Bianchi Type I Viscous Universe with Variable G and Λ

Exact solutions for an anisotropic Bianchi type I model with bulk viscosity and variable G and are obtained. We have found some solutions that correspond to our earlier work for the isotropic one. Unlike Kalligas et al., an inflationary solution with a variable energy density has been found where the anisotropy energy decreases exponentially with time. There is a period of hyper-inflation during which the energy density remains constant.     

Generation of Bianchi type V cosmological models with varying Λ-term

Bianchi type V perfect fluid cosmological models are investigated with cosmological term varying with time. Using a generation technique (Camci et al., 2001), it is shown that Einsteins field equations are solvable for any arbitrary cosmic scale function. Solutions for particular forms of cosmic scale functions are also obtained. The cosmological constant is found to be a decreasing function of time, which is supported by results from recent type Ia supernovae observations. Some physical aspects of the models are also discussed.     

Anisotropic Universe Models with Perfect Fluid and Dark Energy with Time Varying G and Λ

We have investigated general Bianchi type I cosmological models which containing a perfect fluid and dark energy with time varying G and Λ that have been presented. The perfect fluid is taken to be one obeying the equation of state parameter, i.e., p=ωρ; whereas the dark energy density is considered to be either modified polytropic or the Chaplygin gas. Cosmological models admitting both power-law which is explored in the presence of perfect fluid and dark energy too. We reconstruct gravitational parameter G, cosmological term Λ, critical density ρ _c , density parameter Ω, cosmological constant density parameter Ω _Λ and deceleration parameter q for different equation of state. The present study will examine non-linear EOS with a general nonlinear term in the energy density.     

Higher Dimensional Λ-CDM Universe: A Phenomenological Approach with Many Possibilities

In this paper we investigate the Λ-CDM universe by selecting the specific time dependent form of Λ, viz. , in the context of 5-dimensional space time. Time-dependent form of equation of state parameter w is derived along with a possible signature flip of deceleration parameter q. It is also observed that a present age of the Universe, calculated for some specific values of parameter agrees very well with the observational data.     

Flat FRW models with variableG and Λ

We consider Einstein's equations with variable gravitational couplingG and cosmological term . For a power-law time-dependence ofG, the cosmological term varies in proportion to the inverse square of the time, provided the equation of state is not that of vacuum. There is then no dimensional constant associated with . For a vacuum equation of state the model is compatible with classical inflation for a wide class of functionsG(t) and (t). For non-power-law behaviour ofG(t), it is possible to have a scale factor that increases exponentially without a vacuum equation of state. For this case the energy density associated with decreases exponentially, while at time zero it is equal with opposite sign to the regular energy density, so there is zero total energy initially.     

Phenomenological Λ-nuclear interactions

Variational Monte-Carlo calculations have been performed for the ligh s-shell hypernuclei, namely, _Λ ⁴ H, _Λ ⁴ H^*, and _Λ ⁵ He. The main aim of the study has been to give more insight into the Λ-nuclear interactions. Our study shows that the three-body ANN force has a larger share in the splitting energy of _Λ ⁴ H (0⁺–1⁺) compared to the two-body AN force. The analyses on Λ-binding to nuclear matter based on our s-shell results demonstrate that the three-body ANN correlations need to be incorporated suitably to describe these systems.     

Anisotropic Viscous Cosmology with Variable G and Λ

Einstein's equations with variable gravitational and cosmological constants are considered in the presence of bulk viscosity for a Bianchi type I model in a way which conserves the energy momentum tensor. Several solutions are found, one of which corresponds to the earlier solution found by Tarkeshwar Singh et al. for the isotropic case. Our approach is compared with that of Arbab.     

Scenario of Accelerating Universe: Role of Phenomenological Λ Models

Dark matter, the major component of the matter content of the Universe, played a significant role at early stages during structure formation. But at present the Universe is dark energy dominated as well as accelerating. Here, the presence of dark energy has been established by including a time-dependent Λ term in the Einstein’s field equations. This model is compatible with the idea of an accelerating Universe so far as the value of the deceleration parameter is concerned. Possibility of a change in sign of the deceleration parameter is also discussed. The impact of considering the speed of light as variable in the field equations has also been investigated by using a well known time-dependent Λ model.     

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.     

Higher Dimensional FRW Cosmology with Variable G and Λ

We have considered higher dimensional cosmological models of the FRW model with variable G and Λ. The solutions have been obtained for flat model with particular form of cosmological constant. The cosmological parameters have also been obtained for dust, radiation and stiff matter. Physical parameters of the models are discussed.     

Viscous Dark Energy Models with Variable G and Λ

We consider a cosmological model with bulk viscosity η and variable cosmological A ∝p^-α, alpha = const and gravitational G constants. The model exhibits many interesting cosmological features. Inflation proceeds du to the presence of bulk viscosity and dark energy without requiring the equation of state p =-p. During the inflationary era the energy density p does not remain constant, as in the de-Sitter type. Moreover, the cosmological and gravitational constants increase exponentially with time, whereas the energy density and viscosity decrease exponentially with time. The rate of mass creation during inflation is found to be very huge suggesting that all matter in the universe is created during inflation.     

Bulk Viscous Cosmological Models with Variable G and Λ

Einstein's equations with variable gravitational and cosmological constants are considered in the presence of bulk viscosity for the spatially flat homogeneous and isotropic universe in a way which conserves the energy momentum tensor. A solution is found in which the cosmological term varies inversely with the square of time. Our approach is compared with that of Arbab.     

Cosmological models with positive scalar spatial curvature and Λ>0

Some exact spherically symmetric solutions of the Einstein field equations with Λ>0 and positive three-curvature are given. They have reasonable physical properties and represent universes which do not undergo inflation but have a non-de Sitter behaviour for large times. This paper extends some previous results in the literature.     

Plane-symmetric Inhomogeneous Magnetized Viscous Fluid Universe with a Variable Λ

The behavior of magnetic field in plane symmetric inhomogeneous cosmological models is investigated for bulk viscous distribution. The coefficient of bulk viscosity is assumed to be a power function of mass density ( = ₀ p ⁿ). The values of cosmological constant for these models are found to be small and positive which is supported by the results from recent supernovae Ia observations. Some physical and geometric aspects of the models are also discussed.     

Cylindrically symmetric cosmological model in the presence of bulk stress with varying Λ

Cylindrically symmetric non-static space–time is investigated in the presence of bulk stress given by Landau and Lifshitz. To get a solution, a supplementary condition between metric potentials is used. The viscosity coefficient of the bulk viscous fluid is assumed to be a power function of mass density whereas the coefficient of shear viscosity is considered as proportional to the scale of expansion in the model. Also some physical and geometrical properties of the model are discussed.     

Phenomenology of Λ-CDM Model: A?Possibility of?Accelerating Universe with Positive Pressure

Among various phenomenological Λ models, a time-dependent model [(L)\dot] ~ H³\dot{\Lambda}\sim H^{3} is selected here to investigate the Λ-CDM cosmology. The model can follow from dynamics, underlying the origin of Λ. Using this model the expressions for the time-dependent equation of state parameter ω and other physical parameters are derived. It is shown that in H ³ model accelerated expansion of the Universe takes place at negative energy density, but with a positive pressure. It has also been possible to obtain the change of sign of the deceleration parameter q during cosmic evolution.     

Bianchi type I cosmological models with variableG and Λ: A comment

We treat in an alternate way a problem recently considered by Beesham [1]. We find that anisotropic Bianchi I inflationary cosmologies with variable gravitational and cosmological constants admit de Sitter expansion at least for late times.     

Spatially Homogeneous Bianchi Type-I Universes with Variable G and Λ

In this paper, we have studied the cosmological models of Bianchi type-I universes in a different basic form filled with bulk viscous fluid, in the presence of time-dependent gravitational as well as cosmological constants. A set of new exact cosmological solutions have been obtained in both full and truncated causal theories. These solutions are suitable for describing the evolution of the universe in its early stages. The physical and dynamical consequences have been discussed in detail.