from the Dark Energy Spectroscopic Instrument (DESI) has reinforced compelling evidence supporting the dynamical nature of dark energy. In this respect, we here explore a two-scalar field cosmological model, dubbed Chiral-Quintom paradigm, that allows for a unified description of early- and late-time cosmic accelerations, namely inflation and dark energy, respectively. Moreover, we show that it provides a mechanism to cross the phantom divide without instabilities. To do so, we first focus on scenarios where the scalar fields evolve under an exponential potential, leading to distinct cosmological behaviors, including tracking solutions and slow-roll or hyperbolic inflation. Afterwards, by considering a nonlinear potential and a mixing in the kinetic sector, we show that the model can also describe the matter-dominated era, offering a potential unification of the dark sector. Accordingly, we place bounds on this double-field paradigm by employing recent low-redshift observational data, including supernovae from the Pantheon+ and Union3.0 compilations, observational Hubble parameter measurements, and baryon acoustic oscillation data from the latest DESI release. By means of the Akaike information criterion, the standard $Λ$CDM scenario and the $w_0w_a$CDM model are thus compared with the Chiral-Quintom approach, showing that, in principle, multiple-field dynamics does not seem to be ruled out, ultimately providing a flexible framework for describing late-time dynamics.

astro-ph.CO↗

arXiv2025

Vector-field spontaneous baryogenesis with Lorentz invariance violation

We extend spontaneous baryogenesis by considering the spontaneous breaking of $U(1)_B$ through a complex vector field. This field interacts with baryons and leptons via a vector-current coupling and, by construction, acquires a nonzero vacuum expectation value. Accordingly, the theory also exhibits a spontaneous violation of Lorentz invariance, effectively realizing a Bumblebee model. In this picture, the pseudo-Nambu-Goldstone boson arising from spontaneous breaking of the $U(1)_B$ global symmetry is the global phase of the Bumblebee vector and, in the broken phase, it results minimally coupled with the baryonic current, guaranteeing the violation of the baryon number. Consequently, we assume that the pseudo-Nambu-Goldstone, arising from spontaneous breaking of $U(1)_B$, plays the role of the inflaton, leading to baryogenesis across the entire inflationary stage, up to when the inflaton decays into baryon-antilepton and antibaryon-lepton pairs through a CP-violating interaction that also violates the Lorentz symmetry. Afterwards, we address the issue of flavor oscillations among baryon and lepton fields, including the oscillation probability in the calculation of the baryon asymmetry. Remarkably, our framework predicts a non-null mixing factor even for massless fermions. This mixing acts on the spatial momenta rather than on the masses of the produced fermions, allowing larger values of the coupling constant even guaranteeing the production of light fermions. The net baryon asymmetry results accordingly modified, and may also reproduce the experimental data for allowed values of the coupling constant.

hep-ph↗

arXiv2025

Multipartite entanglement features of primordial non-gaussianities

We discuss some entanglement features associated with cubic non-Gaussian perturbations in single-field inflationary scenarios. We adopt standard momentum-space techniques to show how multipartite entanglement arises for inflationary perturbation modes, focusing on the dynamics of the comoving curvature perturbation. In particular, we quantify entanglement generation via the recently proposed Entanglement Distance, which introduces a geometric interpretation of quantum correlations in terms of the Fubini-Study metric. In the continuum limit, we show that the Entanglement Distance arising from displacement transformations is proportional to the total number of excitations in the quantum state for cubic perturbations, thus providing an upper bound on the von Neumann entanglement entropy of any reduced state compatible with such excitations. Within the interaction picture, we further observe that the quantum correlations arising from cubic gravitational interactions are typically much larger than the standard squeezing contribution, in agreement with previous studies focusing on von Neumann entropy generation across the Hubble horizon. We further show how the inflationary parameters affect the total amount of such correlations, highlighting in particular their dependence on the inflationary energy scales and the number of e-foldings during slow-roll.

gr-qc↗

arXiv2025

Temperature definitions and phase transitions within non-minimal large and small inflationary potentials

We explore and compare two distinct temperature definitions for scalar field inflation in the context of small- and large-field potentials. The first is based on a real gas, fluid-like temperature, $T_{RG}$, while the second corresponds to a relativistic species-like temperature, $T_{RS}$. We derive the fundamental thermodynamic relations for both and analyze their implications for the most viable inflationary potentials, consistent with Planck constraints. We also investigate non-minimally coupled scenarios, finding that $T_{RS}$ is the most self-consistent choice, as it decreases during inflation, satisfies standard thermodynamic laws, and exhibits frame-independent behavior in both the Jordan and Einstein frames. Remarkably, the $T_{RS}$ approach shows that the inflaton's dynamics is well-described by Van der Waals-like isotherms, linking inflationary evolution to thermodynamic phase transitions. We find that the onset of inflation is associated with a phase transition acting as the ``trigger'' of the inflationary epoch. Our analysis highlights inconsistencies in the hilltop potential and, more generally, in small-field potentials unless a non-minimal coupling is introduced. Conversely, the Starobinsky and $α$-attractor models emerge as the most suitable paradigms. We further show that \emph{frame independence} is achieved only for coupling values $ζ\leq 1/6$, supporting very small values. Finally, our study of natural inflation with non-minimal coupling reveals a strong dependence on the coupling parameter, where bounds associated with thermodynamic phase transitions coincide with observationally viable ranges, suggesting that thermodynamic considerations may provide an additional criterion to discriminate among inflationary scenarios.

gr-qc↗

arXiv2025

Challenging the $ω_0ω_a$CDM parametrization through rational expansions in view of DESI data release

In view of the new Dark Energy Spectroscopic Instrument (DESI) 2025 results, we analyze three types of \emph{Padé cosmology}, based on rational series making use of Padé approximants over the equations of state, namely Padé$^ω$ (0,1) and Padé$^ω$ (1,1), plus a Padé$^{q}$ (0,1), i.e., a rational expansion on the dark energy deceleration parameter, in which where the numerator and denominator orders are incorporated into the above brackets. These scenarios appear alternative dark energy parameterizations with respect to the well-known $ω_0ω_a$CDM model, claimed as the most viable model by DESI. Accordingly, we perform Monte Carlo Markov chain (MCMC) analyses with the publicly available \texttt{CLASS} Boltzmann code, including the three Padé cosmology, along with the $ω_0ω_a$CDM and $Λ$CDM standard pictures. To this end, we combine independent probes from high to low redshifts to obtain reliable constraints on the cosmological parameters of these models and compare them using statistical selection criteria. \emph{Our results show that Padé cosmology is neither statistically excluded nor worse than the $ω_0ω_a$CDM parametrization}. On the contrary, the Akaike Information Criterion (AIC) identifies Padé$^{q}$ (0,1) as \emph{the best-fit model}, with weak evidence against the $ω_0ω_a$CDM parameterization, while the Deviance Information Criterion (DIC) provides \emph{strong evidence against the $ω_0ω_a$CDM model, favoring Padé (1,1)}. Based on our bounds, we further investigate the evolution of the squared sound speed, revealing that the Padé$^{q}$ (0,1) and Padé$^ω$ (0,1) parameterizations exhibit enhanced stability compared with the other cases here considered and, therefore, describe robust alternatives for the cosmological background.

astro-ph.CO↗

arXiv2025

Investigating the cosmic distance duality relation with gamma-ray bursts

Deviations from the so-called {\it cosmic distance duality relation} may result from systematic errors in distance measurements or, more interestingly, hint at new physics. Further, it can also be related to the Hubble constant tension between early and local measurements of $H_0$. Based on this, we test validity of this relation through a model-independent parameterization of the Hubble rate via the well-estabilished Bézier polynomials approach. We seek for possible departures from the relation considering three parametrizations, i) a power-law correction, ii) a logarithmic correction and iii) a Padé series $P_{n,m}(z)$ of order (1;2) with $n=1$ being the order of the numerator while $m=2$ is the order of the denominator. Then, assuming a flat scenario, we test them through Monte Carlo -- Markov chain analyses that combine low- and intermediate/high-$z$ data sets, such as observational Hubble data, the Pantheon catalog of type Ia supernovae, galaxy clusters, the second data release from the DESI Collaboration and gamma-ray bursts. In particular, we distinguish between \emph{Analysis A} and \emph{Analysis C}, depending whether the prompt emission $E_{iso}-E_p$ or the prompt-afterglow $L_0-E_p-T$ gamma-ray burst correlations, respectively, is fit together with the other probes previously described. Our results seem to point towards a \emph{no violation} of the cosmic distance duality relation and a preference towards Planck's value of $H_0$.

astro-ph.CO↗

arXiv2025

Exploring the cosmic microwave background dipole direction using gamma-ray bursts

We search for dipole variations in the Hubble constant $H_0$ using gamma-ray burst (GRB) data, as such anisotropies may shed light on the Hubble tension. We employ the most recent and reliable GRB catalogs from the $E_{p}-E_{iso}$ and the $L_0-E_{p}-T$ correlations. Despite their large uncertainties, GRBs are particularly suited for this analysis due to their redshift coverage up to $z\sim9$, their isotropic sky distribution that minimizes directional bias, and their strong correlations whose normalizations act as proxies for $H_0$. To this aim, a whole sky scan - partitioning GRB data into hemispheres - enabled to define dipole directions by fitting the relevant GRB correlation and cosmological parameters. The statistical significance across the full $H_0$ dipole maps, one per correlation, is then evaluated through the normalization differences between hemispheres and compared against the CMB dipole direction. The method is then validated by simulating directional anisotropies via Markov Chain Monte Carlo analyses for both correlations. Comparison with previous literature confirms the robustness of the method, while no significant dipole evidence is detected, consistently with the expected isotropy of GRBs. This null result is discussed in light of future analyses involving larger datasets.

astro-ph.CO↗

arXiv2025

Geometric multipartite entanglement from gravitational particle production

We explore novel generation of genuine multipartite entanglement within gravitational particle production processes during inflationary stages. To this end, we focus on perturbative production mechanisms, considering a non-minimally coupled scalar inflaton field with quartic self-coupling potential and computing probability amplitudes arising from its gravitational interaction with background perturbations. The corresponding entanglement amount is quantified using the recently proposed Entanglement Distance, that provides a \emph{geometric interpretation of particle entanglement, in terms of the Fubini-Study metric}. We observe that, in the limit of negligible squeezing, the total amount of entanglement is dominated by the infrared cutoff scale, in agreement with previous studies analyzing the von Neumann entropy within bipartite scenarios. We then show that \emph{non-negligible multipartite entanglement signatures may emerge across inflation, even during the latest stages of slow-roll}, highlighting their dependence on inflationary momentum scales. Generalizations to regimes with non-negligible squeezing, cubic non-Gaussianities, additional spectator fields and possible observational signatures are also discussed.

gr-qc↗

arXiv2025

Bondi accretion disk luminosity around neutral and charged Simpson-Visser spacetimes

We investigate relativistic Bondi accretion in the Simpson-Visser spacetime, which, via a single parameter $\ell$, interpolates between the Schwarzschild, regular black hole, extremal and wormhole regimes. First, we analyze the neutral Simpson-Visser geometry, recovering Schwarzschild at $\ell=0$, and then its charged extension of the Reissner-Nordström metric. In both these cases, we derive the conservation equations and analyze two representative fluid models: a barotropic perfect fluid and a constituent with an exponential density profile. By varying the parameters across regimes, we locate critical (sonic) points and integrate velocity, density and pressure profiles. While near-horizon inflow velocities are similar across the different solutions, we find that the critical radius and the resulting accretion rates and luminosities severely change, depending on the value of the parameter and type of fluid. Remarkably, the barotropic and exponential cases exhibit different trends in the outer regions. Moreover, by extending the analysis to the charged SV spacetime, we find that the presence of a central charge $Q$ produces additional, albeit modest, shifts in the sonic radius which, in combination with those induced by the regularization parameter $\ell$, could provide a double observational marker. In particular, while $\ell$ acts predominantly on the position of the critical point, in the barotropic fluid case, the electromagnetic contribution of $Q$ slightly dampens the inflow velocity near the horizon.

gr-qc↗

arXiv2025

Effects of matter with anisotropic pressure on the Fan-Wang regular black hole shadows

We here investigate the consequences of an exotic fluid, exhibiting negative radial and tangential pressures, \emph{de facto} violating the Zel'dovich limit, on a regular solution that easily generalizes the Schwarzschild black hole. More precisely, we focus on the regular Fan-Wang spacetime, computing how the black hole shadow images, surrounded by the quoted fluid, is modified through the presence of \emph{negative} equations of state for the two pressure components. Even though quite different from quintessence, we consider constant radial and tangential equations of state with the aim of emulating, but not reproducing, dark energy effects. Moreover, we explore the main properties of infalling spherical accretion flows and, accordingly, the influence of the equations of state on the horizons, photosphere, and impact parameter of the Fan-Wang black hole. Afterwards, we examine the luminosities of the shadow and the photon ring in two distinct spherically accretion flows, as well as the observed specific intensity of the shadow itself. Last but not least, we physically interpret the impact of negative pressures on our findings and discuss possible extensions to the isotropic case.

gr-qc↗

arXiv2025

Impact of flavor condensate dark matter on accretion disk luminosity in spherical spacetimes

We investigate the impact of dark matter condensates on the emission and thermodynamic properties of accretion disks, in a spherically-symmetric and static background. We focus on a class of models where dark matter originates from a genuine mass mixing among neutrino fields and compute the corrections to the dark matter's potential within galactic halo. We find a corresponding Yukawa correction induced by the dark matter energy-momentum tensor over the Newtonian potential. In so doing, employing Schwarzschild coordinates, and adopting the Novikov-Thorne formalism, we compute the geodesic structure and the corresponding disk-integrated luminosity profiles. Assuming a constant mass accretion rate, constituted solely by baryonic matter, we find non-negligible deviations in both the disk structure and radiative output, as compared to the standard Schwarzschild case. Afterwards, we discuss physical consequences of our Yukawa correction, comparing it with recent literature, predicting similar potentials, albeit derived from extended theories of gravity. Accordingly, we thus speculate to use our results to distinguish among candidates of dark matter. Indeed, our findings suggest that incoming high-precision observations of accretion disk spectra may provide a tool to probe dark matter's nature under the form of particles, extended theories of gravity or condensates.

gr-qc↗

arXiv2025

Impact of a complex scalar spectator field on baryon asymmetry within spontaneous baryogenesis

We extend the framework of spontaneous baryogenesis by investigating the generation of baryon asymmetry when the inflaton, $θ$, is minimally coupled with a complex spectator scalar field $ϕ$, as $θ^2|ϕ|^2$. To do so, we also consider $ϕ$ non-minimally coupled with the Ricci scalar curvature $R$ through a Yukawa-like interaction. We do not consider further interactions of the spectator field with the fermions of the Standard Model, considering it \emph{de facto} as a dark scalar field. In evaluating the violation of the baryon-number conservation during the reheating epoch, in a perfectly homogeneous and isotropic universe, we follow a semiclassical approach, where $θ$, $ϕ$ and gravity are considered as classical fields, whereas the fermions are quantized. We solve the equations of motion for the inflaton and spectator fields, respectively at first and zero-order in perturbation theory, neglecting at first stage the expansion of the universe. Afterwards, we quantify how the spectator field modifies the inflationary dynamics and thus find the baryon asymmetry produced via the inflaton decays into fermion-antifermion pairs by computing the corresponding decay amplitudes. We therefore obtain small first order correction to standard spontaneous baryogenesis and finally discuss the mass-mixing between fermions. Accordingly, the effects of considering the universe expansion are accounted, showing when the coupling between $ϕ$ and $R$ becomes noticeable in altering the overall baryon asymmetry.

gr-qc↗

arXiv2025

Addressing the $H_0$ tension through matter with pressure and no early dark energy

We propose that the Hubble tension arises due to an unaccounted additional component, that behaves as \emph{matter with pressure}. We demonstrate that this fluid remains subdominant compared to both dust and radiation throughout nearly the entire universe expansion history. Specifically, the additional fluid satisfies the Zel'dovic limit with a constant equation of state, $ω_s > 0$, and a quite small normalized energy density, $Ω_s$. Accordingly, this component modifies both the sound horizon and the background expansion rate, \emph{acting quite differently from early dark energy models}, without significantly affecting the other cosmological parameters. To show this, we perform a Monte Carlo Markov chain analysis of our model, hereafter dubbed $Λ_{ω_s}$CDM paradigm, using the publicly available \texttt{CLASS} Boltzmann code. Our results confirm the presence of this fluid, with properties that closely resemble those of radiation. We find best-fit values that satisfy $ω_s \lesssim ω_γ$ and a relative energy density $Ω_s / Ω_γ= 0.45$, with $ω_r$ and $Ω_r$ the equation of state and density of photons, respectively. The effective fluid may be associated with generalized K-essence models or, alternatively, with Proca-type vector fields, albeit we do not exclude \emph{a priori} more exotic possibilities, i.e., dark radiation, axions, and so on. Physical implications of our results are analyzed in detail, indicating a statistical preference for the $Λ_{ω_s}$CDM scenario over the conventional $Λ$CDM background.

astro-ph.CO↗

arXiv2025

Quantum entanglement in cosmology

We discuss recent progress in the study of entanglement within cosmological frameworks, focusing on both momentum and position-space approaches and also reviewing the possibility to directly extract entanglement from quantum fields. Entanglement generation in expanding spacetimes can be traced back to the phenomenon of gravitational particle production, according to which the background gravitational field may transfer energy and momentum to quantum fields. The corresponding entanglement amount and its mode dependence are both sensitive to the field statistics and to the details of spacetime expansion, thus encoding information about the background. Gravitational production processes also play a key role in addressing the quantum-to-classical transition of cosmological perturbations. In order to directly extract entanglement from quantum fields, local interactions with additional quantum systems, working as detectors, have been suggested, leading to the formulation of the entanglement harvesting protocol. Despite harvesting procedures are currently unfeasible from an experimental point of view, various proposals for implementation exist and a proper modeling of detectors and local interactions is crucial to address entanglement extraction via realistic setups. In the final part of the work, we address entanglement characterization in position space, primarily focusing on black hole spacetimes. We first investigate a possible interpretation of Bekenstein-Hawking black hole entropy in terms of the entanglement entropy arising in discrete quantum field theories, on account of the area law. Then, we discuss the resolution of the black hole information paradox via the gravitational fine-grained entropy formula, which provides a new way to compute the entropy of Hawking radiation and allows to preserve unitarity in black hole evaporation processes.

gr-qc↗