arXiv subjects

J. A. S. Lima

Publications and source records attributed to J. A. S. Lima.

At least 109 records · Page 6Linked to original sources

arXiv2008

Transition Redshift: New Kinematic Constraints from Supernovae

The transition redshift (deceleration/acceleration) is discussed by expanding the deceleration parameter to first order around its present value. A detailed study is carried out by considering two different parameterizations: $q=q_0 + q_1z$ and $q=q_0 + q_1 z(1+z)^{-1}$, and the associated free parameters ($q_o, q_1$) are constrained by 3 different supernova samples. The previous analysis by Riess {\it{et al.}} [ApJ 607, 665, 2004] using the first expansion is slightly improved and confirmed in light of their recent data ({\emph{Gold}}07 sample). However, by fitting the model with the Supernova Legacy Survey (SNLS) type Ia sample we find that the best fit to the redshift transition is $z_t = 0.61$ instead of $z_t = 0.46$ as derived by the High-z Supernovae Search (HZSNS) team. This result based in the SNLS sample is also in good agreement with the Davis {\it{et al.}} sample, $z_t=0.60^{+0.28}_{-0.11}$ ($1σ$). Such results are in line with some independent analyzes and accommodates more easily the concordance flat model ($Λ$CDM). For both parameterizations, the three SNe type Ia samples considered favor recent acceleration and past deceleration with a high degree of statistical confidence level. All the kinematic results presented here depend neither on the validity of general relativity nor the matter-energy contents of the Universe.

astro-ph↗

arXiv2008

Constraining the Nonextensive Mass Function of Halos from BAO, CMB and X-ray data

Clusters of galaxies are the most impressive gravitationally-bound systems in the Universe and its abundance (the cluster mass function) is one important statistics to probe the matter density parameter ($Ω_m$) and the amplitude of density fluctuations ($σ_8$). The cluster mass function is usually described in terms of the Press-Schecther (PS) formalism where the primordial density fluctuations are assumed to be a Gaussian random field. In previous works we have proposed a non-Gaussian analytical extension of the PS approach with basis on the $q$-power law distribution (PL) of the non-extensive kinetic theory. In this paper, by applying the PL distribution to fit the observational mass function data from X-ray highest flux-limited sample (HIFLUGCS) we find a strong degeneracy among the cosmic parameters, $σ_8$, $Ω_m$, and the $q$ parameter from the PL distribution. A joint analysis involving recent observations from baryon acoustic oscillation (BAO) peak and Cosmic Microwave Background (CMB) shift parameter is carried out in order to break these degeneracy and better constrain the physically relevant parameters. The present results suggest that the next generation of cluster surveys will be able to probe the quantities of cosmological interest ($σ_8, Ω_m$) and the underlying cluster physics quantified by the $q$-parameter.

astro-ph↗

arXiv2008

On the Quintessence Scalar Field Potential

In this work we propose a new analytical method for determining the scalar field potential $V(ϕ)$ in FRW type cosmologies containing a mixture of perfect fluid plus a quintessence scalar field. By assuming that the equation of state parameters of the perfect fluid, $γ-1 \equiv p_γ/ρ_γ$ and the quintessence, $ω\equiv p/ρ$ are constants, it is shown that the potential for the flat case is $V(ϕ) = Aρ_{ϕ_{0}}\sinh^{B}(λϕ)$, where $A$, $B$ and $λ$ are functions of $γ$ and $ω$. This general result is a pure consequence of the Einstein field equations and the constancy of the parameters. Applying the same method for closed and open universes, the corresponding scalar field potentials are also explicitly obtained for a large set of values of the free parameters $γ$ and $ω$. A formula yielding the transition redshift from a decelerating to an accelerating regime is also determined and compared to the $Λ$CDM case.

gr-qc↗

arXiv2008

Clustering, Angular Size and Dark Energy

The influence of dark matter inhomogeneities on the angular size-redshift test is investigated for a large class of flat cosmological models driven by dark energy plus a cold dark matter component (XCDM model). The results are presented in two steps. First, the mass inhomogeneities are modeled by a generalized Zeldovich-Kantowski-Dyer-Roeder (ZKDR) distance which is characterized by a smoothness parameter $α(z)$ and a power index $γ$, and, second, we provide a statistical analysis to angular size data for a large sample of milliarcsecond compact radio sources. As a general result, we have found that the $α$ parameter is totally unconstrained by this sample of angular diameter data.

astro-ph↗

arXiv2008

Constraining the dark energy and smoothness-parameter with supernovae

The presence of inhomogeneities modifies the cosmic distances through the gravitational lensing effect, and, indirectly, must affect the main cosmological tests. Assuming that the dark energy is a smooth component, the simplest way to account for the influence of clustering is to suppose that the average evolution of the expanding Universe is governed by the total matter-energy density whereas the focusing of light is only affected by a fraction of the total matter density quantified by the $α$ Dyer-Roeder parameter. By using two different samples of SNe type Ia data, the $Ω_m$ and $α$ parameters are constrained by applying the Zeldovich-Kantowski-Dyer-Roeder (ZKDR) luminosity distance redshift relation for a flat ($Λ$CDM) model. A $χ^{2}$-analysis using the 115 SNe Ia data of Astier {\it et al.} sample (2006) constrains the density parameter to be $Ω_m=0.26_{-0.07}^{+0.17}$($2σ$) while the $α$ parameter is weakly limited (all the values $\in [0,1]$ are allowed even at 1$σ$). However, a similar analysis based the 182 SNe Ia data of Riess {\it et al.} (2007) constrains the pair of parameters to be $Ω_m= 0.33^{+0.09}_{-0.07}$ and $α\geq 0.42$ ($2σ$). Basically, this occurs because the Riess {\it et al.} sample extends to appreciably higher redshifts. As a general result, even considering the existence of inhomogeneities as described by the smoothness $α$ parameter, the Einstein-de Sitter model is ruled out by the two samples with a high degree of statistical confidence ($11.5σ$ and $9.9σ$, respectively). The inhomogeneous Hubble-Sandage diagram discussed here highlight the necessity of the dark energy, and a transition deceleration/accelerating phase at $z\sim 0.5$ is also required.

astro-ph↗

arXiv2008

Chemical Potential and the Nature of the Dark Energy: The case of phantom

The influence of a possible non zero chemical potential $μ$ on the nature of dark energy is investigated by assuming that the dark energy is a relativistic perfect simple fluid obeying the equation of state (EoS), $p=ωρ$ ($ω<0, constant$). The entropy condition, $S \geq 0$, implies that the possible values of $ω$ are heavily dependent on the magnitude, as well as on the sign of the chemical potential. For $μ>0$, the $ω$-parameter must be greater than -1 (vacuum is forbidden) while for $μ< 0$ not only the vacuum but even a phantomlike behavior ($ω<-1$) is allowed. In any case, the ratio between the chemical potential and temperature remains constant, that is, $μ/T=μ_0/T_0$. Assuming that the dark energy constituents have either a bosonic or fermionic nature, the general form of the spectrum is also proposed. For bosons $μ$ is always negative and the extended Wien's law allows only a dark component with $ω< -1/2$ which includes vacuum and the phantomlike cases. The same happens in the fermionic branch for $μ<0$. However, fermionic particles with $μ>0$ are permmited only if $-1 < ω< -1/2$. The thermodynamics and statistical arguments constrain the EoS parameter to be $ω< -1/2$, a result surprisingly close to the maximal value required to accelerate a FRW type universe dominated by matter and dark energy ($ω\lesssim -10/21$).

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↗

arXiv2007

ZKDR Distance, Angular Size and Phantom Cosmology

The influence of mass inhomogeneities on the angular size-redshift test is investigated for a large class of flat cosmological models driven by dark energy plus a cold dark matter component. The results are presented in two steps. First, the mass inhomogeneities are modeled by a generalized Zeldovich-Kantowski-Dyer-Roeder (ZKDR) distance which is characterized by a smoothness parameter $α(z)$ and a power index $γ$, and, second, we provide a statistical analysis to angular size data for a large sample of milliarcsecond compact radio sources. By marginalizing over the characteristic angular size $l$, fixing $Ω_M = 0.26$ and assuming a Gaussian prior on $ω$, i.e., $ω= -1 \pm 0.3$, the best fit values are $ω= -1.03$ and $α= 0.9$. By assuming a Gaussian prior on the matter density parameter, i.e., $Ω_M = 0.3 \pm 0.1$, the best fit model for a phantom cosmology with $ω= -1.2$ occurs at $Ω_M = 0.29$ and $α= 0.9$ when we marginalize over the characteristic size of the compact radio sources. The results discussed here suggest that the ZKDR distance can give important corrections to the so-called background tests of dark energy.

astro-ph↗

arXiv2007

Age of the Universe, Average Deceleration Parameter and Possible Implications for the End of Cosmology

A new expression to the total age of the Universe is derived in terms of the average deceleration parameter. This kinematic result holds regardless of the curvature of the universe as well as of the underlying gravity theory. It remains valid even in the context of brane-world motivated cosmologies. Since the present age parameter of the Universe is accurately adjusted to $H_0t_0 = 1$, it is shown that the time averaged value of the deceleration parameter is zero. This also means that the cosmic age today is exactly the one predicted by a relativistic flat cosmological model filled by K-matter, a fluid satisfying the equation of state $p = -{1/3}ρ$. By assuming the validity of this relation (in an average long time meaning), it is argued that the decelerating stages of the expansion must exactly be compensated by the accelerated phases, as if the observed Universe coasts forever. If this is true, the present accelerating stage must be followed by a subsequent decelerating phase as predicted by some recent scalar field and brane-world motivated cosmologies.

astro-ph↗

arXiv2007

Bulk viscosity and deflationary universes

We analyze the conditions that make possible the description of entropy generation in the new inflationary model by means of a nearequilibrium process. We show that there are situations in which the bulk viscosity cannot describe particle production during the coherent field oscillations phase.

astro-ph↗

arXiv2007

Cosmology, Thermodynamics and Matter Creation

Several approaches to the matter creation problem in the context of cosmological models are summarily reviewed. A covariant formulation of the general relativistic imperfect simple fluid endowed with a process of matter creation is presented. By considering the standard big bang model, it is shown how the recent results of Prigogine et alii \cite{1} can be recovered and, at the same time their limits of validity are explicited.

astro-ph↗

arXiv2007

CMB and LSS constraints on a single-field model of inflation

A new inflationary scenario whose exponential potential $V(Φ)$ has a quadratic dependence on the field $Φ$ in addition to the standard linear term is confronted with the tree-year observations of the Wilkinson-Microwave Anisotropy Probe and the Sloan Digital Sky Survey data. The number of e-folds ($N$), the ratio of tensor-to-scalar perturbations ($r$), the spectral scalar index of the primordial power spectrum ($n_s$) and its running ($dn_s/d\ln k$) depend on the dimensionless parameter $α$ multiplying the quadratic term in the potential. In the limit $α\to 0$ all the results of the standard exponential potential are fully recovered. For values of $α\neq 0$, we find that the model predictions are in good agreement with the current observations of the Cosmic Microwave Background (CMB) anisotropies and Large-Scale Structure (LSS) in the Universe.

astro-ph↗

arXiv2007

Rayleigh's collapsing time of a spherical cavity: the $Δ$-factor

New corrections to the equation of motion and total collapsing time of an empty spherical cavity immersed in an infinite incompressible medium are proposed on the assumption of a non-uniform density. The dimensionless number quantifying the corrections with respect to the standard Rayleigh results (coined the $Δ$-factor) is fully independent of other possible contributions like surface tension and viscous terms. The $Δ$-factor effect advocated here can be seen as a direct consequence of a mass-shell non-trivial solution to the continuity equation. The consistency of the corrections with respect to the Bernoulli theorem and some physical consequences in the framework of the Rayleigh-Plesset equation are also discussed.

physics.flu-dyn↗

arXiv2006

Constraining $H_0$ from Sunyaev-Zel'dovich effect, Galaxy Clusters X-ray data, and Baryon Oscillations

Estimates of $H_0$ from Sunyaev-Zel'dovich effect (SZE) and X-ray surface brightness of galaxy clusters depends on the underlying cosmology. In the current $Λ$CDM flat cosmology, a possible technique to broke the degenerescency on the mass density parameter ($Ω_{m}$) is to apply a joint analysis involving the baryon acoustic oscillations (BAO). By adopting this technique to the ($H_0, Ω_m$) parameter space, we obtain new constraints on the Hubble constant $H_0$ from BAO signature as given by the Sloan Digital Sky Survey (SDSS) catalog. Our analysis based on the SZE/X-ray data for a sample of 25 clusters yields $H_0= 74^{+4}_{-3.5}$ km s$^{-1}$ Mpc$^{-1}$ ($1σ$, neglecting systematic uncertainties). This result is 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 three independent phenomena provides an interesting method to constrain the Hubble constant.

astro-ph↗

arXiv2006

Hamilton-Jacobi Approach for Power-Law Potentials

The classical and relativistic Hamilton-Jacobi approach is applied to the one-dimensional homogeneous potential, $V(q)=αq^n$, where $α$ and $n$ are continuously varying parameters. In the non-relativistic case, the exact analytical solution is determined in terms of $α$, $n$ and the total energy $E$. It is also shown that the non-linear equation of motion can be linearized by constructing a hypergeometric differential equation for the inverse problem $t(q)$. A variable transformation reducing the general problem to that one of a particle subjected to a linear force is also established. For any value of $n$, it leads to a simple harmonic oscillator if $E>0$, an "anti-oscillator" if $E<0$, or a free particle if E=0. However, such a reduction is not possible in the relativistic case. For a bounded relativistic motion, the first order correction to the period is determined for any value of $n$. For $n >> 1$, it is found that the correction is just twice that one deduced for the simple harmonic oscillator ($n=2$), and does not depend on the specific value of $n$.

gr-qc↗

arXiv2006

A simplified approach for Chaplygin-type cosmologies

A new class of accelerating cosmological models driven by a one-parameter version of the general Chaplygin-type equation of state is proposed. The simplified version is naturally obtained from causality considerations with basis on the adiabatic sound speed $v_S$ plus the observed accelerating stage of the universe. We show that very stringent constraints on the unique free parameter $α$ describing the simplified Chaplygin model can be obtained from a joint analysis involving the latest SNe type Ia data and the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations (BAO). In our analysis we have considered separately the SNe type Ia gold sample measured by Riess et al. (2004) and the Supernova Legacy Survey (SNLS) from Astier et al. (2006). At 95.4% (c.l.), we find for BAO + \emph{gold} sample, $0.91 \leq α\leq 1.0$ and $Ω_{\rm{M}}= 0.28^{+0.043}_{-0.048}$ while BAO + SNLS analysis provides $0.94 \leq α\leq 1.0$ and $Ω_{\rm M}=0.27^{+0.048}_{-0.045}$.

astro-ph↗

arXiv2006

Scalar-field-dominated cosmology with a transient accelerating phase

A new cosmological scenario driven by a slow rolling homogeneous scalar field whose exponential potential $V(Φ)$ has a quadratic dependence on the field $Φ$ in addition to the standard linear term is discussed. The derived equation of state for the field predicts a transient accelerating phase, in which the Universe was decelerated in the past, began to accelerate at redshift $z \sim 1$, is currently accelerated, but, finally, will return to a decelerating phase in the future. This overall dynamic behavior is profoundly different from the standard $Λ$CDM evolution, and may alliviate some conflicts in reconciling the idea of a dark energy-dominated universe with observables in String/M-theory. The theoretical predictions for the present transient scalar field plus dark matter dominated stage are confronted with cosmological observations in order to test the viability of the scenario.

astro-ph↗

arXiv2006

Constraining nonextensive statistics with plasma oscillation data

We discuss experimental constraints on the free parameter of the nonextensive kinetic theory from measurements of the thermal dispersion relation in a collisionless plasma. For electrostatic plane-wave propagation, we show through a statistical analysis that a good agreement between theory and experiment is possible if the allowed values of the $q$-parameter are restricted by $q=0.77 \pm 0.03$ at 95% confidence level (or equivalently, $2-q = 1.23$, in the largely adopted convention for the entropy index $q$). Such a result rules out (by a large statistical margin) the standard Bohm-Gross dispersion relation which is derived assuming that the stationary Maxwellian distribution ($q=1$) is the unperturbed solution.

cond-mat.stat-mech↗