Isotropic and anisotropic dark energy models

In this review we discuss the evolution of the universe filled with dark energy with or without perfect fluid. In doing so we consider a number of cosmological models, namely Bianchi type I, III, V, VI₀, VI and FRW ones. For the anisotropic cosmological models we have used proportionality condition as an additional constrain. The exact solutions to the field equations in quadrature are found in case of a BVI model. It was found that the proportionality condition used here imposed severe restriction on the energy-momentum tensor, namely it leads to isotropic distribution of matter. Anisotropic BVI₀, BV, BIII and BIDE models with variable EoS parameter ω have been investigated by using a law of variation for the Hubble parameter. In this case the matter distribution remains anisotropic, though depending on the concrete model there appear different restrictions on the components of energy-momentum tensor. That is why we need an extra assumption such as variational a law for the Hubble parameter. It is observed that, at the early stage, the EoS parameter v is positive i.e. the universe was matter dominated at the early stage but at later time, the universe is evolving with negative values, i.e., the present epoch. DE model presents the dynamics of EoS parameter ω whose range is in good agreement with the acceptable range by the recent observations. A spatially homogeneous and anisotropic locally rotationally symmetric Bianchi-I space time filled with perfect fluid and anisotropic DE possessing dynamical energy density is studied. In the derived model, the EoS parameter of DE (ω^(de)) is obtained as time varying and it is evolving with negative sign which may be attributed to the current accelerated expansion of Universe. The distance modulus curve of derived model is in good agreement with SNLS type Ia supernovae for high redshift value which in turn implies that the derived model is physically realistic. A system of two fluids within the scope of a spatially flat and isotropic FRW model is studied. The role of the two fluids, either minimally or directly coupled in the evolution of the dark energy parameter, has been investigated. In doing so we have used three different ansatzs regarding the scale factor that gives rise to a variable decelerating parameter. It is observed that, in the non-interacting case, both the open and flat universes can cross the phantom region whereas in the interacting case only the open universe can cross the phantom region. The stability and acceptability of the obtained solution are also investigated.     

Interacting Viscous Modified Chaplygin Gas Cosmology in Presence of Cosmological Constant

Here, we consider interacting viscous modified Chaplygin gas in presence of cosmological constant. We assumed bulk viscosity as a function of density. We consider interaction between modified Chaplygin gas and baryonic matter. Then, the effects of viscosities on the cosmological parameters such as energy, density, Hubble expansion parameter, scale factor and deceleration parameter investigated. This model may be considered as a toy model of our universe.     

A matter-dominated cosmological model with variable G and Λ and its confrontation with observational data

In the framework of renormalization-group improved cosmologies, we analyze both theoretically and observationally the exact and general solution of the matter-dominated cosmological equations, by using the expression of the cosmological term as a function of the Newton parameter already determined by the integration method employed in a previous paper. A rough comparison between such a model and the concordance ΛCDM model from the point of view of the magnitude-redshift relationship has been already considered, without showing any appreciable differences. Here we test our model by using astrophysical data (the Union2 type Ia supernovae (SNIa) dataset, the Hubble diagram constructed from some gamma ray bursts luminosity distance indicator), to constrain its parameters. We also apply a cosmographic approach to our cosmological model. In order to estimate the cosmographic parameters we fit a large dataset, including not only the Hubble diagram, as traced by SNIa and gamma ray bursts, but also the H(z) measurements from passively evolving galaxies, baryon acoustic oscillations and the distance priors from the cosmic microwave background radiation anisotropy spectrum. We show that this matter-dominated cosmological model with variable Newton parameter and variable cosmological term is indeed compatible with the observations above. The cosmographic approach adopted confirms such conclusions. Last, it seems possible to include radiation into the model, since numerical integration of the equations derived by the presence of both radiation and matter shows that, after inflation, the total density parameter is initially dominated by the radiation contribution and later by the matter one.     

Cosmological Model with Strange Quark Matter Attached to Cosmic String for Axially Symmetric Space-Time

We have investigated cosmological model with strange quark matter attached to the string cloud in general theory of gravitation for Axially Symmetric space time. The model is obtained with the help of special law of variation for Hubble parameter proposed by Bermann (Nuovo Cimento B 74:182, 1983). Also, some physical and kinematics properties of the model are discussed.     

A Cyclic Model of Universe with Energy Exchange between Radiation,Matter and Vacuum Energy

We further extend the cosmological scenario with energy exchange by Barrow and Clifton and our previous work to the more complex case with energy exchange between three fluids: radiation, matter and vacuum energy. By prescribing the form of energy exchange function, we construct an infinitely cyclic cosmological model, in which the universe undergoes an endless sequence of cosmic epoch and each consisting of expansion and contraction, and the cosmological parameters, such as the Hubble parameter H, deceleration parameter q, transition red-shift Z _T, and densities ρ _r ,ρ _m , and ρ _Λ are consistent with the present observed values.     

Bulk viscosity,interaction and the viability of phantom solutions

We study the dynamics of a bulk viscosity model in the Eckart approach for a spatially flat Friedmann–Robertson–Walker (FRW) Universe. We have included radiation and dark energy, assumed as perfect fluids, and dark matter treated as an imperfect fluid having bulk viscosity. We also introduce an interaction term between the dark matter and dark energy components. Considering that the bulk viscosity is proportional to the dark matter energy density and imposing a complete cosmological dynamics, we find bounds on the bulk viscosity in order to reproduce a matter-dominated era (MDE). This constraint is independent of the interaction term. Some late time phantom solutions are mathematically possible. However, the constraint imposed by a MDE restricts the interaction parameter, in the phantom solutions, to a region consistent with a null value, eliminating the possibility of late time stable solutions with \(w<-1\). From the different cases that we study, the only possible scenario, with bulk viscosity and interaction term, belongs to the quintessence region. In the latter case, we find bounds on the interaction parameter compatible with latest observational data.     

Stability analysis and observational measurement in 5-D Brans–Dicke cosmology

This Letter is a study of the effects of higher dimensional gravity and Brans–Dicke (BD) scalar field on cosmic acceleration in 5-D BD cosmological model. We assume a flat cosmological model in which the matter content of the universe is either cold dark matter or radiation. In a framework to study attractor solutions in the phase space we simultaneously constrain the model parameters with the observational data for distance modulus. The phase space analysis illustrates that the universe begins from an unstable state in the past and eventually reaches an asymptotically stable state (attractor). We examine the model by performing Hubble parameter test in addition to statefinder diagnosis. We also reconstruct the equation of state parameter, the scale factor in 3-D space and along extra dimension. The results show that due to the presence of extra dimension and Brans–Dicke scalar field in the model, the universe undergoes a period of acceleration.     

Two-Fluid Cosmological Model of Bianchi Type-V with Negative Constant Deceleration Parameter

This paper deals with a two-fluid Bianchi type-V anisotropic cosmological model with negative constant deceleration parameter. Exact solution of Einstein’s field equations for interacting matter and radiation field is presented which represents an expanding shearing and nonrotating cosmological model of the universe. This model describes the accelerated phase of the expanding universe. The physical and kinematical behaviors of the model are discussed.     

A Specific Case of Generalized Einstein-aether Theories

With the dark energy phenomena explored over a decade,in this present work we discuss a specific case of the generalized Einstein-aether theories,in which the modified Friedmann equation is similar to that in the Dvali-Gabadadze-Porrati(DGP) brane world model.We compute the joint statistic constraints on model parameters in this specific case by using the recent type Ia supernovae(SNe Ia) data,the cosmic microwave background(CMB) shift parameter data,and the baryonic acoustic oscillations(BAOs) data traced by the Sloan Digital Sky Survey(SDSS).Furthermore,we analyze other constrains from the observational Hubble parameter data(OHD).The comparison with the standard cosmological model(cosmological constant Λ cold dark matter(ΛCDM) model) is clearly shown;also we comment on the interesting relation between the coupling constant M in this model and the special accelerate scale in the modified Newtonian dynamics(MOND) model initially given by Milgrom with the hope for interpreting the galaxy rotation curves without introducing mysterious dark matter.     

Quintessential Inflation with Dissipative Fluid

We have investigated cosmological models with a self-interacting scalar field and a dissipative matter fluid as the sources of matter. Different variables are expressed in terms of a generating function. Exact solutions are obtained for one particular choice of the generating function. The potential corresponding to this generating function is a standard tree-level potential arising in the perturbative regime in quantum field theory. With suitable choice of parameters, the scale factor in our model exhibits both decelerating behaviour in the early time as well as an accelerating phase at late times. For certain choices of the parameter the solution also exhibits an attractor nature towards an asymptotic de-Sitter universe.     

On the resolution of cosmic coincidence problem and phantom crossing with triple interacting fluids

We here investigate a cosmological model in which three fluids interact with each other, involving certain coupling parameters and energy exchange rates. The motivation of the problem stems from the puzzling ‘triple coincidence problem’ which naively asks why the cosmic energy densities of matter, radiation and dark energy are almost of the same order of magnitude at the present time. In our model, we determine the conditions under which triple interacting fluids will cross the phantom divide.     

Modern Cosmological Data and Rotation of the Universe

A new nonstationary rotating cosmological model is developed which describes the evolution of the observed Universe, since its properties are in good agreement with recent astronomical observations. It is demonstrated that the energy density of cosmological rotation can play a role of dark energy and induce the accelerated expansion of the Universe detected recently. In some situations when the causality parameter of the rotating cosmological model is negative, the cosmological rotational energy can be represented as phantom matter that violates the weak energy condition p + ε ≥ 0. The suggested cosmological model has no initial singularity, that is, the cosmological rotation can prevent the formation of the singularity.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 3–6, March, 2005.     

Cosmological model with interactions in the dark sector

A cosmological model for the present Universe is analyzed whose constituents are a non-interacting baryonic matter field and interacting dark matter and dark energy fields. The dark energy and dark matter are coupled through their effective barotropic indexes, which are considered as functions of the ratio of their energy densities. Two asymptotically stable cases are investigated for the ratio of the dark energy densities which have their parameters adjusted by considering best fits to Hubble function data. It is shown that the deceleration parameter, the density parameters, and the luminosity distance have the correct behavior which is expected for a viable present scenario of the Universe.     

Dark energy interacting with dark matter and a third fluid: Possible EoS for this component

A cosmological model of dark energy interacting with dark matter and another general component of the universe is considered. The equations for the coincidence parameters r and s, which represent the ratios between dark energy and dark matter and the other cosmic fluid respectively, are analyzed in terms of the stability of stationary solutions. The obtained general results allow to shed some light on the equations of state of the three interacting fluids, due to the constraints imposed by the stability of the solutions. We found that for an interaction proportional to the sum of the dark energy density and the third fluid density, the hypothetical fluid must have positive pressure, which leads naturally to a cosmological scenario with radiation, unparticle or even some form of warm dark matter as the third interacting fluid.     

Dark energy interacting with two fluids

A cosmological model of dark energy interacting with dark matter and another general component of the universe is investigated. We found general constraints on these models imposing an accelerated expansion. The same is also studied in the case for holographic dark energy.     

A model of the universe with decaying vacuum energy

The consequences of taking the total active gravitational mass of the universe phasewise constant together with a decaying vacuum energy in the background of Robertson-Walker space-time are investigated. The model so determined admits a contracted Ricci-collineation along the fluid flow vectorν ⁱ. It is geometrically closed but ever-expanding and does not possess the initial singularity, horizon, entropy, monopole or cosmological constant problems of the standard big bang cosmology. Estimates of the present matter; radiation and vacuum energy densities, the age of the universe and the present values of the deceleration parameter and the scale factor are also obtained.     

Two-Fluid Anisotropic Cosmological Model with Variable G and Λ

Two-fluid anisotropic Bianchi type-III cosmological model is investigated with variable gravitational constant G and cosmological constant Λ in the framework of Einstein’s general relativity. In the two-fluid model, one fluid represents the matter content of the universe and another fluid is chosen to model the cosmic microwave background radiation. The dynamics of the anisotropic universe with variable G and Λ are discussed. We also discussed in detail the behavior of associated fluid parameters and kinematical parameters.