arXiv subjects

A. R. Gomes

Publications and source records attributed to A. R. Gomes.

At least 19 recordsLinked to original sources

arXiv2025

Note on bulk viscosity as an alternative to dark energy

Bulk viscosity, which characterizes the irreversible dissipative resistance of a fluid to volume changes, has been proposed as a potential mechanism for explaining both early- and late-time accelerated expansion of the Universe. In this work, we investigate two distinct physical scenarios for the origin of bulk viscosity: (1) nonminimal interactions between two fluids, and (2) elastic collisions in an ideal gas. In both cases, we demonstrate that while the associated energy-momentum exchange can significantly influence fluid dynamics, overall energy-momentum conservation precludes such exchange from having any direct gravitational effect in the context of General Relativity. In case (1), we show that the standard bulk viscous energy-momentum tensor can be obtained for the two-fluid system only at the cost of the violation of all classical energy conditions: null, weak, dominant, and strong. In case (2), we consider a single fluid composed of point particles undergoing instantaneous, energy- and momentum-conserving collisions, and find that the proper pressure remains strictly non-negative, with the equation-of-state parameter confined to the interval $[0,1/3]$. In both scenarios, achieving a sufficiently negative effective pressure to drive cosmic acceleration requires assumptions that compromise the physical viability of the model. Our results highlight some of the key physical challenges involved in modeling dark energy through bulk viscous effects.

astro-ph.CO↗

arXiv2015

Trapping Spin-$0$ particles on $p-$balls in $(D,1)$ dimensions

p-Balls are topological defects in $(D,1)$ dimensions constructed with $\mathcal{M}\ge 1$ scalar fields which depend radially on only $2 \le p\le D-2$ spatial dimensions. Such defects are characterized by an action that breaks translational invariance and are inspired on the physics of a brane with $D-p$ extra dimensions. Here we consider the issue of localization of bosonic states described by a scalar field $Φ$ sufficiently weak to not disturb sensibly the defect configuration. After describing the general formalism, we consider some specify examples with $\mathcal{M}=1,2$ and $3$, looking for some region of parameters where bound and resonant bosonic states can be found. We investigate the way the influence of the defect structure, number of radial dimensions and coupling between the fields are related to the occurrence of bound and resonant states.

hep-th↗

arXiv2014

Kink-antikink collisions for twin models

In this work we consider kink-antikink collisions for some classes of $(1,1)$-dimensional nonlinear models. We are particularly interested to investigate in which aspect the presence of a general kinetic content in the Lagrangian could be revealed in a collision process. We consider a particular class of models known as twin theories, where different models lead to same solutions for the equations of motion and same energy density profile. The theories can be distinguished in the level of linear stability of defect structure. We study a class of k-defect theories depending on a parameter $M$ which is the twin theory of the usual $ϕ^4$ theory with standard dynamics. For $M\to\infty$ both models are characterized by the same potential. In the regime $1/M^2<<1$, we obtain analytically the spectrum of excitations around the kink solution. It is shown that with the increasing on the parameter $1/M^2$: i) the gap between the zero-mode and the first-excited mode increases and ii) the tendency of one-bounce collision between kink-antikink increases. We numerically investigate kink-antikink scattering, looking for the influence of the parameter changing for the thickness and number of two-bounce windows, and confronting the results with our analytical findings.

hep-th↗

arXiv2014

Trapping of Spin-0 fields on tube-like topological defects

We have considered the localization of resonant bosonic states described by a scalar field $Φ$ trapped in tube-like topological defects. The tubes are formed by radial symmetric defects in $(2,1)$ dimensions, constructed with two scalar fields $ϕ$ and $χ$, and embedded in the $(3,1)-$dimensional Minkowski spacetime. The general coupling between the topological defect and the scalar field $Φ$ is given by the potential $ηF(ϕ,χ)Φ^2$. After a convenient decomposition of the field $Φ$, we find that the amplitudes of the radial modes satisfy Schrödinger-like equations whose eigenvalues are the masses of the bosonic resonances. Specifically, we have analyzed two simple couplings: the first one is $F(ϕ,χ)=χ^2$ for a fourth-order potential and, the second one is a sixth-order interaction characterized by $F(ϕ,χ)=(ϕχ)^2$% . In both cases the Schrödinger-like equations are numerically solved with appropriated boundary conditions. Several resonance peaks for both models are obtained and the numerical analysis showed that the fourth-order potential generates more resonances than the sixth-order one.

hep-th↗

arXiv2014

Towards scaling cosmological solutions with full coupled Horndeski Lagrangian: the KGB model

We study a general scalar field Lagrangian coupled with matter and linear in $\Boxϕ$ (also called KGB model). Within this class of models, we find the most general form of the Lagrangian that allows for cosmological scaling solutions, i.e. solutions where the ratio of matter to field density and the equation of state remain constant. Scaling solutions of this kind may help solving the coincidence problem since in this case the presently observed ratio of matter to dark energy does not depend on initial conditions, but rather on the theoretical parameters. Extending previous results we find that it is impossible to join in a single solution a matter era and the scaling attractor. This is an additional step towards finding the most general scaling Lagrangian within the Horndeski class, i.e. general scalar-tensor models with second order equations of motion.

astro-ph.CO↗

arXiv2013

Trapping Dirac fermions in tubes generated by two scalar fields

In this work we consider $(1,1)-$dimensional resonant Dirac fermionic states on tube-like topological defects. The defects are formed by rings in $(2,1)$ dimensions, constructed with two scalar field $ϕ$ and $χ$, and embedded in the $(3,1)-$dimensional Minkowski spacetime. The tube-like defects are attained from a lagrangian density explicitly dependent with the radial distance $r$ relative to the ring axis and the radius and thickness of the its cross-section are related to the energy density. For our purposes we analyze a general Yukawa-like coupling between the topological defect and the fermionic field $ηF(ϕ,χ)\barψψ$. With a convenient decomposition of the fermionic fields in left- and right- chiralities, we establish a coupled set of first order differential equations for the amplitudes of the left- and right- components of the Dirac field. After decoupling and decomposing the amplitudes in polar coordinates, the radial modes satisfy Schrödinger-like equations whose eigenvalues are the masses of the fermionic resonances. With $F(ϕ,χ)=ϕχ$ the Schrödinger-like equations are numerically solved with appropriated boundary conditions. Several resonance peaks for both chiralities are obtained, and the results are confronted with the qualitative analysis of the Schrödinger-like potentials.

hep-th↗

arXiv2012

Highly interactive kink solutions

In this work we present a new class of real scalar field models admitting strongly interactive kink solutions. Instead of the usual exponential asymptotic behavior these topological solutions exhibit a power-law one. We investigate the interaction force between a pair of kink/anti-kink solutions both analytically and numerically, by integrating the time dependent field equations of the model. Furthermore, working within the first-order framework, we analyze the linear stability of these solutions. The stability analysis leads to Schödinger-like equations with potentials which, despite admitting no bound states, lead to strong resonance peaks. We argue that these properties are important for some possible physical applications.

hep-th↗

arXiv2011

Fermions on deformed thick branes

In this work we investigate the issue of fermion localization and resonances in $(4,1)$-deformed branes constructed with one scalar field coupled with gravity. Such models provide us branes with internal structures that turns the gravitational interaction more effective for fermions aside the brane, increasing their lifetime. The coupling between the scalar field and spinors is a necessary condition for fermions to be localized on such branes. After performing a chiral decomposition of the five-dimensional spinor we found resonances with both chiralities. The correspondence between the spectra for left and right chirality is guaranteed and Dirac fermions are realized on the brane.

hep-th↗

arXiv2011

Graviton resonances on deformed branes

Plane wave solutions of Schrodinger-like equations obtained from the metric perturbations in 5D braneworld scenarios can present resonant modes. The search for those structures is important because they can provide us massive modes with not suppressed couplings with the membrane. We propose in this paper the study of graviton Kaluza-Klein spectrum in a special kind of membrane that possesses internal structure. The interest in study of these deformed defects is because they have a more rich internal structure that has implications in the matter-energy density along the extra dimensions an this produces a space-time background whose curvature has a splitting, if compared to the usual kink-like models. Such models arise from $(4,1)$-branes constructed with one scalar field coupled with gravity where we find two-kink solutions from deformations of a $ϕ^4$ potential. The main objective of this work is to observe the effects of deformation process in the resonant modes as well as in the coupling between the graviton massive modes and the brane.

hep-th↗

arXiv2011

Twinlike Models in Scalar Field Theories

This work deals with the presence of defect structures in models described by real scalar field in a diversity of scenarios. The defect structures which we consider are static solutions of the equations of motion which depend on a single spatial dimension. We search for different models, which support the same defect solution, with the very same energy density. We work in flat spacetime, where we introduce and investigate a new class of models. We also work in curved spacetime, within the braneworld context, with a single extra dimension of infinite extent, and there we show how the brane is formed from the static field configuration.

hep-th↗

arXiv2010

Lorentz-violating effects on topological defects generated by two real scalar fields

The influence of a Lorentz-violation on soliton solutions generated by a system of two coupled scalar fields is investigated. Lorentz violation is induced by a fixed tensor coefficient that couples the two fields. The Bogomol'nyi method is applied and first-order differential equations are obtained whose solutions minimize energy and are also solutions of the equations of motion. The analysis of the solutions in phase space shows how the stability is modified with the Lorentz violation. It is shown explicitly that the solutions preserve linear stability despite the presence of Lorentz violation. Considering Lorentz violation as a small perturbation, an analytical method is employed to yield analytical solutions.

hep-th↗

arXiv2009

Resonances on deformed thick branes

In this work we investigate the issue of gravity and fermion localization and resonances in $(4,1)$-branes constructed with one scalar field coupled with gravity in deformed models. Such models give solutions for the scalar field that is the usual kink solution in the extra dimension for a parameter $p=1$ and deformations with a two-kink profile for odd $p>1$. Gravity is localized and resonant modes are found for small values of $p$. The coupling between the scalar field and spinors is a necessary condition for fermions to be localized on such branes. After performing a chiral decomposition of the five-dimensional spinor we found resonances with both chiralities for all odd $p$'s. The correspondence between the spectra for left and right chirality is guaranteed and Dirac fermions are realized on the brane. The increasing of $p$ characterizes the formation of branes with internal structure that turns the gravitational interaction more effective for fermions aside the brane, increasing their lifetime. The influence of the internal structure of the branes and the presence of resonances for gravity and fermionic modes is addressed.

hep-th↗

arXiv2009

Fermion localization and resonances on two-field thick branes

We consider $(4,1)$-dimensional branes constructed with two scalar fields $ϕ$ and $χ$ coupled to a Dirac spinor field by means of a general Yukawa coupling. The equation of motion for the coefficients of the chiral decomposition of the spinor in curved spacetime leads to a Schödinger-like equation whose solutions allow to obtain the masses of the fermionic modes. The simplest Yukawa coupling $\barΨϕχΨ$ is considered for the Bloch brane model and fermion localization is studied. We found resonances for both chiralities and related their appearance to branes with internal structure.

hep-th↗

arXiv2009

Stationary solutions for the parity-even sector of the CPT-even and Lorentz-covariance-violating term of the standard model extension

In this work, we focus on some properties of the parity-even sector of the CPT-even electrodynamics of the standard model extension. We analyze how the six non-birefringent terms belonging to this sector modify the static and stationary classical solutions of the usual Maxwell theory. We observe that the parity-even terms do not couple the electric and magnetic sectors (at least in the stationary regime). The Green's method is used to obtain solutions for the field strengths E and B at first order in the Lorentz- covariance-violating parameters. Explicit solutions are attained for point-like and spatially extended sources, for which a dipolar expansion is achieved. Finally, it is presented an Earth-based experiment that can lead (in principle) to an upper bound on the anisotropic coefficients as stringent as $(\widetildeκ_{e-}) ^{ij}<2.9\times10^{-20}.$

hep-th↗

arXiv2009

Extended solutions via the trial-orbit method for two-field models

In this work we investigate the presence of defect structures in models described by two real scalar fields. The coupling between the two fields is inspired on the equations for a multimode laser, and the minimum energy trivial configurations are shown to be structurely dependent on the parameters of the models. The trial orbit method is then used and several non-trivial analytical solutions corresponding to topological solitons are obtained.

physics.optics↗

arXiv2008

Braneworld Models of Scalar Fields with Generalized Dynamics

This work deals with braneworld models driven by real scalar fields with nonstandard dynamics. We develop the first-order formalism for models with standard gravity but with the scalar fields having generalized dynamics. We illustrate the results with examples of current interest, and we find analytical and numerical solutions for warp factors and scalar fields. The results indicate that the generalized braneworld scenario is classically stable, and capable of localizing gravity

hep-th↗

arXiv2008

Lorentz violating effects on a quantized two-level system

In this work, we consider the effects of the Lorentz-violating (LV) term $ v_μ\barψγ^μψ$ belonging to the fermion sector of the extended standard model on the dynamics of a quantum two-level system. We examine how its non-relativistic counterpart, ${(\mathbf{p}-e \mathbf{A})\cdot }\mathbf{v}/{m_{e},}$ affects the Rabi oscillations of a two-level atom coupled with a quantum cavity electromagnetic field. Taking an initial coherent field state in a resonant cavity, it was found that the LV background increases the Rabi frequency and the time interval between collapses and revivals of the population inversion function. It was found that initial field states with low mean number of photons are better probes in order to establish more stringent upper bounds on the background magnitude. In particular, for an initial vacuum state in the cavity the upper limit $\text{v}_{x}<10^{-10}eV$ was attained.

hep-th↗

arXiv2007

Gravity localization on thick branes: a numerical approach

We introduce a numerical procedure to investigate the spectrum of massive modes and its contribution for gravity localization on thick branes. After considering a model with an analytically known Schroedinger potential, we present the method and discuss its applicability. With this procedure we can study several models even when the Schroedinger potential is not known analytically. We discuss both the occurrence of localization of gravity and the correction to the Newtonian potential given by the massive modes.

hep-th↗