arXiv subjects

Orlando Luongo

Publications and source records attributed to Orlando Luongo.

At least 73 records · Page 4Linked to original sources

arXiv2024

Dark energy constraints using gamma-ray burst correlations with DESI 2024 data

Even though the Dark Energy Spectroscopic Instrument (DESI) mission does not exclude a dynamical dark energy evolution, the concordance paradigm, i.e., the $Λ$CDM model, remains statistically favored, as it depends on the fewest number of free parameters. In this respect, high redshift astrophysical sources, such as gamma-ray bursts, represent a formidable tool to model the form of dark energy, since they may provide a link between early and local redshift regimes. Hence, the use of these objects as possible distance indicators turns out to be essential to investigate the cosmological puzzle. To this end, we adopt two gamma-ray burst linear correlations, namely the $L_p-E_p$ and $L_0-E_p-T$ relations, to test the flat and non-flat $Λ$CDM, $ω_0$CDM, and $ω_0ω_1$CDM cosmological models, i.e., those directly examined by the DESI collaboration. The inferred correlation coefficients and cosmological parameters are thus obtained by considering two independent Monte Carlo Markov chain analyses, the first considering the whole DESI data set and the second excluding a seemingly problematic data point placed at $z_{eff} = 0.51$. Using model selection criteria, the two above correlations do not show a preference on a precise cosmological model although, when the data point at $z_{eff}$ is included, the concordance paradigm appears to be the least favored among the tested cosmological models.

astro-ph.CO↗

arXiv2024

Dark energy reconstructions combining BAO data with galaxy clusters and intermediate redshift catalogs

Cosmological parameters and dark energy (DE) behavior are generally constrained assuming \textit{a priori} models. We work out a model-independent reconstruction to bound the key cosmological quantities and the DE evolution. Through the model-independent \textit{Bézier interpolation} method, we reconstruct the Hubble rate from the observational Hubble data and derive analytic expressions for the distances of galaxy clusters, type Ia supernovae, and uncorrelated baryonic acoustic oscillation (BAO) data. In view of the discrepancy between Sloan Digital Sky Survey (SDSS) and Dark Energy Spectroscopic Instrument (DESI) BAO data, they are kept separate in two distinct analyses. Correlated BAO data are employed to break the baryonic--dark matter degeneracy. All these interpolations enable us to single out and reconstruct the DE behavior with the redshift $z$ in a totally model-independent way. In both analyses, with SDSS-BAO or DESI-BAO data sets, the constraints agree at $1$--$σ$ confidence level (CL) with the flat $Λ$CDM model. The Hubble constant tension appears solved in favor of the Planck satellite value. The reconstructed DE behavior exhibits deviations at small $z$ ($>1$--$σ$ CL), but agrees ($<1$--$σ$ CL) with the cosmological constant paradigm at larger $z$. Our method hints for a slowly evolving DE, consistent with a cosmological constant at early times.

astro-ph.CO↗

arXiv2024

Geometric and topological corrections to Schwarzschild black hole

In this paper, we compute departures in the black hole thermodynamics induced by either geometric or topological corrections to general relativity. Specifically, we analyze the spherically symmetric spacetime solutions of two modified gravity scenarios with Lagrangians $\mathcal{L}\sim R^{1+ε}$ and $\mathcal{L}\sim R+ε\, \mathcal{G}^2$, where $\mathcal{G}$ is the Euler density in four dimensions, while $ 0<ε\ll 1$ measures the perturbation around the Hilbert-Einstein action. Accordingly, we find the expressions of the Bekenstein-Hawking entropy by the Wald formula, and the black hole temperature and horizon of the obtained solutions. We then investigate the heat capacities in terms of the free parameters of the theories under study. In doing so, we show that healing the problem of negative heat capacities can be possible under particular choices of the free constants, albeit with limitations on the masses allowed for the black hole solutions.

gr-qc↗

arXiv2024

Particle motion and accretion disk around rotating accelerating black holes

We investigate the motion of test particles and the properties of accretion disks around rotating, accelerating black hole spacetimes with non-zero cosmological constants. To do so, we explore the impact of both rotation and acceleration on massive particle dynamics in motion around these configurations. In this respect, we derive the geodesic equations in the Pleba{ń}ski-Demia{ń}ski spacetime and analyse the characteristics of circular orbits, including the innermost and outermost stable circular orbits. Accordingly, we find that the acceleration parameter significantly influences the radii and angular momenta of these orbits. Further, we examine the precession of non-circular orbits and find that the precession is in the same direction as the black hole rotation regardless of the direction of the black hole spin. We also investigate the thermal spectra of geometrically thin, optically thick accretion disks described by the Novikov-Thorne model. We consider both co-rotating and counter-rotating disks and show that the radiative flux and luminosity are significantly larger for co-rotating disks. We compare the spectral features and luminosity of accretion disks around accelerating black holes with those around non-accelerating Kerr black holes, revealing that acceleration generally reduces the luminosity and flux of emitted light. Our results provide insights into the complex interplay between black hole acceleration, rotation, and the surrounding accretion disk dynamics.

astro-ph.HE↗

arXiv2024

Cosmological constraints from calibrated $E_p-E_{iso}$ gamma-ray burst correlation by using DESI 2024 data release

Recent outcomes by the DESI Collaboration have shed light on a possible slightly evolving dark energy, challenging the standard $Λ$CDM paradigm. To better understand dark energy nature, high-redshift observations like gamma-ray burst data become essential for mapping the universe expansion history, provided they are calibrated with other probes. To this aim, we calibrate the $E_p-E_{iso}$ (or Amati) correlation through model-independent Bézier interpolations of the updated Hubble rate and the novel DESI data sets. More precisely, we provide two Bézier calibrations: i) handling the entire DESI sample, and ii) excluding the point at $z_{eff}=0.51$, criticized by the recent literature. In both the two options, we let the comoving sound horizon at the drag epoch, $r_d$, vary in the range $r_d \in [138, 156]$ Mpc. The Planck value is also explored for comparison. By means of the so-calibrated gamma-ray bursts, we thus constrain three dark energy frameworks, namely the standard $Λ$CDM, the $ω_0$CDM and the $ω_0ω_1$CDM models, in both spatially flat and non-flat universes. To do so, we worked out Monte Carlo Markov chain analyses, making use of the Metropolis-Hastings algorithm. Further, we adopt model selection criteria to check the statistically preferred cosmological model finding a preference towards the concordance paradigm only whether the spatial curvature is zero. Conversely, and quite interestingly, the flat $ω_0$CDM and both the cases, flat/non-flat, $ω_0ω_1$CDM model, leave evidently open the chance that dark energy evolves at higher redshifts.

astro-ph.CO↗

arXiv2024

How cosmological expansion affects communication between distant quantum systems

A quantum communication protocol between harmonic oscillator detectors, interacting with a quantum field, is developed in a cosmological expanding background. The aim is to see if the quantum effects arising in an expanding universe, such as the cosmological particle production, could facilitate the communication between two distant parts or if they provide an additive noisy effect. By considering a perfect cosmic fluid, the resulting expansion turns out to increase the classical capacity of the protocol. This increasing occurs for all the cosmological expansions unless the latter is sharpened just before the receiver's detector interacts with the field. Moreover, the classical capacity turns out to be sensible to the barotropic parameter $w$ of the perfect fluid and to the coupling between the field and the scalar curvature $ξ$. As a consequence, by performing this protocol, one can achieve information about the cosmological dynamics and its coupling with a background quantum field.

gr-qc↗

arXiv2024

Horizon entanglement area law from regular black hole thermodynamics

We investigate the thermodynamics of regular black hole configurations via quantum analogs of entropy and energy -- namely, the entanglement entropy and entanglement energy -- near the event horizon of Bardeen and Hayward black holes. Following standard approaches, we introduce a quantum scalar field propagating in such black hole spacetimes and discretize the field degrees of freedom on a lattice of spherical shells. We observe that, at leading order, the entanglement entropy associated with the scalar field is proportional to Bekestein-Hawking entropy, while the corresponding entanglement energy scales proportionally to Komar energy. We then compute the heat capacity in both scenarios, discussing the black hole stability conditions and the possible appearance of second-order phase transitions. Finally, we extend our analysis to the black hole core, showing that in this sector entanglement energy serves as a valuable tool towards discriminating between singular and regular solutions.

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arXiv2024

A thermodynamic model of inflation without inflaton field

In the framework of geometrothermodynamics, we explore how to construct an inflationary cosmic fluid without requiring inflaton fields. To do so, we employ standard radiation and matter as barotropic background, speculating on the existence of a further real fluid, interpreted as a possible matter with pressure constituent. We thus illustrate its main characteristics, showing how it may face the flatness and horizon problems, yielding an equivalent number of e-foldings, compatible with theoretical expectations. Moreover, we calculate the associated equilibrium and kinetic temperatures, and compute out-of-equilibrium properties, in terms of particle production and effective reheating mechanism. Accordingly, we revise how inflation ends and how to fix the free parameters of our model from observations. Last but not least, we propose how to obtain a bare cosmological constant that could relate the late-time acceleration to inflation, aiming to unify the two approaches. Finally, we discuss the corresponding theoretical implications of our treatment in contrast to the standard approach reliant on scalar field potentials, emphasizing the main advantages and disadvantages.

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arXiv2024

Preserving quantum information in $f(Q)$ cosmology

The effects of cosmological expansion on quantum bosonic states are investigated, using quantum information theory. In particular, a generic Bogoliubov transformation of bosonic field modes is considered and the state change on a single mode is regarded as the effect of a quantum channel. Properties and capacities of this channel are thus explored in the framework of $f(Q)$ theories. As immediate result, we obtain that the information on a single-mode state appears better preserved, whenever the number of particles produced by the cosmological expansion is small. Hence, similarly to general relativity, we show that analogous particle productions result even if we consider symmetric teleparallel gravity theories. Thus, we investigate a power law $f(Q)$ model, leaving unaltered the effective gravitational coupling, and minimise the corresponding particle production. We thus show how to optimise the preservation of classical and quantum information, stored in a bosonic mode states in the remote past. Finally, we compare our findings with those obtained in general relativity.

gr-qc↗

arXiv2024

Alleviating the cosmological constant problem from particle production

We explore a toy model mechanism of geometric cancellation, alleviating the (classical) cosmological constant problem. To do so, we assume at primordial times that vacuum energy fuels an inflationary quadratic hilltop potential nonminimally coupled to gravity through a standard Yukawa-like interacting term, whose background lies on a perturbed Friedmann-Robertson-Walker metric. We demonstrate how vacuum energy release transforms into geometric particles, adopting a quasi-de Sitter phase where we compute the expected particle density and mass ranges. Perturbations are introduced by means of the usual external-field appproximation, so that the back-reaction of the created particles on the geometry is not considered here. We discuss the limitations of this approach and we also suggest possible refinements. We then propose the most suitable dark matter candidates, showing under which circumstances we can interpret dark matter as constituted by geometric quasiparticles. We confront our predictions with quantum particle production and constraints made using a Higgs portal. In addition, the role of the bare cosmological constant is reinterpreted to speed up the universe today. Thus, consequences on the standard $Λ$CDM paradigm are critically highlighted, showing how both coincidence and fine-tuning issues can be healed requiring the Israel-Darmois matching conditions between our involved inhomogeneous and homogeneous phases.

gr-qc↗

arXiv2024

Particle production from non-minimal coupling in a symmetry breaking potential transporting vacuum energy

We propose an inflationary scenario where the inflaton field is non-minimally coupled to spacetime curvature and inflation is driven by a vacuum energy symmetry breaking potential without specifying \emph{a priori} whether the inflaton field is small or large. As we incorporate vacuum energy into our analysis, we further explore the implications of a non-zero potential offset in relation to the emergence of inflationary dynamics. Thus, we propose that vacuum energy can transform into particles as a result of the transition triggered by spontaneous symmetry breaking. This entails a vacuum energy cancellation that yields an effective cosmological constant during inflation by virtue of a quasi-de Sitter evolution and shows that the vacuum energy contribution can manifest as \emph{geometric particles} produced by inflaton fluctuations, with particular emphasis on super-Hubble modes. We conjecture these particles as \emph{quasi-particles} arising from interaction between the inflaton and spacetime geometry, enhanced by non-minimal coupling. Specifically, we propose that dark matter arises from a pure geometric quasi-particle contribution, and we quantify the corresponding dark matter candidate ranges of mass. In this scenario, we further find that a zero potential offset leads to a bare cosmological constant at the end of inflation, while a negative offset would require an additional kinetic (or potential) contribution in order to be fully-canceled. In this regard, we conclude that the scenario of large field inflaton is preferred since it necessitates a more appropriate selection of the offset. Our conclusion is reinforced as small field inflaton would lead to a significant screening of the Newtonian gravitational constant as inflation ends.

gr-qc↗

arXiv2024

Phase-space analysis in non-minimal symmetric-teleparallel dark energy

We modify the symmetric-teleparallel dark energy through the addition of a further Yukawa-like term, in which the non-metricity scalar, $Q$, is non-minimally coupled to a scalar field Lagrangian where the phion acts as quintessence, describing dark energy. We investigate regions of stability and find late-time attractors. To do so, we conduct a stability analysis for different types of physical potentials describing dark energy, namely the power-law, inverse power-law, and exponential potentials. Within these choices, we furthermore single out particular limiting cases, such as the constant, linear and inverse potentials. For all the considered scenarios, regions of stability are calculated in terms of the signs of the coupling constant and the exponent, revealing a clear degeneracy among coefficients necessary to ensure stability. We find that a generic power-law potential with $α> 0$ is not suitable as a non-minimal quintessence potential and we put severe limits on the use of inverse potential, as well. In addition, the equations of state of each potential have been also computed. We find the constant potential seems to be favored than other treatments, since the critical point appears independent of the non-minimal coupling.

gr-qc↗

arXiv2024

Entanglement entropy for spherically symmetric regular black holes

The Bardeen and Hayward spacetimes are here considered as standard configurations of spherically symmetric regular black holes. Assuming the thermodynamics of such objects to be analogous to standard black holes, we compute the island formula in the regime of small topological charge and large vacuum energy, respectively for Bardeen and Hayward spacetimes. Late and early-time domains are separately discussed, with particular emphasis on the island formations. We single out conditions under which it is not possible to find out islands at early-times and how our findings depart from the standard Schwarzschild case. Motivated by the fact that those configurations extend Reissner-Nordström and Schwarzschild-de Sitter metrics through the inclusion of regularity behavior at $r=0$, we show how the effect of regularity induces modifications on the overall entanglement entropy. Finally, the Page time is also computed, showing its asymptotic behavior. Specifically, the Page time exhibits slight departures with respect to the Schwarzschild case, more evident in the Hayward metric only. Conversely, our findings on Page time show that the Bardeen regular black hole is quite indistinguishable from the Schwarzschild case.

gr-qc↗

arXiv2024

Anisotropic Generalized Polytropic Spheres: Regular 3D Black Holes

We model gravitating relativistic 3D spheres composed of an anisotropic fluid in which the radial and transverse components of the pressure correspond to the vacuum energy and a generalized polytropic equation-of-state, respectively. By using the generalized Tolman-Oppenheimer-Volkoff (TOV) equation, and solving the complete system of equations for these anisotropic generalized polytropic spheres, for a given range of model parameters, we find three novel classes of asymptotically AdS black hole solutions with regular core. We show the regularity of the solutions using curvature scalars and the formalism of geodesic completeness. Then, using the eigenvalues of the Riemann curvature tensor, we consider the effects of repulsive gravity in the three static 3D regular black holes, concluding that their regular behavior can be explained as due to the presence of repulsive gravity near the center of the objects. Finally, we study the stability of the regular black holes under the flow of the energy through the Cauchy horizon.

gr-qc↗

arXiv2024

De Sitter-like configurations with asymptotic quintessence environment

We examine a spherically-symmetric class of spacetimes carrying vacuum energy, while considering the influence of an external dark energy environment represented by a non-dynamical quintessence field. Our investigation focuses on a specific set of solutions affected by this field, leading to distinct kinds of spacetime deformations, resulting in regular, singular, and wormhole solutions. We thoroughly discuss the underlying physics associated with each case and demonstrate that more complex deformations are prone to instability. Ultimately, we find that our results lead to an \emph{isotropic de Sitter-like solution} that behaves as a quintessence fluid. To achieve this, we investigate the nature of the corresponding fluid, showing that it cannot provide the sound speed equal to a constant equation of state {parameter} near the center. Consequently, we reinterpret the fluid as a slow-roll quintessence by investigating its behavior in asymptotic regimes. Further, we explore the potential implications of violating the isotropy condition on the pressures and we finally compare our findings with the de Sitter and Hayward solutions, highlighting both the advantages and disadvantages of our scenarios.

gr-qc↗

arXiv2024

Accretion disks properties around regular black hole solutions obtained from non-linear electrodynamics

We investigate a family of spherically symmetric, static, charged regular black hole solutions derived within the framework of Einstein-nonlinear electrodynamics. Our study focuses on examining the characteristics of accretion disks in the spacetimes described by the Dymnikova and Fan-Wang solutions. We explore circular geodesics of test particles and calculate various properties, including the radius of the innermost stable circular orbit, radiant energy, temperature, and conversion efficiency of accretion mass into radiation. We employ the Novikov-Thorne-Page thin accretion disk model as a background. By comparing our findings with those obtained in the Schwarzschild black hole case, we reveal significant modifications in the overall spectral properties. Specifically, we observe an increase in the energy emitted from the disk surface, resulting in higher temperatures for the accretion disks under certain values of the free parameters. Consequently, we note an enhanced efficiency of mass conversion into radiation compared to the Schwarzschild spacetime.

gr-qc↗

arXiv2024

Does dark energy really revive using DESI 2024 data?

We investigate the impact of the Dark Energy Spectroscopic Instrument (DESI) 2024 data on dark energy scenarios. We thus analyze three typologies of models, the first in which the cosmic speed up is related to thermodynamics, the second associated with Taylor expansions of the barotropic factor, whereas the third based on \emph{ad hoc} dark energy parameterizations. In this respect, we perform Monte Carlo Markov chain analyses, adopting the Metropolis-Hastings algorithm, of 12 models. To do so, we first work at the background, inferring \emph{a posteriori} kinematic quantities associated with each model. Afterwards, we obtain early time predictions, computing departures on the growth evolution with respect to the model that better fits DESI data. We find that the best model to fit data \emph{is not} the Chevallier-Polarski-Linder (CPL) parametrization, but rather a more complicated log-corrected dark energy contribution. To check the goodness of our findings, we further directly fit the product, $r_d h_0$, concluding that $r_d h_0$ is anticorrelated with the mass. This treatment is worked out by removing a precise data point placed at $z=0.51$. Surprisingly, in this case the results again align with the $Λ$CDM model, \emph{indicating that the possible tension between the concordance paradigm and the CPL model can be severely alleviated}. We conclude that future data points will be essential to clarify whether dynamical dark energy is really in tension with the $Λ$CDM model.

astro-ph.CO↗

arXiv2024

Model independent cosmographic constraints from DESI 2024

In this study, we explore model independent constraints on the universe kinematics up to the snap and jerk hierarchical terms. To do so, we consider the latest Baryon Acoustic Oscillation (BAO) release provided by the DESI collaboration, tackling the $r_d$ parameter to span within the range $[144,152]$ Mpc, with fixed step, $δr_d=2$ Mpc, aligning with Planck and DESI results. Thus, employing Monte Carlo Markov chain analyses, we place stringent constraints on the cosmographic series, incorporating three combinations of data catalogs: the first BAO with observational Hubble data, the second BAO with type Ia supernovae, and the last including all three data sets. Our results conclusively constrain the cosmographic series, say the deceleration $q_0$, the jerk $j_0$, and the snap $s_0$ parameters, at the $2$--$σ$ level, showcasing a significant departure on $j_0$ even at $1$--$σ$ confidence level, albeit being compatible with the $Λ$CDM paradigm on $q_0$ and $s_0$, at $2$--$σ$ level. Analogously, the $h_0$ tension appears alleviated in the second hierarchy, say including snap. Finally, a direct comparison with the $Λ$CDM, $w$CDM models and the Chevallier-Polarski-Linder parametrization is reported, definitively favoring the wCDM scenario.

astro-ph.CO↗