arXiv subjects

J. A. S. Lima

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

At least 145 records · Page 8Linked to original sources

arXiv2002

Deflationary cosmology: constraints from angular size and ages of globular clusters

Observational constraints to a large class of decaying vacuum cosmologies are derived using the angular size data of compact radio sources and the latest age estimates of globular clusters. For this class of deflationary $Λ(t)$ models, the present value of the vacuum energy density is quantified by a positive $β$ parameter smaller than unity. In the case of milliarcsecond compact radio-sources, we find that the allowed intervals for $β$ and the matter density parameter $Ω_m$ are heavily dependent on the value of the mean projected linear size $l$. For $l \simeq 20h^{-1} - 30h^{-1}$ pc, the best fit occurs for $β\sim 0.58$, $Ω_{\rm{m}} \sim 0.58$, and $β\sim 0.76$, $Ω_{\rm{m}} \sim 0.28$, respectively. This analysis shows that if one minimizes $χ^{2}$ for the free parameters $l$, $Ω_{\rm{m}}$ and $β$, the best fit for these angular size data corresponds to a decaying $Λ(t)$ with $Ω_{\rm{m}} = 0.54$ $β=0.6$ and $l = 22.64h^{-1}$ pc. Constraints from age estimates of globular clusters and old high redshift galaxies are not so restrictive, thereby suggesting that there is no age crisis for this kind of $Λ(t)$ cosmologies.

astro-ph↗

arXiv2002

Dark energy and the angular size - redshift diagram for milliarcsecond radio sources

We investigate observational constraints on the cosmic equation of state from measurements of angular size for a large sample of milliarcsecond compact radio sources. The results are based on a flat Friedmann-Robertson-Walker (FRW) type models driven by non-relativistic matter plus a smooth dark energy component parametrized by its equation of state $p_x = ωρ_x$ ($-1 \leq ω< 0$). The allowed intervals for $ω$ and $Ω_{\rm{m}}$ are heavily dependent on the value of the mean projected linear size $l$. For $l \simeq 20h^{-1} - 30h^{-1}$ pc, we find $Ω_{\rm{m}} \leq 0.62$, $ω\leq -0.2$ and $Ω_{\rm{m}} \leq 0.17$, $ω\leq -0.65$ (68% c.l.), respectively. As a general result, this analysis shows that if one minimizes $χ^{2}$ for the parameters $l$, $Ω_{\rm{m}}$ and $ω$, the conventional flat $Λ$CDM model ($ω= -1$) with $Ω_{\rm{m}} = 0.2$ and $l = 22.6 h^{-1}$pc is the best fit for these angular size data.

astro-ph↗

arXiv2002

Deflationary $Λ(t)$ cosmology: observational expressions

We discuss the classical cosmological tests for a large class of FRW type models driven by a decaying vacuum energy density. Analytic expressions for the lookback time, age of the universe, luminosity distance, angular diameter, and galaxy number counts versus redshift are derived and their meaning discussed in detail. It is found that the standard FRW results are significantly altered, showing that such tests may constrain the physical parameters of these models which are also in agreement to the accelerated expansion suggested by the latest SNe type Ia observations.

astro-ph↗

arXiv2002

Transport coefficients and nonextensive statistics

We discuss the basic transport phenomena in gases and plasmas obeying the $q$-nonextensive velocity distribution (power-law). Analytical expressions for the thermal conductivity ($K_q$) and viscosity ($η_q$) are derived by solving the Boltzmann equation in the relaxation-time approximation. The available experimental results to the ratio {$K_q$}/$η_q$ constrains the $q$-parameter on the interval $0.74 \leq q \leq 1$. In the extensive limiting case, the standard transport coefficients based on the local Gaussian distribution are recovered, and due to a surprising cancellation, the electric conductivity of a neutral plasma is not modified.

cond-mat.stat-mech↗

arXiv2002

A Maxwellian Path to the q-Nonextensive Velocity Distribution Function

Maxwell's first derivation of the equilibrium distribution function for a dilute gas is generalized in the spirit of the nonextensive q-statistics proposed by Tsallis. As an application, the q-Doppler broadening of spectral lines due to the random thermal motion of the radiating atoms is derived.

cond-mat.stat-mech↗

arXiv2002

Temperature Evolution Law of Imperfect Relativistic Fluids

The first-order general relativistic theory of a generic dissipative (heat-conducting, viscous, particle-creating) fluid is rediscussed from a unified covariant frame-independent point of view. By generalizing some previous works in the literature, we derive a formula for the temperature variation rate, which is valid both in Eckart's (particle) and in the Landau-Lifshitz (energy) frames. Particular attention is paid to the case of gravitational particle creation and its possible cross-effect with the bulk viscosity mechanism.

gr-qc↗

arXiv2002

Thermodynamic Approach to Warm Inflation

We study the thermodynamic behavior of a decaying scalar field coupled to a relativistic simple fluid. It is shown that if the decay products are represented by a thermalized bath, its temperature evolution law requires naturally a new phenomenological coupling term. This ``energy loss'' term is the product between the enthalpy density of the thermalized bath and the decay width of the scalar field. We also argue that if the field $ϕ$ decays "adiabatically" some thermodynamic properties of the fluid are preserved. In particular, for a field decaying into photons, the radiation entropy production rate is independent of the specific scalar field potential $V(ϕ)$, and the energy density $ρ$ and average number density of photons n scale as $ρ\sim T^{4}$ and $n \sim T^{3}$. To illustrate these results, a new warm inflationary scenario with no slow roll is proposed.

astro-ph↗

arXiv2002

Cosmological Constant, Conical Defect and Classical Tests of General Relativity

We investigate the perihelion shift of the planetary motion and the bending of starlight in the Schwarzschild field modified by the presence of a $Λ$-term plus a conical defect. This analysis generalizes an earlier result obtained by Islam (Phys. Lett. A 97, 239, 1983) to the case of a pure cosmological constant. By using the experimental data we obtain that the parameter $ε$ characterizing the conical defect is less than $10^{-9}$ and $10^{-7}$, respectively, on the length scales associated with such phenomena. In particular, if the defect is generated by a cosmic string, these values correspond to limits on the linear mass densities of $10^{19}g/cm$ and $10^{21}g/cm$, respectively.

gr-qc↗

arXiv2002

Cosmological implications of old galaxies at high redshifts

Old high-$z$ galaxies are important tools for understanding the structure formation problem and may become the key to determine the ultimate fate of the Universe. In this {\it letter}, the inferred ages of the three oldest galaxies at high redshifts reported in the literature are used to constrain the first epoch of galaxy formation and to reanalyse the high-z time scale crisis. The lower limits on the formation redshift $z_f$ depends on the quantity of cold dark matter in the Universe. In particular, if $Ω_m\geq 0.37$ these galaxies are not formed in FRW cosmologies with no dark energy. This result is in line with the Supernovae type Ia measurements which suggest that the bulk of energy in the Universe is repulsive and appears like an unknown form of dark energy component. In a complementar analysis, unlike recent claims favoring the end of the age problem, it is shown that the Einstein-de Sitter model is excluded at high-z by $\sim 3σ$.

astro-ph↗

arXiv2002

Scalar field description of decaying-$Λ$ cosmologies

The conditions under which cosmologies driven by time varying cosmological terms can be described by a scalar field coupled to a perfect fluid are discussed. An algorithm to reconstruct potentials dynamically and thermodynamically analogue to given phenomenological $Λ$ models is presented. As a worked example, the deflationary cosmology, which evolves from a pure de Sitter vacuum state to a slightly modified FRW cosmology is considered. It is found that this is an example of nonsingular warm inflation with an asymptotic exponential potential. Differences with respect to other scalar field descriptions of decaying vacuum cosmologies are addressed and possible extensions are indicated.

astro-ph↗

arXiv2001

Note on Solving for the Dynamics of the Universe

In a recent article, Faraoni proposed an alternative procedure to solve the Friedman-Lemaitre-Robertson-Walker (FLRW) cosmological equations. The basic result of that paper was obtained long ago through a different approach, which seems to be little known and deserves closer attention due to its pedagogical interest. The broad importance of this method is readily recognized by examining some additional cases not considered by Faraoni. Its instructive potential for introductory courses on cosmology has been positively verified by the author in many lectures during the last decade.

astro-ph↗

arXiv2001

Dark energy and the epoch of galaxy formation

The influence of a dark component on the first epoch of galaxy formation is analysed by using the ages of the three oldest high-redshift galaxies known in the literature. Our results, based on a spatially flat accelerated Universe driven by a "quintessence" component ($p_x = ωρ_x$), show that if the inferred ages of these objects are correct the first formation era is pushed back to extremely high redshifts. For the present best-fit quintessence model ($Ω_{x}= 0.7$, $ω< -0.6$), we find a lower bound of $z_f \geq 7.7$, whereas in the extreme case of $Λ$CDM model ($ω= -1$) the limit is slightly smaller ($z_f \geq 5.8$). The case for open cold dark matter models (OCDM) has also been discussed. For $Ω_m \simeq 0.3$, the formation redshift is restricted by $z_f \geq 18$. As a general result, if $Ω_m \geq 0.37$, these galaxies are not formed in FRW cosmologies with no dark energy since for all these cases $z_f \to \infty$.

astro-ph↗

arXiv2000

Constraining the cosmic equation of state from old galaxies at high redshift

New limits on the cosmic equation of state are derived from age measurements of three recently reported old high redshift galaxies (OHRG). The results are based on a flat FRW type cosmological model driven by nonrelativistic matter plus a smooth component parametrized by its equation of state $p_{x} = ωρ_{x}$ ($ω\geq -1$). The range of $ω$ is strongly dependent on the matter density parameter. For $Ω_{M} \sim 0.3$, as indicated from dynamical measurements, the age estimates of the OHRG restricts the cosmic parameter to $ω\leq - 0.27$. However, if $Ω_{M}$ is the one suggested by some studies of field galaxies, i.e, $Ω_{M} \simeq 0.5$, only a cosmological constant ($ω=-1$) may be compatible with these data.

astro-ph↗

arXiv2000

Angular size in quintessence cosmology

We investigate the influence of an exotic fluid component ("quintessence") on the angular size-redshift relation for distant extragalactic sources. Particular emphasis is given for the redshif $z_{m}$ at which the angular size takes its minimal value. We derive an analytical closed form which determines how $z_m$ depends on the parameter of the equation of state describing the exotic component. The results for a flat model dominated by a "quintessence" are compared in detail with the ones for the standard open model dominated by cold dark matter. Some consequences of systematic evolutionary effects on the values of $z_{m}$ are also briefly discussed. It is argued that the critical redshift, for all practical purposes, may completely be removed if such effects are taken into account.

astro-ph↗

arXiv1999

Extended Warm Inflation

A bidimensional parameter space of unitary area is introduced to study phenomenologically the dynamic and thermodynamic behavior of inflationary models driven by a scalar field coupled to a thermal component. In this enlarged context the isentropic and warm pictures are just extreme cases of an infinite two-parametric family of possible inflationary scenarios. It is also shown that strong couplings are responsible by an alteration in the standard slow roll conditions which relax the smallness constraint on the first and the second derivatives of the potential.

astro-ph↗

arXiv1999

Minimal Angular Size of Distant Sources in Open, $Λ$CDM, and Scalar Field Cosmologies

We propose a simple method for determining the redshift $z_{m}$ at which the angular size of an extragalactic source with fixed proper diameter takes its minimal value. A closed analytical expression, which is quite convenient for numerical evaluation is derived. The method is exemplified with the following FRW type expanding universes: the open matter dominated models ($Ω_Λ= 0$), a critical density model with cosmological constant ($Ω_Λ \neq 0$), and the class of scalar field cosmologies proposed by Ratra and Peebles. The influence of systematic evolutionary effects is briefly discussed.

astro-ph↗

arXiv1999

Closed and open FRW cosmologies with matter creation: Kinematic tests

The limits imposed by the classical cosmological tests on closed and open FRW universes driven by adiabatic matter creation are investigated. Exact expressions for the lookback time, age of the universe, luminosity distance, angular diameter, and galaxy number counts versus redshift are derived and their meaning discussed in detail. An interesting consequence of these cosmological models is the possibility of an accelerated expansion today (as indicated from supernovae observations) with no need to invoke either a cosmological constant or an exotic "quintessence" component.

astro-ph↗

arXiv1999

Spectrum of $γ$-Fluids: A Statistical Derivation

The spectrum of massless bosonic and fermionic fluids satisfying the equation of state $p=(γ-1)ρ$ is derived using elementary statistical methods. As a limiting case, the Lorentz invariant spectrum of the vacuum ($γ=0, p=-ρ$) is deduced. These results are in agreement with our earlier derivation for bosons using thermodynamics and semiclassical considerations.

hep-th↗