Showing papers on "Deceleration parameter published in 2002"

Generalized Chaplygin Gas, Accelerated Expansion and Dark Energy-Matter Unification

Abstract: We consider the scenario emerging from the dynamics of a generalized Born-Infeld theory. The equation of state describing this system is given in terms of the energy density $\ensuremath{\rho}$ and pressure p by the relationship $p=\ensuremath{-}A/{\ensuremath{\rho}}^{\ensuremath{\alpha}},$ where A is a positive constant and $0l\ensuremath{\alpha}l~1.$ We discuss the conditions under which homogeneity arises and show that this equation of state describes the evolution of a universe evolving from a phase dominated by nonrelativistic matter to a phase dominated by a cosmological constant via an intermediate period where the effective equation of state is given by $p=\ensuremath{\alpha}\ensuremath{\rho}.$

Bulk Viscous Cosmological Models in Barber's Second Self Creation Theory

Abstract: Barber's second self creation theory with bulk viscous fluid source for an LRS Bianchi type-I metric is considered by using deceleration parameter to be constant where the metric potentials are taken as function of $x$ and $t$. The coefficient of bulk viscosity is assumed to be a power function of the mass density. Some physical and geometrical features of the models are discussed

Braneworld models of dark energy

Abstract: We explore a new class of braneworld models in which the scalar curvature of the (induced) brane metric contributes to the brane action. The scalar curvature term arises generically on account of one-loop effects induced by matter fields residing on the brane. Spatially flat braneworld models can enter into a regime of accelerated expansion at late times. This is true even if the brane tension and the bulk cosmological constant are tuned to satisfy the Randall--Sundrum constraint on the brane. Braneworld models admit a wider range of possibilities for dark energy than standard LCDM. In these models the luminosity distance can be both smaller and larger than the luminosity distance in LCDM. Whereas models with $d_L \leq d_L(\rm LCDM)$ imply $w = p/\rho \geq -1$ and have frequently been discussed in the literature, models with $d_L > d_L(\rm LCDM)$ have traditionally been ignored, perhaps because within the general-relativistic framework, the luminosity distance has this property {\em only if} the equation of state of matter is strongly negative ($w < -1$). Matter with $w < -1$ is beset with a host of undesirable properties, which makes this model of dark energy unattractive within the conventional framework. Braneworld models, on the other hand, have the capacity to endow dark energy with exciting new possibilities without suffering from the problems faced by models with $w < -1$. For a subclass of parameter values, braneworld dark energy and the acceleration of the universe are {\em transient} phenomena. In these models, the universe, after the current period of acceleration, re-enters the matter dominated regime so that the deceleration parameter $q(t) \to 0.5$ when $t \gg t_0$, where $t_0$ is the present epoch. Such models could help reconcile an accelerating universe with the requirements of string/M-theory.

Bulk viscous solutions to the field equations and the deceleration parameter-revisited

Abstract: We utilise a form for the Hubble parameter to generate a number of solutions to the Einstein field equations with variable cosmological constant and variable gravitational constant in the presence of a bulk viscous fluid. The Hubble law utilised yields a constant value for the deceleration parameter. A new class of solutions is presented in the Robertson–Walker spacetimes. The coefficient of bulk viscosity is assumed to be a power function of the mass density. For a class of solutions, the deceleration parameter is negative which is consistent with the supernovae Ia observations.

Lrs bianchi type-i cosmological models in barber's second self creation theory

Abstract: Barber's second self creation theory with perfect fluid source for an LRS Bianchi type-I metric is considered using deceleration parameter to be constant where the metric potentials are taken as functions of x and t. In particular, some exact solutions have also been obtained for the vacuum universe, Zel'dovich universe and radiation universe. Some physical properties of the models are also discussed.

Cosmology with a long-range repulsive force

Abstract: We discuss the properties of a cosmology dominated by a charged scalar field with a repulsive, long-range self-interaction. The interaction, in the form of a vector field with a tiny mass, can have a dramatic effect on the evolution of the universe, with interesting consequences — including in some cases accelerated expansion. One characteristic of the model is an oscillating deceleration parameter, which would potentially allow it to be distinguished from other scalar field models such as quintessence.

Brane dynamics in the Randall-Sundrum model, inflation and graceful exit

Abstract: We study the averaged action of the Randall-Sundrum model with a time-dependent metric ansatz. It can be reformulated in terms of a Brans-Dicke action with time-dependent Newton's constant. We show that the physics of the early universe, particularly inflation, is governed by the Brans-Dicke theory. The Brans-Dicke scalar, however, quickly settles to its equilibrium value and decouples from the post-inflationary cosmology. The deceleration parameter is negative to start with but changes sign before the Brans-Dicke scalar settles to its equilibrium value. Consequently, the brane metric smoothly exits inflation. We have also studied the slow-roll inflation in our model and investigated the spectra of the density perturbation generated by the radion field and find them consistent with the current observations.

The dynamical parameters of the universe

Abstract: The results of different analyses of the dynamical parameters of the Universe are converging towards agreement. Remaining disagreements reflect systematic errors coming either from the observations or from differences in the methods of analysis. Compiling the most precise parameter values with our estimates of such systematic errors added, we find the following best values: the baryonic density parameter Omega_bh^2 = 0.019 +/- 0.02, the density parameter of the matter component Omega_m = 0.29 +/- 0.06, the density parameter of the cosmological constant Omega_lambda = 0.71 +/- 0.07, the spectral index of scalar fluctuations n_s = 1.02 +/- 0.08, the equation of state of the cosmological constant w_lambda < -0.86, and the deceleration parameter q_0 = -0.56 +/- 0.04. We do not modify the published best values of the Hubble parameter H_0 = 0.73 +/- 0.07 and the total density parameter Omega_0 ^{+0.03}_{-0.02}.

Iterative solutions for relativistic dissipative cosmologies

Abstract: Iterative solutions of the gravitational field equations for a homogeneous flat Friedmann-Robertson-Walker Universe filled with a causal bulk viscous fluid in the framework of the full Israel-Stewart-Hiscock theory are presented. The general solution of the field equations are presented in a parametric form in the zeroth-, first-, second-, and mth-order approximation by the method of iteration. The time evolution of the scale factor, energy density, deceleration parameter, entropy, Kretschmann scalar, and bulk viscous pressure-thermodynamic pressure ratio for a Zeldovich fluid-filled space-time are discussed.

Bianchi type I universes with dilaton and magnetic fields

Abstract: We consider the dynamics of a Bianchi type I spacetime in the presence of dilaton and magnetic fields. The general solution of the Einstein-Maxwell dilaton field equations can be obtained in an exact parametric form. Depending on the numerical values of the parameters of the model there are three distinct classes of solutions. The time evolution of the mean anisotropy, shear and deceleration parameter is considered in detail and it is shown that a magnetic-dilaton anisotropic Bianchi type I geometry does not isotropize, the initial anisotropy being present in the universe for all times.

Bulk Viscous Solutions to the Field Equations and the Deceleration Parameter-Revisited

Abstract: We utilise a form for the Hubble parameter to generate a number of solutions to the Einstein field equations with variable cosmological constant and variable gravitational constant in the presence of a bulk viscous fluid. The Hubble law utilised yields a constant value for the deceleration parameter. A new class of solutions is presented in the Robertson-Walker spacetimes. The coefficient of bulk viscosity is assumed to be a power function of the mass density. For a class of solutions, the deceleration parameter is negative which is consistent with the supernovae Ia observations.

Cosmology: the Cosmos as a Whole

Abstract: Having studied our Milky Way Galaxy, various other galaxies of the most diverse kinds, and the arrangement of the galaxies into clusters and superclusters, we now turn to the Universe as a whole, its spatial structure and its evolution in time. We begin, in Sect. 13.1, with E. Hubble’s discovery of the expansion of the Universe, and then discuss the possible cosmological models on the basis of the theory of General Relativity. Now, in order to determine the correct model of our Universe and its parameters, we must make use of a wide variety of different observations. To this end, we investigate somewhat more carefully the propagation of electromagnetic radiation over cosmic distances and describe the isotropic background radiation at 3 K, discovered by A.A. Penzias and R. W. Wilson, and the hot, radiation-dominated initial state of the Universe where the lightest elements, in particular helium, were produced (Sect. 13.2). Finally, in Sect. 13.3, we give an overview of the most important relevant facts in order to arrive at a picture of the evolution of the Universe from its earliest stages, which are closely connected with our knowledge of the fundamental physical interactions, through the production of the 3 K radiation, on to the formation of the galaxies and galaxy clusters, and finally to the present-day state of the cosmos.

Inflationary phase with time varying fundamental constants

Abstract: Barrow, and Barrow and collaborators, have put forward theoretical models with variable fundamental constants including JBD theories. The experimental evidence for an accelerating Universe points out to a deceleration parameter approximately equal to -- 1. On the other hand, there is evidence for a time varying fine structure constant alpha . We have included the above results in a JBD cosmological model modified by J.D. Barrow by including a time varying speed of light, thus, finding an exponential inflationary phase with variables G, alpha and c . This means that the primordial value of alpha was exponentially larger than its present value. Planck`s time may not be then approximately 10 to the power - 43 s ; the same may happen to other Planck`s quantities. We found an exponentially time-decaying Cosmological term.

Inflationary Lambda-Universe with Time Varying Fundamental Constants

Abstract: Barrow, and Barrow and collaborators, have put forward theoretical models with variable fundamental constants including JBD theories. The experimental evidence for an accelerating Universe points out to a deceleration parameter approximately equal to -- 1. On the other hand, there is evidence for a time varying fine structure constant alpha . We have included the above results in a JBD cosmological model modified by J.D. Barrow by including a time varying speed of light, thus, finding an exponential inflationary phase with variables G, alpha and c . This means that the primordial value of alpha was exponentially larger than its present value. Planck`s time may not be then approximately 10 to the power - 43 s ; the same may happen to other Planck`s quantities. We found an exponentially time-decaying Cosmological term.

Acceleration of the Universe in Type-0 Non-Critical Strings: a Review

Abstract: I review cosmology within the framework of type-0 non-critical strings, proposed in collaboration with G Diamandis, B Georgalas, E Papantonopoulos and I Pappa The instabilities of the tachyonic backgrounds, due to the absence of space-time supersymmetry, are treated in this framework as a necessary ingredient to ensure cosmological flow The model involves D3 brane worlds, whose initial quantum fluctuations induce the non criticality I argue that this model is compatible with the current astrophysical observations pointing towards acceleration of the Universe A crucial r\^ole for the correct ``phenomenology'' of the model, in particular the order-one value of the deceleration parameter, is played by the relative magnitude of the flux of the five form of the type-0 string as compared to the size of the volume of five of the extra dimensions, transverse to the direction of the flux-field

Future Destiny of Quintessential Universe and Constraint on Model from Deceleration Parameter

Abstract: The evolution of the quintessence in various stages of the universe, i.e., the radiation-, matter-, and quintessence-dominated stages, is closely related with the tracking behavior and the deceleration parameter of the universe. We gave the explicit relation between the equation-of-state of the quintessence in the epoch of the matter-quintessence equality and the inverse power index of the quintessence potential, obtained the constraint on this potential parameter coming from the present deceleration parameter, i.e., a low inverse power index. We point out that the low inverse power-law potential with a single term cannot work for the tracking solution. In order to have both of the tracker and the suitable deceleration parameter it is necessary to introduce at least two terms in the quintessence potential. We give the future evolution of the quintessential universe.