arXiv subjects

J. A. S. Lima

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

159 records · Page 9Linked to original sources

arXiv1999

New limits on $Ω_Λ$ and $Ω_M$ from old galaxies at high redshift

The ages of two old galaxies (53W091, 53W069) at high redshifts are used to constrain the value of the cosmological constant in a flat universe ($Λ$CDM) and the density parameter $Ω_M$ in Friedmann-Robertson-Walker (FRW) models with no $Λ$-term. In the case of $Λ$CDM models, the quoted galaxies yield two lower limits for the vacuum energy density parameter, $Ω_Λ\geq 0.42$ and $Ω_Λ\geq 0.5$, respectively. Although compatible with the limits from statistics of gravitational lensing (SGL) and cosmic microwave background (CMB), these lower bounds are more stringent than the ones recently determined using SNe Ia as standard candles. For matter dominated universes ($Ω_Λ=0$), the existence of these galaxies imply that the universe is open with the matter density parameter constrained by $Ω_M \leq 0.45$ and $Ω_M \leq 0.37$, respectively. In particular, these results disagree completely with the analysis of field galaxies which gives a lower limit $Ω_M \geq 0.40$.

astro-ph↗

arXiv1999

Flat FRW Cosmologies with Adiabatic Matter Creation: Kinematic tests

Some observational consequences of a cosmological scenario 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 redshift relations are derived and their meaning discussed in detail. The expressions of the conventional FRW models are significantly modified and provide a powerful method to limit the parameters of the models.

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arXiv1997

Big-Bang Cosmology with Photon Creation

The temperature evolution law is determined for an expanding FRW type Universe with a mixture of matter and radiation where "adiabatic" creation of photons has taken place. Taking into account this photon creation we discuss the physical conditions for having a hot big bang Universe. We also compare our results to the ones obtained from the standard FRW model.

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arXiv1996

On the Newtonian Cosmology Equations with Pressure

The basic equations describing a Newtonian universe with uniform pressure are reexamined. We argue that in the semi-classical formulation adopted in the literature the continuity equation has a misleading pressure gradient term. When this term is removed, the resulting equations give the same homogeneous background solutions with pressure and the same evolution equation for the density contrast as are obtained using the full relativistic approach.

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arXiv1996

D-Dimensional Radiative Plasma: A Kinetic Approach

The covariant kinetic approach for the radiative plasma, a mixture of a relativistic moving gas plus radiation quanta (photons, neutrinos, or gravitons) is generalized to D spatial dimensions. The operational and physical meaning of Eckart's temperature is reexamined and the D-dimensional expressions for the transport coefficients (heat conduction, bulk and shear viscosity) are explicitly evaluated to first order in the mean free time of the radiation quanta. Weinberg's conclusion that the mixture behaves like a relativistic imperfect simple fluid (in Eckart's formulation) depends neither on the number of spatial dimensions nor on the details of the collisional term. The case of Thomson scaterring is studied in detail, and some consequences for higher dimensional cosmologies are also discussed.

gr-qc↗

arXiv1996

Accretion of Cold and Hot Dark Matter onto Cosmic String Filaments

The Zeldovich approximation is applied to study the accretion of hot and cold dark matter onto moving long strings. It is assumed that such defects carry a substantial amount of small-scale structure, thereby acting gravitationally as a Newtonian line source whose effects dominate the velocity perturbations. Analytical expressions for the turn-around surfaces are derived and the mass inside of these surfaces is calculated. Estimates are given for the redshift dependence of $Ω_{nl}$, the fraction of mass in nonlinear objects. Depending on parameters, it is possible to obtain $Ω_{nl} = 1$ at the present time. Even with hot dark matter, the first nonlinear filamentary structures form at a redshift close to 100, and there is sufficient nonlinear mass to explain the observed abundance of high redshift quasars and damped Lyman alpha systems. These results imply that moving strings with small-scale structure are the most efficient seeds to produce massive nonlinear objects in the cosmic string model.

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arXiv1996

Remarks on a Decrumpling Model of the Universe

It is argued that when the dimension of space is a constant integer the full set of Einstein's field equations has more information than the spatial components of Einstein's equation plus the energy conservation law. Applying the former approach to the decrumpling FRW cosmology recently proposed, it is shown that the spacetime singularity cannot be avoided and that turning points are absent. This result is in contrast to the decrumpling nonsingular spacetime model with turning points previously obtained using the latter approach.

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arXiv1996

Deflationary Models Driven by Matter Creation

A nonsingular deflationary cosmology driven by adiabatic matter creation is proposed. In this scenario there is no preinflationary stage as happens in conventional inflationary models. Deflation starts from a de Sitter spacetime characterized by an arbitrary time scale $H_{I}^{-1}$, which also pins down an initial value for the temperature of the universe. The model evolves continuously towards a slightly modified Friedman-Robertson-Walker universe. The horizon and other well known problems of the standard model are then solved but, unlike in microscopic models of inflation, there is no supercooling and subsequent reheating. Entropy generation is concomitant with deflation and if $H_{I}^{-1}$ is of the order of the Planck time, the present day value of the radiation temperature is deduced. It is also shown that the ``age problem'' does not exist here. In particular, the theoretically favored FRW flat model is old enough to agree with the observations even given the high values of $H_{o}$ suggested by recent measurements.

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arXiv1996

Inflationary Models Driven by Adiabatic Matter Creation

The flat inflationary dust universe with matter creation proposed by Prigogine and coworkers is generalized and its dynamical properties are reexamined. It is shown that the starting point of these models depends critically on a dimensionless parameter $Σ$, closely related to the matter creation rate $ψ$. For $Σ$ bigger or smaller than unity flat universes can emerge, respectively, either like a Big-Bang FRW singularity or as a Minkowski space-time at $t=-\infty$. The case $Σ=1$ corresponds to a de Sitter-type solution, a fixed point in the phase diagram of the system, supported by the matter creation process. The curvature effects have also been investigated. The inflating de Sitter is a universal attractor for all expanding solutions regardless of the initial conditions as well as of the curvature parameter.

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arXiv1996

Graviton Production in Elliptical and Hyperbolic Universes

The problem of cosmological graviton creation for homogeneous and isotropic universes with elliptical ($\vae =+1$) and hyperbolical ($\vae =-1$) geometries is addressed. The gravitational wave equation is established for a self-gravitating fluid satisfying the barotropic equation of state $p=(γ-1)ρ$, which is the source of the Einstein's equations plus a cosmological $Λ$-term. The time dependent part of this equation is exactly solved in terms of hypergeometric functions for any value of $γ$ and spatial curvature $\vae$. An expression representing an adiabatic vacuum state is then obtained in terms of associated Legendre functions whenever $γ\neq \frac{2}{3}\; \frac{(2n+1)}{(2n-1)}$, where n is an integer. This includes most cases of physical interest such as $γ=0,\;4/3\;,1$. The mechanism of graviton creation is reviewed and the Bogoliubov coefficients related to transitions between arbitrary cosmic eras are also explicitly evaluated.

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arXiv1996

Cosmologies with Photon Creation and the 3K Relic Radiation Spectrum

A new Planckian distribution for cosmologies with photon creation is derived using thermodynamics and semiclassical considerations. This spectrum is preserved during the evolution of the universe and compatible with the present spectral shape of the cosmic microwave background radiation(CMBR). Accordingly, the widely spread feeling that cosmologies with continuous photon creation are definitely ruled out by the COBE limits on deviation of the CMBR spectrum from blackbody shape should be reconsidered. It is argued that a crucial test for this kind of cosmologies is provided by measurements of the CMBR temperature at high redshifts. For a given redshift $z$ greater than zero, the temperature is smaller than the one predicted by the standard FRW model.

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arXiv1996

Thermodynamics of Decaying Vacuum Cosmologies

The thermodynamic behavior of vacuum decaying cosmologies is investigated within a manifestly covariant formulation. Such a process corresponds to a continuous irreversible energy flow from the vacuum component to the created matter constituents. It is shown that if the specific entropy per particle remains constant during the process, the equilibrium relations are preserved. In particular, if the vacuum decays into photons, the energy density $ρ$ and average number density of photons $n$ scale with the temperature as $ρ\sim T^{4}$ and $n \sim T^{3}$. The temperature law is determined and a generalized Planckian type form of the spectrum, which is preserved in the course of the evolution, is also proposed. Some consequences of these results for decaying vacuum FRW type cosmologies as well as for models with ``adiabatic'' photon creation are discussed.

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arXiv1995

FRW Cosmologies with Adiabatic Matter Creation

Some properties of cosmological models with matter creation are investigated in the framework of the Friedman-Robertson-Walker (FRW) line element. For adiabatic matter creation, as developed by Prigogine and coworkers, we derive a simple expression relating the particle number density $n$ and energy density $ρ$ which holds regardless of the matter creation rate. The conditions to generate inflation are discussed and by considering the natural phenomenological matter creation rate $ψ=3 βnH$, where $β$ is a pure number of the order of unity and $H$ is the Hubble parameter, a minimally modified hot big-bang model is proposed. The dynamic properties of such models can be deduced from the standard ones simply by replacing the adiabatic index $γ$ of the equation of state by an effective parameter $γ_{*} = γ(1 - β)$. The thermodynamic behavior is determined and it is also shown that ages large enough to agree with observations are obtained even given the high values of $H$ suggested by recent measurements.

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arXiv1995

Decaying Vacuum Energy and Deflationary Cosmology in Open and Closed Universes

We consider a nonsingular deflationary cosmological model with decaying vacuum energy density in universes of arbitrary spatial curvature. Irrespective of the value of $k$, the models are characterized by an arbitrary time scale $H_I^{-1}$ which determines the initial temperature of the universe and the largest value of the vacuum energy density, the slow decay of which generates all the presently observed matter-energy of the universe. If $H_I^{-1}$ is of the order of the Planck time, the models begin with the Planck temperature and the present day value of the cosmological constant satisfies $Λ_I/Λ_0 \simeq 10^{118}$ as theoretically suggested. It is also shown that all models allow a density parameter $Ω_0 <2/3$ and that the age of the universe is large enough to agree with observations even with the high value of $H_0$ suggested by recent measurements.

astro-ph↗

arXiv1995

THERMODYNAMIC PROPERTIES OF $γ$--FLUIDS AND THE QUANTUM VACCUM

The thermodynamic behaviour of a relativistic perfect simple fluid obeying the equation of state $p=(γ-1)ρ$, where $0 \le γ\le 2$ is a constant, has been investigated. Particular cases include: vacuum($p=-ρ$, $γ=0$), a randomly oriented distribution of cosmic strings ($p=-{1 \over 3} ρ$, $γ=2/3$), blackbody radiation ($p={1\over 3} ρ$, $γ=4/3$) and stiff matter ($p=ρ$, $γ=2$). Fluids with $γ<1$ become hotter when they expand adiabatically ($T\propto V^{1- γ}$). By assuming that such fluids may be regarded as a kind of generalized radiation, the general Planck's type form of the spectrum is deduced. As a limiting case, a new Lorentz invariant spectrum of the vacuum which is compatible with the equation of state and other thermodynamic constraints is proposed. Some possible consequences to the early universe physics are also discussed.

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