Evolution of a spherically symmetric dust-like cloud in ΛCDM models

We investigate the evolution of a spherically symmetric dust-like cloud at the linear and nonlinear stages in the framework of ΛCDM models of the universe with nonzero three-space curvature. The evolution conditions are expressed for any redshift z ≥ 0 in terms of the amplitude δ_min of the fluctuation which stops to expand at infinite time, the amplitude δ _ta of the fluctuation which stops to expand at a given moment, and the amplitude δ _c of the fluctuation which collapses at a given moment. These amplitudes are calculated as functions of cosmological model parameters and redshift. The ratios D _nl ^r /D _l of nonlinear amplitude estimates to linear ones and the typical fluctuation scales k _nl /k are approximated by a function of the linear amplitude δ _z .     

Anisotropic viscous universe with variableG and Λ

An anisotropic model with variableG and and bulk viscosity is considered. The model exhibits an inflationary behavior during which the coefficient of bulk viscosity varies lineraly with the energy density. This allows the anisotropy energy to decrease exponentially with time. Other results overlap with our earlier work with a different ansatz for . The gravitational constant was found to increase during the radiation and matter epochs.     

On the spin distributions of ΛCDM haloes

We used merger trees realizations, predicted by the extended Press-Schechter theory, in order to study the growth of angular momentum of dark matter haloes. Our results showed that:

1. The spin parameter λ′ resulting from the above method, is an increasing function of the present day mass of the halo. The mean value of λ′ varies from 0.0343 to 0.0484 for haloes with present day masses in the range of 10⁹h^?1 M _⊙ to 10¹⁴h^?1 M _⊙.
2. The distribution of λ′ is close to a log-normal, but, as it is already found in the results of N-body simulations, the match is not satisfactory at the tails of the distribution. A new analytical formula that approximates the results much more satisfactorily is presented.
3. The distribution of the values of λ′ depends only weakly on the redshift.
4. The spin parameter of an halo depends on the number of recent major mergers. Specifically the spin parameter is an increasing function of this number.

     

ΛCDM, ΛDGP and extended phantom-like cosmologies

In this paper we compare outcomes of some extended phantom-like cosmologies with each other and also with ΛCDM and ΛDGP. We focus on the variation of the luminosity distances, the age of the universe and the deceleration parameter versus the redshift in these scenarios. In a dynamical system approach, we show that the accelerating phase of the universe in the f(R)-DGP scenario is stable if one consider the curvature fluid as a phantom scalar field in the equivalent scalar-tensor theory, otherwise it is a transient and unstable phenomenon. Up to the parameters values adopted in this paper, the extended F(R,ϕ)-DGP scenario is closer to the ΛCDM scenario than other proposed models. All of these scenarios explain the late-time cosmic speed-up in their normal DGP branches, but the redshift at which transition to the accelerating phase occurs are different: while the ΛDGP model transits to the accelerating phase much earlier, the F(R,ϕ)-DGP model transits to this phase much later than other scenarios. Also, within the parameter spaces adopted in this paper, the age of the universe in the f(R)-DGP model is larger than ΛCDM, but this age in F(G,ϕ)-DGP is smaller than ΛCDM.     

Accelerating universe with time variation of G and Λ

We study a gravitational model in which scale transformations play the key role in obtaining dynamical G and Λ. We take a non-scale invariant gravitational action with a cosmological constant and a gravitational coupling constant. Then, by a scale transformation, through a dilaton field, we obtain a new action containing cosmological and gravitational coupling terms which are dynamically dependent on the dilaton field with Higgs type potential. The vacuum expectation value of this dilaton field, through spontaneous symmetry breaking on the basis of anthropic principle, determines the time variations of G and Λ. The relevance of these time variations to the current acceleration of the universe, coincidence problem, Mach’s cosmological coincidence and those problems of standard cosmology addressed by inflationary models, are discussed. The current acceleration of the universe is shown to be a result of phase transition from radiation toward matter dominated eras. No real coincidence problem between matter and vacuum energy densities exists in this model and this apparent coincidence together with Mach’s cosmological coincidence are shown to be simple consequences of a new kind of scale factor dependence of the energy momentum density as ρ∼a ⁻⁴. This model also provides the possibility for a super fast expansion of the scale factor at very early universe by introducing exotic type matter like cosmic strings.     

Cosmological Implications of The Bulk Viscous Model With Variable G and Λ

We have investigated the cosmological implications of the bulk viscous cosmological model with variable G and Λ. These results are found to be compatible with the present observations. The classical cosmological tests for this model encompass the Freese et al. ones. The model has some spirits of the Standard Model. The inflationary solution which solves the Standard problems is obtained as a special solution. The influence of viscosity is shown to affect the past and the future of the Universe. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cosmological scalar fields that mimic the ΛCDM cosmological model

We look for cosmologies with a scalar field (dark energy without cosmological constant), which mimic the standard ΛCDM cosmological model yielding exactly the same large-scale geometry described by the evolution of the Hubble parameter (i.e. photometric distance and angular diameter distance as functions on z). Asymptotic behavior of the field solutions is studied in the case of spatially flat Universe with pressureless matter and separable scalar field Lagrangians; the cases of power-law kinetic term and power-law potential are considered. Exact analytic solutions are found in some special cases. A number of models have the field solutions with infinite behavior in the past or even singular behavior at finite redshifts. We point out that introduction of the cosmological scalar field involves some degeneracy leading to lower precision in determination of Ω _m . To remove this degeneracy additional information is needed besides the data on large-scale geometry. The article is published in the original.     

Turbulent viscosity and Λ-effect from numerical turbulence models

Homogeneous anisotropic turbulence simulations are used to determine off-diagonal components of the Reynolds stress tensor and its parameterization in terms of turbulent viscosity and Λ-effect. The turbulence is forced in an anisotropic fashion by enhancing the strength of the forcing in the vertical direction. The Coriolis force is included with a rotation axis inclined relative to the vertical direction. The system studied here is significantly simpler than that of turbulent stratified convection which has often been used to study Reynolds stresses. Certain puzzling features of the results for convection, such as sign changes or highly concentrated latitude distributions, are not present in the simpler system considered here. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Some Homogeneous Bianchi type IX Viscous Fluid Cosmological Models with a Varying Λ

Some Bianchi type IX viscous fluid cosmological models are investigated. To get a solution, a supplementary condition between metric potentials is used. The viscosity coefficient of bulk viscous fluid is assumed to be a power function of mass density, whereas the coefficient of shear viscosity is considered as proportional to scale of expansion in the model. The cosmological constant Λ is found to be positive and is a decreasing function of time, which is supported by results from recent supernovae observations. Some physical and geometric properties of the models are also discussed.     

Multiple ΛCDM cosmology with string landscape features and future singularities

Multiple ΛCDM cosmology is studied in a way that is formally a classical analog of the Casimir effect. Such cosmology corresponds to a time-dependent dark fluid model or, alternatively, to its scalar field presentation, and it motivated by the string landscape picture. The future evolution of the several dark energy models constructed within the scheme is carefully investigated. It turns out to be almost always possible to choose the parameters in the models so that they match the most recent and accurate astronomical values. To this end, several universes are presented which mimic (multiple) ΛCDM cosmology but exhibit Little Rip, asymptotically de Sitter, or Type I, II, III, and IV finite-time singularity behavior in the far future, with disintegration of all bound objects in the cases of Big Rip, Little Rip and Pseudo-Rip cosmologies.     

A New Consistency Test for ΛCDM Cosmology Using Galaxy Surveys

We propose a new consistency test for the ΛCDM cosmology using baryonic acoustic oscillations(BAO) and redshift space distortion(RSD) measurements from galaxy redshift surveys. Specifically, we determine the peak position of fσ8(z) in redshift z offered by an RSD measurement, and compare it to the one predicted by the BAO observables assuming a flat ΛCDM cosmology. We demonstrate this new test using the simulated data for the DESI galaxy survey, and argue that this test complements those using t...     

Bianchi type-I cosmological models with time dependent G and Λ

Einstein field equations with variable gravitational and cosmological constants are considered in the presence of perfect fluid for Bianchi type-I universe by assuming the cosmological term proportional to the Hubble parameter. This variation law for vacuum density has recently been proposed by Schützhold on the basis of quantum field estimations in the curved and expanding background. The model obtained approaches isotropy. The cosmological term tends asymptotically to a genuine cosmological constant, and the model tends to a deSitter universe. We obtain that the present universe is accelerating with a large fraction of cosmological density in the form of cosmological term.     

Bianchi type-I cosmological models with time dependent q and Λ-term in general relativity

On getting motivation from increasing evidence for the need of a geometry that resembles Bianchi morphology to explain the observed anisotropy in the WMAP data, Einstein’s field equations with variable cosmological “constant” are considered in presence of perfect fluid for a homogeneous and anisotropic Bianchi type-I space-time. Einstein’s field equations are solved by considering a time dependent deceleration parameter which affords a late time acceleration in the universe. The cosmological constant Λ is found to be a decreasing function of time and it approaches a small positive value at the present epoch which is corroborated by consequences from recent supernovae Ia observations. From recently developed Statefinder pair, the behavior of different stages of the evolution of the universe has been studied. The physical significance of the cosmological models have also been discussed.     

Bulk viscous fluid hypersurface–homogeneous cosmological models with time varying G and Λ

Hypersurface–homogeneous cosmological models containing a bulk viscous fluid with time varying G and Λ have been presented. We have shown that the field equations are solvable for any arbitrary cosmic scale function. The viscosity coefficient of bulk viscous fluid is assumed to be a power function of the energy density. Exact solutions of Einstein’s field equations are obtained which represent an expanding, shearing and accelerating/decelerating models of the universe. The physical and kinematical behaviours of the models are also discussed.     

Bianchi type-I cosmological models with variable G and Λ-term in general relativity

Einstein’s field equations with variable gravitational and cosmological “constant” are considered in presence of perfect fluid for Bianchi type-I space-time. Consequences of the four cases of the phenomenological decay of Λ have been discussed which are consistent with observations. The physical significance of the cosmological models have also been discussed.      

Bulk viscous cosmological models of universe with variable deceleration parameter in Lyra’s Manifold

FRW models of universe in the presence of viscous fluid are investigated in the cosmological theory based on Lyra’s Manifold. By considering the deceleration parameter to be a variable and the viscosity coefficient of bulk viscous fluid to be a constant, exacts solutions have been obtained from which three forms of model of the universe are derived. The physical properties of the models are also investigated.     

Five dimensional Bianchi type-V cosmological models described by a binary mixture of perfect fluid and dark energy with Λ-term

Some recent experimental observations have been shown that inclusion of electron collisions damping in inertial Alfvén wave (IAW) dynamics may be important for laboratory as well as space plasmas. This paper presents the numerical simulation of model equation governing the nonlinear dynamics of IAW in low-beta plasmas. When the nonlinearity arises due to the ponderomotive force and Joule heating driven density perturbations, the model equation turns out to be a modified nonlinear Schr?dinger equation (MNLS). The electron collisions are introduced only in the electron momentum equation. The damped localized structures of IAW with sidebands are obtained. Also, the effect of collisional damping on power spectra of magnetic fluctuations with different scaling laws has been studied. These turbulent structures may be responsible for particle acceleration in laboratory and space plasmas.