arXiv subjects

Júlio C. Fabris

Publications and source records attributed to Júlio C. Fabris.

At least 19 recordsLinked to original sources

arXiv2026

Wormhole Reconstruction in Non-Conservative Unimodular Gravity: Ricci Scalar as an Independently Prescribed Curvature Profile

The absence of an algebraic relation between the Ricci scalar and the trace of the energy-momentum tensor in Non-Conservative Unimodular Gravity ($\mathrm{NUG}$) opens new possibilities for constructing spacetime geometries from prescribed curvature profiles. Exploiting this property, we develop a curvature reconstruction framework in which traversable wormhole geometries are generated directly from a prescribed curvature profile rather than from the matter sector or the metric functions. The reconstruction procedure is implemented for a power-law Ricci scalar combined with three different redshift functions, leading to distinct classes of static and spherically symmetric wormhole solutions. We investigate their geometrical properties, asymptotic behavior and null energy condition. Although the radial null energy condition remains necessarily violated at the throat, we show that the dimensionless curvature parameter controls the intensity of the exotic matter supporting the wormhole, while the curvature exponent determines the global asymptotic structure of the reconstructed spacetime. In particular, the critical configuration separating asymptotically flat solutions from geometries with residual asymptotic deformation naturally emerges from the reconstruction scheme. These results establish curvature reconstruction from prescribed Ricci scalar profiles as a viable geometrical framework for generating compact-object spacetimes within non-conservative unimodular gravity.

gr-qc↗

arXiv2026

Sources of matter for wormholes in a k-essence theory

In this work, we analyze some matter sources associated with wormhole models within a k-essence theory coupled to the gravitational sector through a phantom scalar field. We adopt a spherically symmetric background in (3+1) dimensions and consider two types of systems: electrically and magnetically charged. In the first case, we consider the generalized Ellis-Bronnikov model, in which we fix the power of the kinetic term in the k-essence Lagrangian function to $n=1/2$ and take the parameter $m\ge 2$, which acts as a generalization factor for the geometry of the wormhole area function. From this, we obtained the expression for the scalar field, the potential, and the associated electromagnetic functions for any values of the parameter $m\geq{2}$. In the second and third models, we consider the scenario of two wormholes that are structured according to the adjustment of the parameters that define the metric component associated with the area function $Σ^2$ (the $g_{22}$ component of the line element), and in both cases we adopt $n=1/2$. We show that the violation of the null energy conditions is conditioned by the parameters of the area function. Finally, we studied the linear stability of the models through the behavior of a test scalar field using both the WKB method and the time-domain evolution method.

gr-qc↗

arXiv2026

Lagrangian Extensions of Newtonian Gravity constrained by Solar System tests

We explore an extension to Newtonian gravity through a generalised Lagrangian function with the introduction of a second dynamical scalar field. Building on previous research into gravity with variable gravitational coupling, the work derives the complete field equations and applies a weak-field approximation. This leads to an effective post-Newtonian gravitational potential that includes key aspects of relativistic theories. The resulting N-body equations of motion highlight differences among inertial and gravitational masses, which can constrain the theory's free parameter through data from the Nordtvedt effect. By employing the method of osculating orbits for a two-body system, the study calculates the secular variation of the orbital pericenter and aligns this with the latest data on Mercury's perihelion shift, for another observational constraint on the model. Furthermore, a few examples of theories are discussed.

gr-qc↗

arXiv2026

Generalized Unimodular Gravity and Cosmological Perturbations

The generalized unimodular theory is revisited and its consequence for cosmology is discussed. The usual matter components of the universe are obtained in a pure geometric way. This result gives a new perspective to the studies of the dark sector of the universe. A background and perturbative analysis are carried out, recovering the corresponding results obtained through the general relativity theory but with a different interpretation.

gr-qc↗

arXiv2025

Anisotropic Bianchi-I cosmological model in non-conservative unimodular gravity

In this article, we propose an anisotropic Bianchi-I type cosmological model in non-conservative Unimodular Gravity ($\mathrm{NUG}$). We show that simply using the Bianchi-I type metric does not resolve a striking characteristic of the field equations in $\mathrm{NUG}$: their underdetermination. This fact led us to implement extra conditions on the combination $\left(ρ+p\right)$ and, consequently, obtain a consistent background cosmological analysis. In the vacuum case, we obtain an analogy between the Kasner solutions and the equations in $\mathrm{NUG}$. We also propose a new analysis of a non-homogeneous equation of state, the combination $\left(ρ+p\right)=l$. We identify that the cosmological dynamics are strictly dependent on the value of the constant $l$. The physically interesting case is at the value $l<0$, which seems to indicate a super-accelerated, ghost-like universe. This case still requires a more detailed analysis, for example, from a thermodynamic point of view, keeping in mind that $\left(ρ+p\right)$ may be interpreted as enthalpy of the system. For the cases $\left(ρ+p\right)\propto a^{-3}$ and $\left(ρ+p\right) \propto a^{-4}$, we obtain a description consistent with the anisotropic cosmological model described by $\mathrm{GR}$. In all cases analyzed, a small value for the anisotropic parameter $Ω_{A}$ (on the order of $10^{-2}$) is required in order to have agreement, for example, with the age of the universe to be approximately $12-14\, \mathrm{Gyr}$, agreeing with the age of globular clusters. As the universe expands an isotropization is verified, with the anistropies going to zero asymptotically, similarly with what happens in an anistropic cosmological model based on $\mathrm{GR}$.

gr-qc↗

arXiv2025

A Five-dimensional Kaluza-Klein Approach to Unimodular Gravity

In this article we present a possibility of imposing the unimodular condition within the 5-dimensional Kaluza-Klein theory including the scalar field. Unimodular gravity became an object of increasing interest in the late 80-ties; and was recently used in primordial Universe modeling with cosmological constant, in the context of the Brans-Dicke gravity including scalar field. A generalization of the unimodularity principle to the 5-dimensional Kaluza-Klein model was discussed in our recent paper, in which variational principle is formulated in 5 dimensions first, and dimensional reduction is applied to the resulting set of equations. A cosmological model based on these equations was then presented and discussed. Here we present further developments of this approach, focussing our attention at perturbative aspects and stability of solutions.

gr-qc↗

arXiv2025

The frozen vanilla model: Exploring dark sector interactions with delta effective theories

In this paper, we introduce a new interacting mechanism within the dark sector, encompassing both dark energy and dark matter, while grounding our analysis in the familiar framework of the $\mathrm{ΛCDM}$ model augmented by baryons and radiation components, including photons and neutrinos. The interaction between dark energy and dark matter is confined to the perturbative level. One significant advantage of this proposal is that all geometric probes yield constraints consistent with the so-called vanilla model or the extended vanilla model, where dark energy has a constant equation of state, $w_{x}$. However, the introduction of this new interacting mechanism affects several theoretical signatures, involving contrast dark matter and dark energy densities. We perform an exploratory analysis of those effects in the CMB power spectra, matter-power spectra, and redshift space distortions. For instance, it allows for a decrease/increment in the integrated Sachs-Wolfe (ISW) effect depending on the value taken by the interaction coupling. This effect could be observationally detected by looking for a cross-correlation between the ISW temperature fluctuations and the distribution of galaxies or quasars. At late times, the interaction in the dark sector becomes very effective, affecting the non-linear scale of structure formation. We discuss how the estimators $fσ_{8}(z)$ and $S_{8}(z)$ are affected by different interacting couplings; indicating that $fσ_{8}(z)$ can show a relative change of up to $15\%$ compared to the concordance model at low redshifts. Finally, we show how the various terms in the dark energy pressure perturbation (both adiabatic and non-adiabatic) are relevant for different scales, demonstrating the absence of large-scale instabilities.

gr-qc↗

arXiv2025

A new framework for cosmological expansion in a reformulated Newtonian-Like gravity with variable G

A Newtonian-like theory inspired by the Brans-Dicke gravitational Lagrangian has been recently proposed in Ref. arXiv:2009.04434(v4). We propose here a new variant of this theory such that the usual Newtonian second law is preserved. The cosmological solutions are analysed and accelerated background expansion can be obtained even in a pure matter dominated universe. This happens due to the dynamic character of the effective gravitational coupling which is sourced by a time evolving scalar field . We also analyse the matter density perturbations and find they exhibit an enhanced growth in comparison with the usual Newtonian like behavior in Einstein-de Sitter model.

gr-qc↗

arXiv2025

Quasar Pairs as Large--Scale Structure tracers

Quasars are effective tracers of the large-scale distribution of galaxies at high redshift thanks to their high luminosity and dedicated surveys. Previous studies have shown that quasars exhibit a bias similar to that of rich groups, indicating that quasar pairs may be linked to higher density environments, serving as protocluster proxies. In this work, we aim to characterize close quasar pairs within the same halo by identifying them in redshift space. We analyze pair-quasar cross correlations and CMB-derived lensing convergence profiles centered in these systems. We have identified 2777 pairs of quasars in the redshift range from 1.2 to 2.8. Quasar pairs exhibit a distribution of relative luminosities that differs from that of random pairs with the same redshift distribution, indicating that quasars in these systems are distinct from isolated ones. The cross-correlation between pairs and quasars shows a larger amplitude than the auto-correlation function of quasars, suggesting these systems are more strongly biased toward the large-scale mass distribution and reside in more massive halos. This is further supported by higher convergence CMB lensing profiles of pairs compared to isolated quasars with a similar redshift distribution. Our results indicate that quasar pairs are suitable precursors to present-day clusters of galaxies, unlike isolated quasars, which are associated with less dense environments.

astro-ph.CO↗

arXiv2025

Black-bounce solutions in a k-essence theory under the effects of bumblebee gravity

In the present study, we analyze the effects of violation of Lorentz symmetry for black-bounce solutions in a $k$-essence theory that has the form of a power law for the configuration $n=1/3$. We perform such analysis for a known model explored in previous work $Σ^2=x^2+a^2$ and complement the proposal with a new black-bounce model for the area functions $Σ^2_1=\sqrt{x^4+d^4}$. This model has the Schwarzschild-de Sitter asymptomatic behavior for $x\to{-\infty}$, and we investigate the scalar field, potential, and energy conditions for both models. We have shown that the violation of Lorentz symmetry can be generated through $k$-essence without the need for an additional field. These results corroborate the validation of other previously investigated wormhole solutions.

gr-qc↗

arXiv2025

Revisiting Unimodular Quantum Cosmology

The quantization of unimodular gravity in minisuperspace leads to a time evolution of states generated by the Hamiltonian, as in usual quantum mechanics. We revisit the analysis made in Ref. \cite{unruh}, extending it to phantom scalar fields. It is argued that only in this case a non-trivial evolution for the scalar field can be obtained. The behavior of the scale factor presents a bounce followed by a de Sitter expansion, reproducing the quantum cosmological scenario in General Relativity when the source is given by a cosmological term described by the Schutz variable. The analysis is extended to the Brans-Dicke scalar tensor theory.

gr-qc↗

arXiv2025

Magnetically charged black-bounce solution via nonlinear electrodynamics in a k-essence theory

In the present work, we obtain and analyze a new class of analytical solutions of magnetically charged black bounces in k-essence theory, spherically symmetric in (3+1)-dimensions, coupled to nonlinear electrodynamics (NED). We consider two metric models, Simpson-Visser and Bardeen, for the k-essence configurations n = 1/3 and n = 1/5. We obtain in an analytical way which scalar field, field potential, and Lagrangian NED are necessary to support the metrics. We analyze the behavior of these quantities and the energy conditions due to the scalar field and the NED.

gr-qc↗

arXiv2025

Fluid and scalar field representations in the Brans-Dicke theory: cosmological scenarios

The Brans-Dicke scalar-tensor cosmological models are studied in both Einstein and Jordan frames, using hydrodynamical and self-interacting scalar field representations of the energy-momentum tensor, leading to the same background solutions. The main features of the corresponding cosmological scenarios are determined. In many cases, we identify a transition from decelerated to accelerated regimes. The properties of the self-interacting scalar field, including the corresponding potential, are determined. Finally, some possible realistic configurations are analyzed.

gr-qc↗

arXiv2025

On the Stability of Non-Singular Solutions in Effective Theory from Kaluza-Klein Unimodular Gravity

Unimodular theory incorporating the Kaluza-Klein construction in five dimensions leads, after reduction to four dimensions, to a new class of scalar-tensor theory. The vacuum cosmological solutions display a bouncing, non singular behavior. From the four dimensional point of view, the solutions are completely regular. However, the propagation of gravitational waves in this geometry displays the presence of instabilities which reflect some features of the original five dimensional structure. Comparison with a four dimensional quantum model with cosmological constant, which has a similar background behavior, is discussed.

gr-qc↗

arXiv2024

A unimodular Kaluza-Klein theory

Unimodular gravity became an object of increasing interest in the late $80$-ties and was recently used in primordial Universe modeling with cosmological constant, in the context of the Brans-Dicke gravity including scalar field. In the present article we investigate the possibility of imposing the unimodular condition within the $5$-dimensional Kaluza-Klein theory including the scalar field. The variational principle is formulated in $5$ dimensions first, and dimensional reduction is applied to the resulting set of equations. A cosmological model based on these equations is then presented and discussed.

gr-qc↗

arXiv2024

New sources of ghost fields in $k$-essence theories for black-bounce solutions

In the present study, we generalize the possible ghost field configurations within the framework of $k$-essence theory to the Simpson-Visser metric area function $Σ^2=x^2+a^2$. Our analysis encompasses field configurations for the region-defined metric function $dA_\pm$ as well as the general solution that asymptotically behaves as Schwarzschild-de Sitter for $x\to-\infty$. Specifically, we investigate two scalar field configurations and define the associated potential for each one. Through rigorous calculations, we verify that all equations of motion are satisfied. Notably, our findings indicate that even when proposing new configurations of ghost scalar fields, the energy conditions remain unchanged. This result serves to validate the wormhole solutions obtained in previous studies.

gr-qc↗

arXiv2024

Celestial mechanics in Newtonian-like gravity with variable $G$

A Newtonian-like theory inspired by the Brans-Dicke gravitational Lagrangian has been recently proposed in Ref. arXiv:2009.04434(v4). This work demonstrates that the modified gravitational force acting on a test particle is analogous to that derived from the Manev potential. Specifically, an additional term $\propto r^{-3}$ emerges alongside the conventional Newtonian component. We analyse the predicted expression for the pericenter advance and the Roche limit and use them to constraint the theory's single free parameter $ω$ which is analogous to the Brans-Dicke parameter. At the same time this theory is able to solve the advance of Mercury's perihelion, we also show that there is no relevant impact on the Roche limit in comparison to the well known Newtonian results.

gr-qc↗

arXiv2024

Holographic Ricci dark energy in Nonconservative Unimodular Gravity

The structure of unimodular gravity (UG) is invariant to a subclass of diffeomorphism, the transverse diffeomorphism, due to the unimodular condition ($\sqrt{-g}=ε=cte$). Consequently, there is a freedom to define how the conservation laws of the energy-momentum tensor in unimodular gravity in the cosmological context. One of the main characteristics of the complete system of equations that describe cosmological dynamics in UG is that they form an underdetermined system if the usual conservation law of the energy-momentum tensor is not used in your structure, that is, it is necessary to insert extra information into the system to solve the complete set of equations. In this article, we propose the construction of a background cosmological model based on the description of a holographic dark energy component with a cutoff of the order of Ricci scalar in non-conservative UG. Although this choice is indeed a new addition of information to the cosmological system, the complete set of equations remains underdetermined, however, the new feature of this cosmological model is the appearance of an interaction between matter and dark energy. Indeed, this is a well-known characteristic of cosmological models in which we have holographic dark energy density. Consequently, we propose an ansatz to the interaction term $Q=βH ρ_{m}$, and obtain the cosmological parameters of our model. We found a viable universe model with similar characteristics to the $Λ\mathrm{CDM}$ model. We performed statistical analysis of the background model using the "Cosmic Chronometer" (CC) data for $H(z)$, and obtain as a result using Akaike Information Criterion (AIC), and the Bayesian Information Criterion (BIC) as model selection criteria that $Λ\mathrm{CDM}$ prevails as the best model. However, the proposed model is competitive when compared to the cosmological model $ω\mathrm{CDM}$.

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