arXiv subjects

J. C. Fabris

Publications and source records attributed to J. C. Fabris.

At least 19 recordsLinked to original sources

arXiv2026

Dynamical systems analysis of unimodular cosmology in $D=4+d$ dimensions

We investigate the effective four-dimensional cosmology induced by unimodular gravity in $D=4+d$ dimensions, where the internal extra-dimensional volume is encoded in a scalar degree of freedom. After dimensional reduction, we show that the resulting FLRW equations admit a natural autonomous formulation whose phase-space structure differs qualitatively from that of general relativity. In the vacuum sector, the reduced system exhibits a continuous family of finite equilibrium points, $λ=dH$, together with well-defined asymptotic Poincaré directions. In the matter sector, we focus on the five-dimensional case $d=1$ and use the reduced Bianchi relation as the consistency condition that links the ordinary matter component to the internal-volume degree of freedom. The system is then closed by adopting the minimal higher-dimensional conservation prescription, according to which matter is diluted by both the external volume and the internal-volume modulus. This leads to a reduced matter--geometry dynamics with isolated critical points and a globally organized compactified flow. Numerical examples illustrate how the internal-volume degree of freedom affects the background evolution and the global phase-space structure. The comparison with $Λ$CDM is used only as a benchmark, while a full observational analysis and more general matter--geometry exchange prescriptions are left for future work.

physics.gen-ph↗

arXiv2026

Perturbative analysis of singularity-free cosmological solutions in unimodular Kaluza-Klein theory

The unimodular version of the Kaluza-Klein theory is briefly recalled, and its projection on the $4$-dimensional spacetime is constructed. Imposing unimodularity condition on the $5$-dimensional Kaluza-Klein metric, det$g_{AB}=1$ is equivalent with introducing cosmological term in Einstein's equations in $4$ dimensions, and with scalar field of the Brans-Dicke type. Singularity-free cosmological solutions with scalar field and with matter sources are constructed, and their basic properties analyzed, along the results obtained in previous publications. In the present paper, attention is focussed on the perturbative analysis of cosmological solutions, providing a clue concerning their stability against small fluctuations.

gr-qc↗

arXiv2025

Dark sector interactions in light of weak lensing data

The current observational tensions in the standard cosmological model have reinforced the research on dynamical dark energy, in particular on models with non-gravitational interaction between the dark components. Analyses of late-time observables like type Ia supernovas (SNe Ia) and large-scale structures (LSS) are not conclusive about the presence of energy flux between dark energy and dark matter, while the anisotropy spectrum of the cosmic microwave background (CMB) is fully consistent with no interaction at all. As background and visible matter tests are less sensitive to the suppression/enhancement in the dark matter power spectrum, which is a characteristic of interacting models, while the CMB spectrum is strongly affected by it, this could be the origin of those results. In order to confirm it and at the same time to rule out the role of possible systematics between early and late-time observations, the use of a low redshift observable sensitive to the gravitational potential generated by dark matter is crucial. In the present paper, we investigate the observational viability of a class of interacting dark energy models, namely with energy exchange between vacuum-type and dust components, in the light of the Dark Energy Survey (DES) observations of galaxy weak lensing, in the context of a spatially-flat Friedmann-Lemaître-Robertson-Walker spacetime. The best fit of our analysis is entirely consistent with null interaction, confirming the CMB based constraints.

astro-ph.CO↗

arXiv2024

Quasinormal modes and the analytical continuation of non-self-adjoint operators

We briefly review the analytical continuation method for determining quasinormal modes (QNMs) and the associated frequencies in open systems. We explore two exactly solvable cases based on the Pöschl-Teller potential to show that the analytical continuation method cannot determine the full set of QNMs and frequencies of a given problem starting from the associated bound state problem in Quantum Mechanics. The root of the problem is that many QNMs are the analytically continued counterparts of solutions that do not belong to the domain where the associated Schrödinger operator is self-adjoint, challenging the application of the method for determining full sets of QNMs. We illustrate these problems through the physically relevant case of BTZ black holes, where the natural domain of the problem is the negative real line.

gr-qc↗

arXiv2024

Generalized Uncertainty Principle for Entangled States of Two Identical Particles

In this work we determine the consequences of the quantum entanglement of a system of two identical particles when a generalized uncertainty principle (GUP) is considered. GUP's are usually associated with the existence of a minimal length. We focus on the main GUP's (KMM, ADV, Pedram and Nouicer) and then we determine the minimal uncertainties in position induced by those modified GUP's. Our results point out that the minimal uncertainty is reduced by half of its usual value independently of the GUP employed. This implies that the minimal length is also reduced by half. On the other hand, it is generally expected that the minimal length must not depend on physical system. We overcome this apparent paradox by realizing that the entangled system is composed by two particles so that an effective parameter related to the minimal length must be employed.

quant-ph↗

arXiv2022

Rapidly rotating neutron stars in $f(R,T)$ gravity

In this work, we study the influence of $f(R,T)$ gravity on rapidly rotating neutron stars. First we discuss the main aspects of this modified theory of gravity where the gravitational Lagrangian is an arbitrary function of the Ricci scalar $R$ and of the trace of the energy-momentum tensor $T$. Then we present the basic equations for neutron stars including the equations of state used in the present work to describe the hadronic matter. Some physical quantities of interest are calculated such as mass-radius relations, moments of inertia, angular momentum, and compactness. By considering four different rotation regimes, we obtain results that indicate substantial modifications in the physical properties of neutron stars in $f(R,T)$ gravity when compared to those in the context of general relativity. In particular, the mass-radius relation for sequences of stars indicates that $f(R,T)$ gravity increases the mass and the equatorial radius of the neutron stars for stars rotating with an angular velocity smaller than Kepler limit.

gr-qc↗

arXiv2022

Self-adjoint extensions for a $p^{4}$-corrected Hamiltonian of a particle on a finite interval

In the present paper we deal with the issue of finding the self-adjoint extensions of a $p^4$-corrected Hamiltonian. The importance of this subject lies on the application of the concepts of quantum mechanics to the minimal-length scale scenario which describes an effective theory of quantum gravity. We work in a finite one dimensional interval and we give the explicit $U(4)$ parametrization that leads to the self-adjoint extensions. Once the parametrization is known, we can choose appropriate $U(4)$ matrices to model physical problems. As examples, we discuss the infinite square-well, periodic conditions, anti-periodic conditions and periodic conditions up to a prescribed phase. We hope that the parametrization we found will contribute to model other interesting physical situations in further works.

math-ph↗

arXiv2021

Constraints on Cosmographic Functions of Cosmic Chronometers Data Using Gaussian Processes

We study observational constraints on the cosmographic functions up to the fourth derivative of the scale factor with respect to cosmic time, i.e., the so-called snap function, using the non-parametric method of Gaussian Processes. As observational data we use the Hubble parameter data. Also we use mock data sets to estimate the future forecast and study the performance of this type of data to constrain cosmographic functions. The combination between a non-parametric method and the Hubble parameter data is investigated as a strategy to reconstruct cosmographic functions. In addition, our results are quite general because they are not restricted to a specific type of functional dependency of the Hubble parameter. We investigate some advantages of using cosmographic functions instead of cosmographic series, since the former are general definitions free of approximations. In general, our results do not deviate significantly from $ΛCDM$. We determine a transition redshift $z_{tr}=0.637^{+0.165}_{-0.175}$ and $H_{0}=69.45 \pm 4.34$. Also assuming priors for the Hubble constant we obtain $z_{tr}=0.670^{+0.210}_{-0.120}$ with $H_{0}=67.44$ (Planck) and $z_{tr}=0.710^{+0.159}_{-0.111}$ with $H_{0}=74.03$(SH0ES). Our main results are summarized in table 2.

astro-ph.CO↗

arXiv2021

Quantum and classical cosmology in the Brans-Dicke theory

In this paper we discuss classical and quantum aspects of cosmological models in Brans-Dicke theory. First, we review cosmological bounce solution in Brans-Dicke theory that obeys energy conditions (without ghost) for a universe filled with radiative fluid. Then we quantize this classical model in a canonical way, establishing the corresponding Wheeler-DeWitt equation in the minisuperspace, and analyze the quantum solutions. When the energy conditions are violated, corresponding to the case $ω<-3/2$, the energy is bounded from below and singularity-free solutions are found. However, in the case $ω> -3/2$ we cannot compute the evolution of the scale factor by evaluating the expectation values because the wave function is not finite (energy spectrum is not bounded from below). But we can analyze this case using Bohmian mechanics and the de Broglie-Bohm interpretation of quantum mechanics. Using this approach, the classical and quantum results can be compared for any value of $ω$.

gr-qc↗

arXiv2021

Quantum Cosmology with Dynamical Vacuum in a Minimal-Length Scenario

In this work, we consider effects of the dynamical vacuum in quantum cosmology in presence of a minimum length introduced by the GUP (generalized uncertainty principle) related to the modified commutation relation $[\hat{X},\hat{P}] := \frac{i\hbar}{ 1 - β\hat{P}^2 }$ . We determine the wave function of the Universe $ ψ_{qp}(ξ,t)$, which is solution of the modified Wheeler-DeWitt equation in the representation of the quasi-position space, in the limit where the scale factor of the Universe is small. Although $ψ_{qp}(ξ,t)$ is a physically acceptable state it is not a realizable state of the Universe because $ ψ_{qp}(ξ,t)$ has infinite norm, as in the ordinary case with no minimal length.

gr-qc↗

arXiv2021

Entropy bound in Einstein-Born-Infeld black holes

We study the validity of Bekenstein's entropy bound for a charged black hole in the context of nonlinear electrodynamics. Bekenstein's inequalities are commonly understood as universal relations between the entropy, the charge, the momentum, and the energy of a physical system but independent of its dynamics. In particular, we consider the Born-Infeld electrodynamics coupled to gravity as described by General Relativity. Following the steps that lead to these inequalities, we study the absorption of a charged test particle by the black hole and verify that the entropy bound is violated. We find a modified upper bound for the entropy that depends on the maximum field parameter of the Born-Infeld theory.

gr-qc↗

arXiv2021

J-PAS: forecasts on interacting vacuum energy models

The next generation of galaxy surveys will allow us to test some fundamental aspects of the standard cosmological model, including the assumption of a minimal coupling between the components of the dark sector. In this paper, we present the Javalambre Physics of the Accelerated Universe Astrophysical Survey (J-PAS) forecasts on a class of unified models where cold dark matter interacts with a vacuum energy, considering future observations of baryon acoustic oscillations, redshift-space distortions, and the matter power spectrum. After providing a general framework to study the background and linear perturbations, we focus on a concrete interacting model without momentum exchange by taking into account the contribution of baryons. We compare the J-PAS results with those expected for DESI and Euclid surveys and show that J-PAS is competitive to them, especially at low redshifts. Indeed, the predicted errors for the interaction parameter, which measures the departure from a $Λ$CDM model, can be comparable to the actual errors derived from the current data of cosmic microwave background temperature anisotropies.

astro-ph.CO↗

arXiv2020

Joint analysis of EDGES $21$-cm line observations with standard candles and rulers in $Λ$CDM and non-adiabatic gCg models

A decomposed generalised Chaplygin gas (gCg) with energy flux from dark energy to dark matter, represented by a negative value for the gas parameter $α$, is shown to alleviate the tension between EDGES data and the cosmological standard model. Using EDGES data and employing a Bayesian statistical analysis, the agreement with the standard model is only marginal. However, if $α$ is negative enough the gCg fits remarkably well the data, even in combination with SNe Ia datasets. On the other hand, when the CMB and BAO acoustic scales are included the preferred value for $α$ is near zero, implying that a small deviation from $Λ$CDM is predicted.

astro-ph.CO↗

arXiv2019

Nonconservative traceless type gravity

Extensions of the gravity theory in order to obtain traceless field equations have been widely considered in the literature. The leading example of such class of theories is the unimodular gravity, but there are other possibilities like the mimetic gravity and the Rastall gravity with a coupling parameter $λ= 1/2$. The unimodular gravity proposal is a very interesting approach in other to address the cosmological constant problem. When coupled to matter such theories may imply that the energy-momentum tensor is not divergence free anymore. In this paper, a unimodular type theory will be developed by evading the conservation $T^{μν}_{\, ; μ}=0$. The cosmological consequences of the later, both at background as well as for scalar and tensor perturbations, are explored. Possible further extensions of this approach are discussed as well as its connection with the traditional unimodular gravity.

gr-qc↗

arXiv2019

Regular Bouncing Solutions, Energy Conditions and the Brans-Dicke Theory

In general, to avoid a singularity in cosmological models involves the introduction of exotic kind of matter fields, for example, a scalar field with negative energy density. In order to have a bouncing solution in classical General Relativity, violation of the energy conditions is required. In this work, we discuss a case of the bouncing solution in the Brans-Dicke theory with radiative fluid that obeys the energy conditions, and with no ghosts.

gr-qc↗

arXiv2019

On the instability of some k-essence space-times

We study the stability properties of static, spherically symmetric configurations in k-essence theories with the Lagrangians of the form $F(X)$, $X \equiv ϕ_{,α} ϕ^{,α}$. The instability under spherically symmetric perturbations is proved for two recently obtained exact solutions for $F(X) =F_0 X^{1/3}$ and for $F(X) = F_0 X^{1/2} - 2 Λ$, where $F_0$ and $Λ$ are constants. The first solution describes a black hole in an asymptotically singular space-time, the second one contains two horizons of infinite area connected by a wormhole. It is argued that spherically symmetric k-essence configurations with $n < 1/2$ are generically unstable because the perturbation equation is not of hyperbolic type.

gr-qc↗

arXiv2019

Stiff Matter Solution in Brans-Dicke Theory and The General Relativity Limit

Generally the Brans-Dicke theory reduces to General Relativity in the limit $ω\rightarrow\infty$ if the scalar field goes as $ϕ\propto1/ω$. However, it is also known that there are examples with $ϕ\propto1/\sqrtω$ that does not tend to GR. We discuss another case: a homogeneous and isotropic universe filled with stiff matter. The power of time dependence of these solutions do not depend on $ω$, and there is no General Relativity limit even though we have $ϕ\propto1/ω$. A perturbative analysis of this exotic case is carried out.

gr-qc↗

arXiv2019

Gravitational waves from binary axionic black holes

In a recent paper we have shown that a minimally coupled, self-interacting scalar field of mass $m$ can form black holes of mass $M=\sqrt{3}/(4m)$ (in Planck units). If dark matter is composed by axions, they can form miniclusters that for QCD axions have masses below this value. In this work it is shown that for a scenario in which the axion mass depends on the temperature as $m \propto T^{-6}$, minicluster masses above $0.32\,M_\odot$, corresponding to an axion mass of $3\times 10^{-10}$ eV, exceed $M$ and can collapse into black holes. If a fraction of these black holes is in binary systems, gravitational waves emitted during the inspiral phase could be detected by advanced interferometers like LIGO or VIRGO and by the planned Einstein Telescope. For a detection rate of one event per year, the lower limits on the binary fraction are $10^{-4}$ and $10^{-6}$ for LIGO and Einstein Telescope respectively.

gr-qc↗