arXiv subjects

S. H. Pereira

Publications and source records attributed to S. H. Pereira.

At least 55 records · Page 3Linked to original sources

arXiv2017

New cosmological solutions in massive gravity theory

In this paper we present some new cosmological solutions in massive gravity theory. Some homogeneous and isotropic solutions correctly describe accelerated evolutions for the universe. The study was realized considering a specific form to the fiducial metric and found different functions and constant parameters of the theory that guarantees the conservation of the energy momentum tensor. Several accelerating cosmologies were found, all of them reproducing a cosmological constant term proportional to the graviton mass, with a de Sitter type solution for the scale factor. We have also verified that when the fiducial metric is close to the physical metric the solutions are absent, except for some specific open cases.

gr-qc↗

arXiv2017

Casimir effect for Elko fields

The Casimir effect for {mass dimension one fermion fields (sometimes called Elko)} in $3+1$ dimension is obtained using Dirichlet boundary conditions. It is shown the existence of a repulsive force four times greater than the case of the scalar field. The precise reason for such differences are highlighted and interpreted, as well as the right parallel of the Casimir effect due to scalar and fermionic fields.

hep-th↗

arXiv2017

Searching for deviations from the General Relativity Theory with gas mass fraction of galaxy clusters and complementary probes

Nowadays, thanks to the improved precision of cosmological data, it has been possible to search for deviation from the general relativity theory with tests on large cosmic scales. Particularly, there is a class of modified gravity theories that breaks the Einstein equivalence principle (EEP) in the electromagnetic sector, generating variations of the fine structure constant, violations of the cosmic distance duality relation and the evolution law of the cosmic microwave background (CMB) radiation. In recent papers, this class of theories has been tested with angular diameter distances from galaxy clusters, type Ia supernovae and CMB temperature. In this work we propose a new test by considering the most recent X-ray surface brightness observations of galaxy clusters jointly with type Ia supernovae and CMB temperature. {The crucial point here is that we take into account the dependence of the X-ray gas mass fraction of galaxy clusters on possible variations of the fine structure constant and violations of the cosmic distance duality relation.} Our basic result is that this new approach is competitive with the previous one and it also does not show significant deviations from the general relativity.

astro-ph.CO↗

arXiv2016

$Λ(t)$ cosmology induced by a slowly varying Elko field

In this work the exact Friedmann-Robertson-Walker equations for an Elko spinor field coupled to gravity in an Einstein-Cartan framework are presented. The torsion functions coupling the Elko field spin-connection to gravity can be exactly solved and the FRW equations for the system assume a relatively simple form. In the limit of a slowly varying Elko spinor field there is a relevant contribution to the field equations acting exactly as a time varying cosmological model $Λ(t)=Λ_*+3βH^2$, where $Λ_*$ and $β$ are constants. Observational data using distance luminosity from magnitudes of supernovae constraint the parameters $Ω_m$ and $β$, which leads to a lower limit to the Elko mass. Such model mimics, then, the effects of a dark energy fluid, here sourced by the Elko spinor field. The density perturbations in the linear regime were also studied in the pseudo-Newtonian formalism.

gr-qc↗

arXiv2016

Creation of Elko particles in asymptotically expanding universe

In the present article we study the process of particle creation for Elko spinor fields as a consequence of expansion of the universe. We study the effect driven by a expanding background that is asymptotically minkowskian in the past and future. The differential equation that governs the time mode function is obtained for the conformal coupling case and, although its solution is non-analytic, within an approximation that preserves the characteristics of the terms that break the analyticity, analytic solutions are obtained. Thus, by means of the Bogolyubov transformations technique, the number density of particles created is obtained, which can be compared to exact solutions already present in literature for scalar and Dirac particles. The spectrum of created particles is obtained and it was found that it is a generalization of the scalar field case, which converges to the scalar field one when the specific terms concerning the Elko field are dropped out. We also found that lighter Elko particles are created in larger quantities than Dirac fermionic particles. By considering the Elko particles as candidate to dark matter in the universe, such result shows that there are more light dark matter (Elko) particles created by gravitational effects in the universe than baryonic (fermionic) matter, in agreement to standard model.

hep-th↗

arXiv2016

Can galaxy clusters, type Ia supernovae and cosmic microwave background rule out a class of modified gravity theories?

In this paper we study cosmological signatures of modified gravity theories that can be written as a coupling between a extra scalar field and the electromagnetic part of the usual Lagrangian for the matter fields. In these frameworks all the electromagnetic sector of the theory is affected and variations of fundamental constants, of the cosmic distance duality relation and of the evolution law of the cosmic microwave background radiation (CMB) are expected and are related each other. In order to search these variations we perform jointly analyses with angular diameter distances of galaxy clusters, luminosity distances of type Ia supernovae and $T_{CMB}(z)$ measurements. We obtain tight constraints with no indication of violation of the standard framework.

astro-ph.CO↗

arXiv2016

Can Dark Matter be a Scalar Field?

In this paper we study a real scalar field as a possible candidate to explain the dark matter in the universe. In the context of a free scalar field with quadratic potential, we have used Union 2.1 SN Ia observational data jointly with a Planck prior over the dark matter density parameter to set a lower limit on the dark matter mass as $m\geq0.12H_0^{-1}$ eV ($c=\hbar=1$). For the recent value of the Hubble constant indicated by the Hubble Space Telescope, namely $H_0=73\pm1.8$ km s$^{-1}$Mpc$^{-1}$, this leads to $m\geq1.56\times10^{-33}$ eV at 99.7\% c.l. Such value is much smaller than $m\sim 10^{-22}$ eV previously estimated for some models. Nevertheless, it is still in agreement with them once we have not found evidences for a upper limit on the scalar field dark matter mass from SN Ia analysis. In practice, it confirms free real scalar field as a viable candidate for dark matter in agreement with previous studies in the context of density perturbations, which include scalar field self interaction.

gr-qc↗

arXiv2016

A framework to a mass dimension one fermionic sigma model

In this paper a mass dimension one fermionic sigma model, realized by the eigenspinors of the charge conjugation operator with dual helicity (Elko spinors), is developed. Such spinors are chosen as a specific realization of mass dimension one spinors, wherein the non-commutative fermionic feature is ruled by torsion. Moreover, we analyse Elko spinors as a source of matter in a background in expansion. Moreover, we analyse Elko spinors as a source of matter in a background in expansion and we have found that such kind of mass dimension one fermions can serve not only as dark matter but they also induce an effective cosmological constant.

hep-th↗

arXiv2015

Cosmological bounds on open FLRW solutions of massive gravity

In this work we have analysed some cosmological bounds concerning an open FLRW solution of massive gravity. The constraints with recent observational $H(z)$ data were found and the best fit values for the cosmological parameters are in agreement with the $Λ$CDM model, and also point to a nearly open spatial curvature, as expected from the model. The graviton mass dependence with the constant parameters $α_3$ and $α_4$, related to the additional lagrangians terms of the model, are also analysed, and we have obtained a strong dependence with such parameters, although the condition $m_g\simeq H_0^{-1}$ seems dominant for a long range of the parameters $α_3$ and $α_4$.

gr-qc↗

arXiv2015

A new approach on the stability analysis in ELKO cosmology

In this work it has been developed a new approach to study the stability of a system composed by an ELKO field interacting with dark matter, which could give some contribution in order to alleviate the cosmic coincidence problem. It is assumed that the potential which characterizes the ELKO field is not specified, but it is related to a constant parameter $δ$. The strength of the interaction between matter and ELKO field is characterized by a constant parameter $β$ and it is also assumed that both ELKO field as matter energy density are related to their pressures by equations of state parameters $ω_ϕ$ and $ω_m$, respectively. The system of equations is analysed by a dynamical system approach. It has been found the conditions of stability between the parameters $δ$ and $β$ in order to have stable fixed points for the system for different values of the equation of state parameters $ω_ϕ$ and $ω_m$, and the results are presented in form of tables. The possibility of decay of ELKO field into dark matter or vice versa can be read directly from the tables, since the parameters $δ$ and $β$ satisfy some inequalities. It allows us to constrain the potential assuming that we have a stable system for different interactions terms between the ELKO field and dark matter. The cosmic coincidence problem can be alleviated for some specific relations between the parameters of the model.

gr-qc↗

arXiv2014

Some remarks on the attractor behaviour in ELKO cosmology

Recent results on the dynamical stability of a system involving the interaction of the ELKO spinor field with standard matter in the universe have been reanalysed, and the conclusion is that such system does not exhibit isolated stable points that could alleviate the cosmic coincidence problem. When a constant parameter $δ$ related to the potential of the ELKO field is introduced in the system however, stable fixed points are found for some specific types of interaction between the ELKO field and matter. Although the parameter $δ$ is related to an unknown potential, in order to satisfy the stability conditions and also that the fixed points are real, the range of the constant parameter $δ$ can be constrained for the present time and the coincidence problem can be alleviated for some specific interactions. Such restriction on the ELKO potential opens possibility to apply the ELKO field as a candidate to dark energy in the universe, and so explain the present phase of acceleration of the universe through the decay of the ELKO field into matter.

gr-qc↗

arXiv2014

CCDM model from quantum particle creation: constraints on dark matter mass

In this work the results from the quantum process of matter creation have been used in order to constrain the mass of the dark matter particles in an accelerated Cold Dark Matter model (Creation Cold Dark Matter, CCDM). In order to take into account a back reaction effect due to the particle creation phenomenon, it has been assumed a small deviation $\varepsilon$ for the scale factor in the matter dominated era of the form $t^{\frac{2}{3}+\varepsilon}$. Based on recent $H(z)$ data, the best fit values for the mass of dark matter created particles and the $\varepsilon$ parameter have been found as $m=1.6\times10^3$ GeV, restricted to a 68.3\% c.l. interval of ($1.5<m<6.3\times10^7$) GeV and $\varepsilon = -0.250^{+0.15}_{-0.096}$ at 68.3\% c.l. For these best fit values the model correctly recovers a transition from decelerated to accelerated expansion and admits a positive creation rate near the present era. Contrary to recent works in CCDM models where the creation rate was phenomenologically derived, here we have used a quantum mechanical result for the creation rate of real massive scalar particles, given a self consistent justification for the physical process. This method also indicates a possible solution to the so called "dark degeneracy", where one can not distinguish if it is the quantum vacuum contribution or quantum particle creation which accelerates the Universe expansion.

astro-ph.CO↗

arXiv2014

Exact solutions to Elko spinors in spatially flat Friedmann-Robertson-Walker spacetimes

In this paper we present exact solutions to the so-called Elko spinors for three models of expanding universe, namely the de Sitter, linear and the radiation type evolution. The study was restrict to flat, homogeneous and isotropic Friedmann-Robertson-Walker backgrounds. Starting with an Elko spinor we present the solutions for these cases and compared to the case of Dirac spinors. Besides, an attempt to use Elko spinors as a dark energy candidate in the cosmological context is investigated.

hep-th↗

arXiv2014

Particle creation in a f(R) theory with cosmological constraints

In this paper we study the creation of super-massive real scalar particles in the framework of a $f(R)=R-β/R^n$ modified gravity theory, with parameters constrained by observational data. The analysis is restrict to a homogeneous and isotropic flat and radiation dominated universe. We compare the results to the standard Einstein gravity with cosmological constant ($ΛCDM$ model), and we show that the total number density of created particles in the $f(R)$ model is very close to the standard case. Another interesting result is that the spectrum of created particles is $β$ independent at early times.

gr-qc↗

arXiv2013

Massless particle creation in a f(R) expanding universe

In this paper we present an exact solution to the spectrum of massless particle creation for a power law expansion of the scale factor of the form $t^{2n/3}$ for real scalar particles in a flat and matter dominated universe. Such an evolution follows from a modified theory of gravity of the type $f(R)=βR^n$, and it is showed that the spectrum of created particles is $β$ independent. We find that greater the value of $n$ smaller is the number of created particles. We study in detail the spectrum of the total particle number created for $n=1\pm \varepsilon$, with $\varepsilon = 0.1$, a very small deviation from the standard general relativity case ($n=1$). We find that such a very small deviation causes a great difference in the total particle number, of about one order of magnitude as compared to the standard general relativity. Our calculations are based on the method of instantaneous Hamiltonian diagonalization, where the vacuum states are defined as those which minimizes the energy at a particular instant of time. The spectrum of the total number and total energy of created particles can be determined exactly for any value of the physical time in a matter dominated universe. We also find that the main contribution to the total number of particles and total energy comes from small wavenumbers $k$ and for particles with large values of $k$ the contribution is very small.

gr-qc↗

arXiv2012

On the dynamics of the universe in $D$ spatial dimensions

In this paper we present the equations of the evolution of the universe in $D$ spatial dimensions, as a generalization of the work of Lima \citep{lima}. We discuss the Friedmann-Robertson-Walker cosmological equations in $D$ spatial dimensions for a simple fluid with equation of state $p=ω_{D}ρ$. It is possible to reduce the multidimensional equations to the equation of a point particle system subject to a linear force. This force can be expressed as an oscillator equation, anti-oscillator or a free particle equation, depending on the $k$ parameter of the spatial curvature. An interesting result is the independence on the dimension $D$ in a de Sitter evolution. We also stress the generality of this procedure with a cosmological $Λ$ term. A more interesting result is that the reduction of the dimensionality leads naturally to an accelerated expansion of the scale factor in the plane case.

gr-qc↗

arXiv2011

On the Sackur-Tetrode equation in an expanding universe

In this work we investigate the thermodynamic properties satisfied by an expanding universe filled with a monoatomic ideal gas. We show that the equations for the energy density, entropy density and chemical potential remain the same of an ideal gas confined to a constant volume V . In particular the Sackur-Tetrode equation for the entropy of the ideal gas is also valid in the case of an expanding universe, provided that the constant value that represents the current entropy of the universe is appropriately chosen.

physics.gen-ph↗

arXiv2009

Quantized fields and gravitational particle creation in f(R) expanding universes

The problem of cosmological particle creation for a spatially flat, homogeneous and isotropic Universes is discussed in the context of f(R) theories of gravity. Different from cosmological models based on general relativity theory, it is found that a conformal invariant metric does not forbid the creation of massless particles during the early stages (radiation era) of the Universe.

astro-ph.CO↗