arXiv subjects

J. F. Jesus

Publications and source records attributed to J. F. Jesus.

48 records · Page 3Linked to original sources

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

Is the transition redshift a new cosmological number?

Observations from Supernovae Type Ia (SNe Ia) provided strong evidence for an expanding accelerating Universe at intermediate redshifts. This means that the Universe underwent a transition from deceleration to acceleration phases at a transition redshift $z_t$ of the order unity whose value in principle depends on the cosmology as well as on the assumed gravitational theory. Since cosmological accelerating models endowed with a transition redshift are extremely degenerated, in principle, it is interesting to know whether the value of $z_t$ itself can be observationally used as a new cosmic discriminator. After a brief discussion of the potential dynamic role played by the transition redshift, it is argued that future observations combining SNe Ia, the line-of-sight (or "radial") baryon acoustic oscillations, the differential age of galaxies, as well as the redshift drift of the spectral lines may tightly constrain $z_t$, thereby helping to narrow the parameter space for the most realistic models describing the accelerating Universe.

astro-ph.CO↗

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↗

arXiv2013

Shear and rotation in Chaplygin cosmology

We study the effect of shear and rotation on results previously obtained dealing with the application of the spherical collapse model (SCM) to generalized Chaplygin gas (gCg) dominated universes. The system is composed of baryons and gCg and the collapse is studied for different values of the parameter $α$ of the gCg. We show that the joint effect of shear and rotation is that of slowing down the collapse with respect to the simple SCM. This result is of utmost importance for the so-called unified dark matter models, since the described slow down in the growth of density perturbation can solve one of the main problems of the quoted models, namely the instability described in previous papers [e.g., H. B. Sandvik {\it et al.}, Phys. Rev. D {\bf 69}, 123524 (2004)] at the linear perturbation level.

astro-ph.CO↗

arXiv2011

Newtonian Perturbations on Models with Matter Creation

Creation of Cold Dark Matter (CCDM) can macroscopically be described by a negative pressure, and, therefore, the mechanism is capable to accelerate the Universe, without the need of an additional dark energy component. In this framework we discuss the evolution of perturbations by considering a Neo-Newtonian approach where, unlike in the standard Newtonian cosmology, the fluid pressure is taken into account even in the homogeneous and isotropic background equations (Lima, Zanchin and Brandenberger, MNRAS {\bf 291}, L1, 1997). The evolution of the density contrast is calculated in the linear approximation and compared to the one predicted by the $Λ$CDM model. The difference between the CCDM and $Λ$CDM predictions at the perturbative level is quantified by using three different statistical methods, namely: a simple $χ^{2}$-analysis in the relevant space parameter, a Bayesian statistical inference, and, finally, a Kolmogorov-Smirnov test. We find that under certain circumstances the CCDM scenario analysed here predicts an overall dynamics (including Hubble flow and matter fluctuation field) which fully recovers that of the traditional cosmic concordance model. Our basic conclusion is that such a reduction of the dark sector provides a viable alternative description to the accelerating $Λ$CDM cosmology.

astro-ph.CO↗

arXiv2010

Note on "Continuous matter creation and the acceleration of the universe: the growth of density fluctuations"

Recently, de Roany & Pacheco (2010) performed a Newtonian analysis on the evolution of perturbations for a class of relativistic cosmological models with Creation of Cold Dark Matter (CCDM) proposed by the present authors. In this note we demonstrate that the basic equations adopted in their work do not recover the specific (unperturbed) CCDM model. Unlike to what happens in the original CCDM cosmology, their basic conclusions refer to a decelerating cosmological model in which there is no transition from a decelerating to an accelerating regime as required by SNe type Ia and complementary observations.

astro-ph.CO↗

arXiv2009

CDM Accelerating Cosmology as an Alternative to LCDM model

A new accelerating cosmology driven only by baryons plus cold dark matter (CDM) is proposed in the framework of general relativity. In this model the present accelerating stage of the Universe is powered by the negative pressure describing the gravitationally-induced particle production of cold dark matter particles. This kind of scenario has only one free parameter and the differential equation governing the evolution of the scale factor is exactly the same of the $Λ$CDM model. For a spatially flat Universe, as predicted by inflation ($Ω_{dm}+Ω_{baryon}=1$), it is found that the effectively observed matter density parameter is $Ω_{meff} = 1- α$, where $α$ is the constant parameter specifying the CDM particle creation rate. The supernovae test based on the Union data (2008) requires $α\sim 0.71$ so that $Ω_{meff} \sim 0.29$ as independently derived from weak gravitational lensing, the large scale structure and other complementary observations.

astro-ph.CO↗

arXiv2009

Can Dark Matter Decay in Dark Energy?

We analyze the interaction between Dark Energy and Dark Matter from a thermodynamical perspective. By assuming they have different temperatures, we study the possibility of occurring a decay from Dark Matter into Dark Energy, characterized by a negative parameter $Q$. We find that, if at least one of the fluids has non vanishing chemical potential, for instance $μ_x<0$ and $μ_{dm}=0$ or $μ_x=0$ and $μ_{dm}>0$, the decay is possible, where $μ_x$ and $μ_{dm}$ are the chemical potentials of Dark Energy and Dark Matter, respectively. Using recent cosmological data, we find that, for a fairly simple interaction, the Dark Matter decay is favored with a probability of $\sim 93%$ over the Dark Energy decay. This result comes from a likelihood analysis where only background evolution has been considered.

astro-ph↗

arXiv2008

New coupled quintessence cosmology

A component of dark energy has been recently proposed to explain the current acceleration of the Universe. Unless some unknown symmetry in Nature prevents or suppresses it, such a field may interact with the pressureless component of dark matter, giving rise to the so-called models of coupled quintessence. In this paper we propose a new cosmological scenario where radiation and baryons are conserved, while the dark energy component is decaying into cold dark matter (CDM). The dilution of CDM particles, attenuated with respect to the usual $a^{-3}$ scaling due to the interacting process, is characterized by a positive parameter $ε$, whereas the dark energy satisfies the equation of state $p_x=ωρ_x$ ($ω< 0$). We carry out a joint statistical analysis involving recent observations from type Ia supernovae, baryon acoustic oscillation peak, and Cosmic Microwave Background shift parameter to check the observational viability of the coupled quintessence scenario here proposed.

astro-ph↗

arXiv2008

Simplified Chaplygin Gas: Deriving $H_0$ From Ages of Old High Redshift Objects and Baryon Acoustic Oscillations

The discovery that the expansion of the Universe is accelerating is the most challenging problem of modern cosmology. In the context of general relativity, there are many dark energy candidates to explain the observed acceleration. In this work we focus our attention on two kinds of simplified Chaplygin gas cosmological accelerating models recently proposed in the literature. In the first scenario, the simplified Chaplygin gas works like a Quintessence model while in the second one, it plays the role of a Quartessence (an unification of the dark sector). Firstly, in order to limit the free parameters of both models, we discuss the age of high redshift objects with special emphasis to the old quasar APM 08279+5255 at $z = 3.91$. The basic finding is that this old high redshift object constrain severely the simplified Chaplygin cosmologies. Secondly, through a joint analysis involving the baryon acoustic oscillations (BAO) and a sample of old high redshift galaxies (OHRGs) we also estimate the value of the Hubble parameter, $H_0$. Our approach suggests that the combination of these two independent phenomena provides an interesting method to constrain the Hubble constant.

astro-ph↗

arXiv2007

Constraints from the Old Quasar APM 08279+5255 on Two Classes of $Λ($t$)$-Cosmologies

The viability of two different classes of $Λ(t)$CDM cosmologies is tested by using the APM 08279+5255, an old quasar at redshift $z = 3.91$. In the first class of models, the cosmological term scales as $Λ(t)\sim R^{-n}$. The particular case $n=0$ describes the standard $Λ$CDM model whereas $n=2$ stands for the Chen and Wu model. For an estimated age of 2 Gyr, it is found that the power index has a lower limit $n> 0.21$, whereas for 3 Gyr the limit is $n > 0.6$. Since $n$ can not be so large as $\sim 0.81$, the $Λ$CDM and Chen and Wu models are also ruled out by this analysis. The second class of models is the one recently proposed by Wang and Meng which describes several $Λ(t)$CDM cosmologies discussed in the literature. By assuming that the true age is 2 Gyr it is found that the $ε$ parameter satisfies the lower bound $ε> 0.11$, while for 3 Gyr, a lower limit of $ε> 0.52$ is obtained. Such limits are slightly modified when the baryonic component is included.

astro-ph↗

arXiv2007

Can Old Galaxies at High Redshifts and Baryon Acoustic Oscillations Constrain H_0?

A new age-redshift test is proposed in order to constrain $H_0$ with basis on the existence of old high redshift galaxies (OHRG). As should be expected, the estimates of $H_0$ based on the OHRG are heavily dependent on the cosmological description. In the flat concordance model ($Λ$CDM), for example, the value of $H_0$ depends on the mass density parameter $Ω_M=1 - Ω_Λ$. Such a degeneracy can be broken trough a joint analysis involving the OHRG and baryon acoustic oscillation (BAO) signature. In the framework of the $ΛCDM$ model our joint analysis yields a value of $H_0=71^{+4}_{-4}\kms$ Mpc$^{-1}$ ($1σ$) with the best fit density parameter $Ω_M=0.27\pm0.03$. Such results are in good agreement with independent studies from the {\it{Hubble Space Telescope}} key project and the recent estimates of WMAP, thereby suggesting that the combination of these two independent phenomena provides an interesting method to constrain the Hubble constant.

astro-ph↗