arXiv subjects

J. A. S. Lima

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

At least 127 records · Page 7Linked to original sources

arXiv2005

Relativity, nonextensivity, and extended power law distributions

A proof of the relativistic $H$-theorem by including nonextensive effects is given. As it happens in the nonrelativistic limit, the molecular chaos hypothesis advanced by Boltzmann does not remain valid, and the second law of thermodynamics combined with a duality transformation implies that the q-parameter lies on the interval [0,2]. It is also proved that the collisional equilibrium states (null entropy source term) are described by the relativistic $q$-power law extension of the exponential Juttner distribution which reduces, in the nonrelativistic domain, to the Tsallis power law function. As a simple illustration of the basic approach, we derive the relativistic nonextensive equilibrium distribution for a dilute charged gas under the action of an electromagnetic field $F^{μν}$. Such results reduce to the standard ones in the extensive limit, thereby showing that the nonextensive entropic framework can be harmonized with the space-time ideas contained in the special relativity theory.

cond-mat.stat-mech↗

arXiv2005

Interpreting Cosmological Vacuum Decay

The cosmological vacuum decay scenario recently proposed by Wang and Meng \cite{wm} is rediscussed. From thermodynamic arguments it is found that the $ε$ parameter quantifying the vacuum decay rate must be positive in the presence of particle creation. If there is no particle creation, the proper mass of Cold Dark Matter (CDM) particles is necessarily a time dependent quantity, scaling as $m(t) = m_o a(t)^ε$. By considering the presence of baryons in the cosmological scenario, it is also shown that their dynamic effect is to alter the transition redshift $z_*$ (the redshift at which the Universe switches from decelerating to accelerating expansion), predicting values of $z_*$ compatible with current estimates based on type Ia supernova. In order to constrain the $Ω_m - ε$ plane, a joint statistical analysis involving the current supernovae observations, gas mass fraction measurements in galaxy clusters and CMB data is performed. At 95% c.l. it is found that the vacuum decay rate parameter lies on the interval $ε= 0.11 \pm 0.12$). The possibility of a vacuum decay into photons is also analyzed. In this case, the energy density of the radiation fluid scales as $ρ_r = ρ_{ro}a^{-4 + ε}$, and its temperature evolution law obeys $T(t) = T_oa(t)^{ε/4 - 1}$.

astro-ph↗

arXiv2005

An old quasar in a young dark energy-dominated universe?

Dark energy is the invisible fuel that seems to drive the current acceleration of the Universe. Its presence, which is inferred from an impressive convergence of high-quality observational results along with some apparently sucessful theoretical predictions, is also supported by the current estimates of the age of the Universe from dating of local and high-$z$ objects. In this paper we test the viability of several dark energy scenarios in the light of the age estimates of the high redshift ($z=3.91$) quasar APM 08279+5255. Using a chemodinamical model for the evolution of spheroids, we first reevaluate its current estimated age, as given by Hasinger et al. (2002). An age of 2.1 Gyr is set by the condition that Fe/O abundance ratio (normalized to solar values) of the model reaches 3.3, which is the best fit value obtained in the above reference. It is shown that for the currently accepted value of the matter density parameter, most of the existing dark energy scenarios cannot accomodate this old high redshift object unless the Hubble parameter is as low as $H_o = 58$ $\rm{km.s^{-1}.Mpc^{-1}}$, as recently advocated by Sandage and collaborators. Even considering less stringent age limits, only cosmological models that predicts a considerably old Universe at high-$z$ can be compatible with the existence of this object. This is the case of the conventional $Λ$CDM scenario and some specific classes of brane world cosmologies.

astro-ph↗

arXiv2005

Decaying Lambda cosmology, varying G and holography

We discuss a class of uniform and isotropic, spatially flat, decaying Lambda cosmologies, in the realm of a model where the gravitation constant G is a function of the cosmological time. Besides the usual de Sitter solution, the models at late times are characterized by a constant ratio between the matter and total energy densities. One of them presents a coasting expansion where the matter density parameter is equal to 1/3, and the age of the universe satisfies Ht = 1. From considerations in line with the holographic conjecture, it is argued that, among the non-decelerating solutions, the coasting expansion is the only acceptable from a thermodynamic viewpoint, and that the time varying cosmological term must be proportional to the square of the Hubble parameter, a result earlier obtained using different arguments.

gr-qc↗

arXiv2004

On the width of the last scattering surface

We discuss the physical effects of some accelerated world models on the width of the last scattering surface (LSS) of the cosmic microwave background radiation (CMBR). The models considered in our analysis are X-matter (XCDM) and a Chaplygin type gas. The redshift of the LSS does not depend on the kind of dark energy (if XCDM of Chaplygin). Further, for a Chaplygin gas, the width of the LSS is also only weakly dependent on the kind of scenario (if we have dark energy plus cold dark matter or the unified picture).

astro-ph↗

arXiv2004

Thermodynamics, Spectral Distribution and the Nature of Dark Energy

Recent astronomical observations suggest that the bulk of energy in the Universe is repulsive and appears like a dark component with negative pressure ($ω\equiv p_x/ρ_x < 0$). In this work we investigate thermodynamic and statistical properties of such a component. It is found that its energy and temperature grow during the evolution of the Universe since work is done on the system. Under the hypothesis of a null chemical potential, the case of phantom energy ($ω< -1$) seems to be physically meaningless because its entropy is negative. It is also proved that the wavelengths of the $ω$-quanta decrease in an expanding Universe. This unexpected behavior explains how their energy may be continuously stored in the course of expansion. The spectrum and the associated Wien-type law favors a fermionic nature with $ω$ naturally restricted to the interval $-1 \leqslant ω< -1/2$. Our analysis also implies that the ultimate fate of the Universe may be considerably modified. If a dark energy dominated Universe expands forever, it will become increasingly hot.

astro-ph↗

arXiv2004

Measuring the Chaplygin gas equation of state from angular and luminosity distances

An exotic dark component, named generalized Chaplygin gas (Cg) and parameterized by an equation of state $p = -A/ρ_{Cg}^α$ where $A$ and $α$ are arbitrary constants, is one of the possible candidates for dark energy as well as for a unified scenario of dark matter/energy. In this paper we investigate qualitative and quantitative aspects of the angular size - redshift test in cosmological models driven by such a dark component. We discuss the prospects for constraining the Cg equation of state from measurements of the angular size at low and high redshift radio sources and also from a joint analysis involving angular size and supernova data. A detailed discussion about the influence of the Cg on the minimal redshift at which the angular size of an extragalactic source takes its minimal value is also presented.

astro-ph↗

arXiv2004

On the existence of turning points in D-dimensionsal Schwarzschild-de Sitter and anti-de Sitter spacetimes

We investigate the motion of a test particle in a d-dimensional, spherically symmetric and static space-time supported by a mass $M$ plus a $Λ$-term. The motion is strongly dependent on the sign of $Λ$. In Schwarzschild-de Sitter (SdS) space-time ($Λ> 0$), besides the physical singularity at $r=0$ there are cases with two horizons and two turning points, one horizon and one turning point and the complete absence of horizon and turning points. For Schwarzschild-Anti de Sitter (SAdS) space-time ($Λ< 0$) the horizon coordinate is associated to a unique turning point.

gr-qc↗

arXiv2004

Power Law Stellar Distributions

The density profiles and other quantities of physical interest for spherically symmetric systems are computed by assuming that a collisionless stellar gas may relax to the non-Gaussian power law distribution suggested by the nonextensive kinetic theory. There are two different classes of solutions. The first class behaves like a subset of the polytropic Lane-Emden spheres, whereas the second one corresponds to a transition between two different polytropic indices. Unlike the isothermal Maxwellian sphere, the total mass and sizes of both classes are finite for a large range of the nonextensive q-parameter.

astro-ph↗

arXiv2004

Nonsingular FRW cosmology and nonlinear electrodynamics

The possibility to avoid the cosmic initial singularity as a consequence of nonlinear effects on the Maxwell eletromagnetic theory is discussed. For a flat FRW geometry we derive the general nonsingular solution supported by a magnetic field plus a cosmic fluid and a nonvanishing vacuum energy density. The nonsingular behavior of solutions with a time-dependent $Λ(t)$-term are also examined. As a general result, it is found that the functional dependence of $Λ(t)$ can uniquely be determined only if the magnetic field remains constant. All these models are examples of bouncing universes which may exhibit an inflationary dynamics driven by the nonlinear corrections of the magnetic field.

astro-ph↗

arXiv2004

Alternative Dark Energy Models: An Overview

A large number of recent observational data strongly suggest that we live in a flat, accelerating Universe composed of $\sim$ 1/3 of matter (baryonic + dark) and $\sim$ 2/3 of an exotic component with large negative pressure, usually named {\bf Dark Energy} or {\bf Quintessence}. The basic set of experiments includes: observations from SNe Ia, CMB anisotropies, large scale structure, X-ray data from galaxy clusters, age estimates of globular clusters and old high redshift galaxies (OHRG's). Such results seem to provide the remaining piece of information connecting the inflationary flatness prediction ($Ω_{\rm{T}} = 1$) with astronomical observations. Theoretically, they have also stimulated the current interest for more general models containing an extra component describing this unknown dark energy, and simultaneously accounting for the present accelerating stage of the Universe. An overlook in the literature shows that at least five dark energy candidates have been proposed in the context of general relativistic models. Since the cosmological constant and rolling scalar field models have already been extensively discussed, in this short review we focus our attention to the three remaining candidates, namely: a decaying vacuum energy density (or ${\bf Λ(t)}$ {\bf models}), the {\bf X-matter}, and the so-called {\bf Chaplygin-type gas}. A summary of their main results is given and some difficulties underlying the emerging dark energy paradigm are also briefly examined.

astro-ph↗

arXiv2004

Cosmological constraints on Chaplygin gas dark energy from galaxy clusters X-ray and supernova data

The recent observational evidences for the present accelerated stage of the Universe have stimulated renewed interest for alternative cosmologies. In general, such models contain an unknown negative-pressure dark component that explains the supernova results and reconciles the inflationary flatness prediction ($Ω_{\rm{T}} = 1$) and the cosmic microwave background measurements with the dynamical estimates of the quantity of matter in the Universe ($Ω_{\rm{m}} \simeq 0.3 \pm 0.1$). In this paper we study some observational consequences of a dark energy candidate, the so-called generalized Chaplygin gas which is characterized by an equation of state $p_{C} = -A/ρ_{C}^α$, where $A$ and $α$ are positive constants. We investigate the prospects for constraining the equation of state of this dark energy component by combining Chandra observations of the X-ray luminosity of galaxy clusters, independent measurements of the baryonic matter density, the latest measurements of the Hubble parameter as given by the HST Key Project and data of the Supernova Cosmology Project. We show that very stringent constraints on the model parameters can be obtained from this combination of observational data.

astro-ph↗

arXiv2003

Reply to ``Comment on Conservative Force Fields in Nonextensive Kinetic Theory"

It is shown that the comment by Costa and Meneses [1] did not point any technical or conceptual flaw in our paper titled ``Conservative Force Fields in Nonextensive Kinetic Theory" (Physica A 316, 289 (2002)). In particular, the application of the nonextensive distribution to the historical problem of the unbounded isothermal atmosphere under constant gravity is theoretically correct. It should be stressed that our solution is somewhat connected with a similar solution to a problem appearing in the astrophysical domain, namely, the so-called singular isothermal sphere. Actually, both problems are solved by a stationary Tsallis q-distribution with a "thermal cut-off" in the allowed values of the energy.

cond-mat.stat-mech↗

arXiv2003

Constraining the dark energy with galaxy clusters X-ray data

The equation of state characterizing the dark energy component is constrained by combining Chandra observations of the X-ray luminosity of galaxy clusters with independent measurements of the baryonic matter density and the latest measurements of the Hubble parameter as given by the HST key project. By assuming a spatially flat scenario driven by a "quintessence" component with an equation of state $p_x = ωρ_x$ we place the following limits on the cosmological parameters $ω$ and $Ω_{\rm{m}}$: (i) $-1 \leq ω\leq -0.55$ and $Ω_{\rm m} = 0.32^{+0.027}_{-0.014}$ (1$σ$) if the equation of state of the dark energy is restricted to the interval $-1 \leq ω< 0$ (\emph{usual} quintessence) and (ii) $ω= -1.29^{+0.686}_{-0.792}$ and $Ω_{\rm{m}} = 0.31^{+0.037}_{-0.034}$ ($1σ$) if $ω$ violates the null energy condition and assume values $< -1$ (\emph{extended} quintessence or ``phantom'' energy). These results are in good agreement with independent studies based on supernovae observations, large-scale structure and the anisotropies of the cosmic background radiation.

astro-ph↗

arXiv2003

Cosmological implications of APM 08279+5255, an old quasar at z = 3.91

The existence of old high-redshift objects provides an important tool for constraining the expanding age of the Universe and the formation epoch of the first objects. In a recent paper, Hasinger {\it et al.} (2002) reported the discovery of the quasar APM 08279+5255 at redshift $z=3.91$ with an extremely high iron abundance, and estimated age of 2 - 3Gyr. By assuming the lower limit for this age estimate and the latest measurements of the Hubble parameter as given by the HST key project, we study some cosmological implications from the existence of this object. In particular, we derive new limits on the dark matter and vacuum energy contribution. Our analysis is also extended to quintessence scenarios in which the dark energy is parameterized by a smooth component with an equation of state $p_x = ω_x ρ_x$ ($-1\leq ω_x < 0$). For flat models with a relic cosmological constant we show that the vacuum energy density parameter is constrained to be $Ω_Λ\geq 0.78$, a result that is marginally compatible with recent observations from type Ia supernovae (SNe Ia) and cosmic microwave background (CMB). For quintessence scenarios the same analysis restricts the cosmic parameter to $ω_x \leq -0.22$. Limits on a possible first epoch of quasar formation are also briefly discussed. The existence of this object pushes the formation era back to extremely high redshifts.

astro-ph↗

arXiv2002

Jeans' gravitational instability and nonextensive kinetic theory

The concept of Jeans gravitational instability is rediscussed in the framework of nonextensive statistics and its associated kinetic theory. A simple analytical formula generalizing the Jeans criterion is derived by assuming that the unperturbed self- gravitating collisionless gas is kinetically described by the $q$-parameterized class of power law velocity distributions. It is found that the critical values of wavelength and mass depend explicitly on the nonextensive $q$-parameter. The standard Jeans wavelength derived for a Maxwellian distribution is recovered in the limiting case $q$=1. For power-law distributions with cutoff, the instability condition is weakened with the system becoming unstable even for wavelengths of the disturbance smaller than the standard Jeans length $λ_J$.

astro-ph↗

arXiv2002

Nonextensive Thermostatistics and the H-Theorem

The kinetic foundations of Tsallis' nonextensive thermostatistics are investigated through Boltzmann's transport equation approach. Our analysis follows from a nonextensive generalization of the ``molecular chaos hypothesis". For $q>0$, the $q$-transport equation satisfies an $H$-theorem based on Tsallis entropy. It is also proved that the collisional equilibrium is given by Tsallis' $q$-nonextensive velocity distribution.

cond-mat↗

arXiv2002

Conservative Force Fields in Nonextensive Kinetic Theory

We investigate the nonextensive $q$-distribution function for a gas in the presence of an external field of force possessing a potential $U({\bf r})$. In the case of a dilute gas, we show that the power law distribution including the potential energy factor term can rigorously be deduced based on kinetic theoretical arguments. This result is significant as a preliminary to the discussion of long range interactions according to nonextensive thermostatistics and the underlying kinetic theory. As an application, the historical problem of the unbounded isothermal planetary atmospheres is rediscussed. It is found that the maximum height for the equilibrium atmosphere is $z_{max} = K_{B}T/mg(1-q)$. In the extensive limit, the exponential Boltzmann factor is recovered and the length of the atmosphere becomes infinite.

cond-mat.stat-mech↗