arXiv subjects

S. H. Pereira

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

At least 37 records · Page 2Linked to original sources

arXiv2021

An unified cosmological model driven by a scalar field nonminimally coupled to gravity

This paper consider an universe dominated by baryonic matter, radiation and a nonminimally coupled massive scalar field under the action of a symmetry breaking potential. Inflation occurs naturally with appropriated slow-roll values. The field evolves up to late times, when it is supposed to enter a slowing varying phase, remaining at rest at the value $ϕ_0$. A mechanism for coupling the scalar field to baryonic and radiation energy densities through the nonminimal coupling $ξ$ is presented, leading naturally to dark matter and dark radiation components. A cosmological constant like term is also present, acting as a dark energy at late times. All the density parameters are in good agreement to observational data of Hubble measurements plus Supernovae Type Ia data. All the desired phases of cosmic evolution appears naturally in this simple model.

astro-ph.CO↗

arXiv2021

On the cosmic distance duality relation and the strong gravitational lens power law density profile

Many new strong gravitational lensing (SGL) systems have been discovered in the last two decades with the advent of powerful new space and ground-based telescopes. The effect of the lens mass model (usually the power-law mass model) on cosmological parameters constraints has been performed recently in literature. In this paper, by using SGL systems and Supernovae type Ia observations, we explore if the power-law mass density profile ($ρ\propto r^{-γ}$) is consistent with the cosmic distance duality relation (CDDR), $D_L(1+z)^{-2}/D_A=η(z)=1$, by considering different lens mass intervals. { It has been obtained that the verification of the CDDR validity is significantly dependent on lens mass interval considered: the sub-sample with $σ_{ap} \geq 300$ km/s (where $σ_{ap}$ is the lens apparent stellar velocity dispersion) is in full agreement with the CDDR validity, the sub-sample with intermediate $σ_{ap}$ values ($200 \leq σ_{ap} < 300)$ km/s is marginally consistent with $η=1$ and, finally, the sub-sample with low $σ_{ap}$ values ($σ_{ap} < 200$ km/s) ruled out the CDDR validity with high statistical confidence. Therefore, if one takes the CDDR as guarantee, our results suggest that using a single density profile is not suitable to describe lens with low $σ_{ap}$ values and it is only an approximate description to lenses with intermediate mass interval. }

astro-ph.CO↗

arXiv2020

Cosmological inflation driven by a scalar torsion function

A viable model for inflation driven by a torsion function in a Friedmann background is presented. The scalar spectral index in the interval $0.92\lesssim n_{s}\lesssim 0.97$ is obtained in order to satisfy the initial conditions for inflation. The post inflationary phase is also studied, and the analytical solutions obtained for scale factor and energy density generalizes that ones for a matter dominated universe, indicating just a small deviation from the standard model evolution. The same kind of torsion function used also describes satisfactorily the recent acceleration of the universe, which could indicate a possible unification of different phases, apart form specific constants.

gr-qc↗

arXiv2020

A low Hubble Constant from galaxy distribution observations

An accurate determination of the Hubble constant remains a puzzle in observational cosmology. The possibility of a new physics has emerged with a significant tension between the current expansion rate of our Universe measured from the cosmic microwave background by the Planck satellite and from local methods. In this paper, new tight estimates on this parameter are obtained by considering two data sets from galaxy distribution observations: galaxy cluster gas mass fractions and baryon acoustic oscillation measurements. Priors from the Big Bang nucleosynthesis (BBN) were also considered. By considering the flat $Λ$CDM and XCDM models, and the non-flat $Λ$CDM model, our main results are: $H_0=65.9^{+1.5}_{-1.5}$ km s$^{-1}$ Mpc$^{-1}$, $H_0=65.9^{+4.4}_{-4.0}$ km s$^{-1}$ Mpc$^{-1}$ and $H_0=64.3^{+ 4.5}_{- 4.4}$ km s$^{-1}$ Mpc$^{-1}$ in $2σ$ c.l., respectively. These estimates are in full agreement with the Planck satellite results. Our analyses in these cosmological scenarios also support a negative value for the deceleration parameter at least in 3$σ$ c.l..

astro-ph.CO↗

arXiv2020

Cosmological Constraints on Scalar Field Dark Matter

This paper aims to put constraints on the parameters of the Scalar Field Dark Matter (SFDM) model, when dark matter is described by a free real scalar field filling the whole Universe, plus a cosmological constant term. By using a compilation of 51 $H(z)$ data and 1048 Supernovae data from Panteon, a lower limit for the mass of the scalar field was obtained, $m \geq 5.1\times 10^{-34} $eV and $H_0=69.5^{+2.0}_{-2.1}\text{ km s}^{-1}\text{Mpc}^{-1}$. Also, the present dark matter density parameter was obtained as $Ω_ϕ= 0.230^{+0.033}_{-0.031}$ at $2σ$ confidence level. The results are in good agreement to standard model of cosmology, showing that SFDM model is viable in describing the dark matter content of the universe.

astro-ph.CO↗

arXiv2020

Gravitational entropy of wormholes with exotic matter and in galactic halos

In this work we study and compare the features of gravitational entropy near the throat of transversable wormholes formed by exotic matter and wormholes in galactic halos. We have verified that gravitational entropy and entropy density of these wormholes in regions near their throats are indistinguishable for objects of same throat, despite the fact they are described by different metrics and by distinct energy-momentum tensors. We have found that the gravitational entropy density diverges near the throat for both cases, probably due to a non-trivial topology at this point, however allowing the interesting interpretation that a maximum flux of information can be carried through the throat of these wormholes. In addition, we have found that both are endowed with an entropic behaviour similar to Hawking-Bekenstein's entropy of non-rotating and null charge black holes.

gr-qc↗

arXiv2019

Acceleration in Friedmann cosmology with torsion

A Friedmann like cosmological model in Einstein-Cartan framework is studied when the torsion function is assumed to be proportional to a single $ϕ(t)$ function coming just from the spin vector contribution of ordinary matter. By analysing four different types of torsion function written in terms of one, two and three free parameters, we found that a model with $ϕ(t)=- αH(t) \big({ρ_{m}(t)}/{ρ_{0c}}\big)^n$ is totally compatible with recent cosmological data, where $α$ and $n$ are free parameters to be constrained from observations, $ρ_m$ is the matter energy density and $ρ_{0c}$ the critical density. The recent accelerated phase of expansion of the universe is correctly reproduced by the contribution coming from torsion function, with a deceleration parameter indicating a transition redshift of about $0.65$.

gr-qc↗

arXiv2019

Gaussian Process Estimation of Transition Redshift

This paper aims to put constraints on the transition redshift $z_t$, which determines the onset of cosmic acceleration, in cosmological-model independent frameworks. In order to do that, we use the non-parametric Gaussian Process method with $H(z)$ and SNe Ia data. The deceleration parameter reconstruction from $H(z)$ data yields $z_t=0.59^{+0.12}_{-0.11}$. The reconstruction from SNe Ia data assumes spatial flatness and yields $z_t=0.683^{+0.11}_{-0.082}$. These results were found with a Gaussian kernel and we show that they are consistent with two other kernel choices.

astro-ph.CO↗

arXiv2019

An unified cosmological evolution driven by a mass dimension one fermionic field

An unified cosmological model for an Universe filled with a mass dimension one (MDO) fermionic field plus the standard matter fields is considered. After a primordial quantum fluctuation the field slowly rolls down to the bottom of a symmetry breaking potential, driving the Universe to an inflationary regime that increases the scale factor for about 71 e-folds. After the end of inflation, the field starts to oscillate and can transfer its energy to the standard model particles through a reheating mechanism. Such a process is briefly discussed in terms of the admissible couplings of the MDO field with the electromagnetic and Higgs fields. We show that even if the field loses all its kinetic energy during reheating, it can evolve as dark matter due a gravitational coupling (of spinorial origin) with baryonic matter. Since the field acquires a constant value at the bottom of the potential, a non-null, although tiny, mass term acts as a dark energy component nowadays. Therefore, we conclude that MDO fermionic field is a good candidate to drive the whole evolution of the Universe, in such a way that the inflationary field, dark matter and dark energy are described by different manifestations of a single field.

gr-qc↗

arXiv2019

Galaxy cluster Sunyaev-Zel'dovich effect scaling-relation and type Ia supernova observations as a test for the cosmic distance duality relation

In this paper, we propose a new test to the cosmic distance duality relation (CDDR), $D_L=D_A(1+z)^2$, where $D_L$ and $D_A$ are the luminosity and angular diameter distances, respectively. The data used correspond to 61 Type Ia Supernova luminosity distances and $Y_{SZE}-Y_X$ measurements of 61 galaxy clusters obtained by the {\it Planck} mission and the deep XMM-Newton X-ray data, where $Y_{SZE}$ is the integrated comptonization parameter obtained via Sunyaev-Zel'dovich effect observations and $Y_X$ is the X-ray counterpart. More precisely, we use the $Y_{SZE}D_{A}^{2}/C_{XSZE}Y_X$ scaling-relation and a deformed CDDR, such as $D_L/D_A(1+z)^2=η(z)$, to verify if $η(z)$ is compatible with the unity. Two $η(z)$ functions are used, namely, $η(z)=1+η_0 z$ and $η(z)=1+η_0 z /(1+z)$. { We obtain that the CDDR validity ($η_0=0$) is verified within $\approx 1.5σ$ c.l. for both $η(z)$ functions.}.

astro-ph.CO↗

arXiv2019

Partition function for a mass dimension one fermionic field and the dark matter halo of galaxies

This work study the finite temperature effects of a mass dimension one fermionic field, sometimes called Elko field. The equilibrium partition function was calculated by means of the imaginary time formalism and the result obtained was the same for a Dirac fermion field, even though the Elko field does not satisfy a Dirac like equation. The high and low temperature limits were obtained, and for the last case the degeneracy pressure due to Pauli exclusion principle can be responsible for the dark matter halos around galaxies to be greater than or of the same order of the galaxy radius. Also, for a light particle of about $0.1$eV and a density of just 0.1 particle per cubic centimeter, the value of the total dark matter mass due to Elko particles is of the same order of a typical galaxy. Such a result satisfactorily explains the dark matter as being formed just by Elko fermionic particles and also the existence of galactic halos that go beyond the observable limit.

physics.gen-ph↗

arXiv2018

Model independent constraints on transition redshift

This paper aims to put constraints on the transition redshift $z_t$, which determines the onset of cosmic acceleration, in cosmological-model independent frameworks. In order to perform our analyses, we consider a flat universe and {assume} a parametrization for the comoving distance $D_C(z)$ up to third degree on $z$, a second degree parametrization for the Hubble parameter $H(z)$ and a linear parametrization for the deceleration parameter $q(z)$. For each case, we show that {type Ia supernovae} and $H(z)$ data complement each other on the parameter {space} and tighter constrains for the transition redshift are obtained. By {combining} the type Ia supernovae observations and Hubble parameter measurements it is possible to constrain the values of $z_t$, for each approach, as $0.806\pm 0.094$, $0.870\pm 0.063$ and $0.973\pm 0.058$ at 1$σ$ c.l., respectively. Then, such approaches provide cosmological-model independent estimates for this parameter.

astro-ph.CO↗

arXiv2018

Effective lagrangian for a mass dimension one fermionic field in curved spacetime

In this work we use momentum-space techniques to evaluate the propagator $G(x,x^{\prime})$ for a spin $1/2$ mass dimension one spinor field on a curved Friedmann-Robertson-Walker spacetime. As a consequence, we built the one-loop correction to the effective lagrangian in the coincidence limit. Going further we compute the effective lagrangian in the finite temperature regime. We arrive at interesting cosmological consequences, as time-dependent cosmological `constant', fully explaining the functional form of previous cosmological models.

hep-th↗

arXiv2018

Cosmic transparency and acceleration

In this paper, by considering an absorption probability independent of photon wavelength, we show that current type Ia supernovae (SNe Ia) and gamma ray burst (GRBs) observations plus high-redshift measurements of the cosmic microwave background (CMB) radiation temperature support cosmic acceleration regardless of the transparent-universe assumption. Two flat scenarios are considered in our analyses: the $Λ$CDM model and a kinematic model. We consider $τ(z)=2\ln(1+z)^{\varepsilon}$, where $τ(z)$ denotes the opacity between an observer at $z=0$ and a source at $z$. This choice is equivalent to deforming the cosmic distance duality relation as $D_LD^{-1}_A = (1 + z)^{2+\varepsilon}$ and, if the absorption probability is independent of photon wavelength, the CMB temperature evolution law is $T_{CMB}(z)=T_0(1+z)^{1+2\varepsilon/3 }$. By marginalizing on the $\varepsilon$ parameter, our analyses rule out a decelerating universe at 99.99 \% c.l. for all scenarios considered. Interestingly, by considering only SNe Ia and GRBs observations, we obtain that a decelerated universe indicated by $Ω_Λ \leq 0.33$ and $q_0 > 0$ is ruled out around 1.5$σ$ c.l. and 2$σ$ c.l., respectively, regardless of the transparent-universe assumption.

astro-ph.CO↗

arXiv2017

Evolution of the universe driven by a mass dimension one fermion field

This paper study the evolution of the universe filled with a neutral mass dimension one fermionic field, sometimes called Elko. The numerical analysis of the coupled system of equations furnish a scale factor growth and energy density evolution that correctly reproduces the inflationary phase of the universe. After that, supposing a mechanism of energy transference to ordinary matter, the initial conditions generated after inflation drives the radiation dominated phase and also the subsequent dark matter evolution, since the Elko field is a good dark matter candidate. The energy density of the field at the end of inflation, at the end of radiation phase and for present time are in agreement to the standard model estimates. The analysis was performed with a potential containing a quadratic mass term plus a quartic self-interaction term, which follows naturally from the theory of mass dimension one fermions. It is interesting to notice that inflation occurs when the field makes a kind of transition around the Planck mass scale. The number of e-foldings during inflation was found to be strongly dependent on the initial conditions of the Elko field, as occurs in chaotic inflationary models. An upper mass limit for Elko field has been obtained as $m<10^9$GeV. A possible interpretation of both inflationary phase and recent cosmic acceleration as a consequence of a kind of Pauli exclusion principle is presented at the end.

gr-qc↗

arXiv2017

From inflation to recent cosmic acceleration: The fermionic Elko field driving the evolution of the universe

In this paper we construct the complete evolution of the universe driven by the mass dimension one dark spinor called Elko, starting with inflation, passing by the matter dominated era and finishing with the recent accelerated expansion. The dynamic of the fermionic Elko field with a symmetry breaking type potential can reproduce all phases of the universe in a natural and elegant way. The dynamical equations in general case and slow roll conditions in the limit $H\ll m_{pl}$ are also presented for the Elko system. Numerical analysis for the number of e-foldings during inflation, energy density after inflation and for present time and also the actual size of the universe are in good agreement with the standard model of cosmology. An interpretation of the inflationary phase as a result of Pauli exclusion principle is also possible if the Elko field is treated as an average value of its quantum analogue.

gr-qc↗

arXiv2017

Improved constraints on violations of the Einstein equivalence principle in the electromagnetic sector with complementary cosmic probes

Recent results have shown that a field non-minimally coupled to the electromagnetic Lagrangian can induce a violation of the Einstein equivalence principle. { This kind of coupling is present in a very wide class of gravitation theories.} In a cosmological context, this would break the validity of the cosmic distance duality relation as well as cause a time variation of the fine structure constant. Here, we improve constraints on this scenario by using four different observables: the luminosity distance of type Ia supernovae, the angular diameter distance of galaxy clusters, the gas mass fraction of galaxy clusters and the temperature of the cosmic microwave background at different redshifts. We consider four standard parametrizations adopted in the literature and show that, due to a high complementarity of the data, the errors are shrunk between 20\% and 40\% depending on the parametrization. We also show that our constraints are weakly affected by the geometry considered to describe the galaxy clusters. In short, no violation of the Einstein equivalence principle is detected up to redshifts $\sim$ 3.

gr-qc↗

arXiv2017

Constraints on a possible evolution of mass density power-law index in strong gravitational lensing from cosmological data

In this work, by using strong gravitational lensing (SGL) observations along with Type Ia Supernovae (Union2.1) and gamma ray burst data (GRBs), we propose a new method to study a possible redshift evolution of $γ(z)$, the mass density power-law index of strong gravitational lensing systems. In this analysis, we assume the validity of cosmic distance duality relation and the flat universe. In order to explore the $γ(z)$ behavior, three different parametrizations are considered, namely: (P1) $γ(z_l)=γ_0+γ_1 z_l$, (P2) $γ(z_l)=γ_0+γ_1 z_l/(1+z_l)$ and (P3) $γ(z_l)=γ_0+γ_1 \ln(1+z_l)$, where $z_l$ corresponds to lens redshift. If $γ_0=2$ and $γ_1=0$ the singular isothermal sphere model is recovered. Our method is performed on SGL sub-samples defined by different lens redshifts and velocity dispersions. For the former case, the results are in full agreement with each other, while a 1$σ$ tension between the sub-samples with low ($\leq 250$ km/s) and high ($>250$ km/s) velocity dispersions was obtained on the ($γ_0$-$γ_1$) plane. By considering the complete SGL sample, we obtain $γ_0 \approx 2$ and $ γ_1 \approx 0$ within 1$σ$ c.l. for all $γ(z)$ parametrizations. However, we find the following best fit values of $γ_1$: $-0.085$, $-0.16$ and $-0.12$ for P1, P2 and P3 parametrizations, respectively, suggesting a mild evolution for $γ(z)$. By repeating the analysis with Type Ia Supernovae from JLA compilation, GRBs and SGL systems this mild evolution is reinforced.

gr-qc↗