Bianchi type I cosmological models with variableG and A

Einstein's equations with variableG and A scalars are considered for a Bianchi type I metric. A solution is found in which the cosmological term varies inversely with the square of time. As in the case of the flat Friedmann-Lemaître-Robertson-Walker (flrw) models discussed recently, there is then no dimensional constant associated with A. However, it is shown that the time behaviour of Bianchi type I inflationary solutions cannot be of the pure de Sitter type. This shows that if the flat FLRW inflationary solutions previously considered are perturbed by the introduction of Bianchi type I anisotropy, then the time evolution may be perturbed from the pure exponential form.     

A Bianchi type-II dark-energy cosmology with a decaying Λ-term in the Brans–Dicke theory of gravity

We present a sequence of anisotropic Bianchi type-II dark-energy models in the framework of the Brans–Dicke theory of gravity with a variable equation of state (EoS) parameter and a constant deceleration parameter. We use power-law relations between the scalar field φ and the scale factor A and between the average Hubble parameter H and the average scale factor A to obtain most of the analytic solutions. The dark-energy EoS parameter ω and its range admitted by the models agrees well with the most recent observational data. It has been observed that the cosmological constant Λ is decreasing with time, which is consistent with recent cosmological observations. We study the dynamical stability and physical features of the models.     

Cosmological models with constant deceleration parameter and bulk viscous mode

Recently Berman and Gomide have presented cosmological models with a constant deceleration parameter in general relativity without assuming a specific equation of state. It is shown that these models are equivalent to those with bulk viscosity. Some general remarks are made on the former models, including implications for violations of causality.     

Early Universe in Scalar-Tensor Theory

We consider the flat Robertson–Walker model in scalar-tensor theory proposed by Lau and Prokhovnik. In this model, the field equations are solved by using “gamma-law” form of equation of state p=(γ−1)ρ, where the adiabatic parameter ‘gamma’ (γ) varies continuously as the universe expands. Our aim is to study how the adiabatic parameter γ should vary so that in the course of its evolution the universe goes through a transition from an inflationary to a radiation-dominated phase. A unified one parameter function of γ has been considered to describe the two early phases of evolution of universe. The solutions show the power-law expansion and cosmological constant is found to be positive and decreasing function of cosmic time. The solutions are compatible with the Dirac’s large number hypothesis. The deceleration parameter has been presented in a unified manner in terms of scale factor, which describes the inflation of the model. The nature of singularity and the physical properties have been discussed in details.     

Locally-rotationally-symmetric Bianchi type-V cosmology with heat flow

In this paper we present a spatially homogeneous locally-rotationally-symmetric (LRS) Bianchi type-V cosmological model with perfect fluid and heat flow. A general approach is introduced to solve Einstein’s field equations using a law of variation for the mean Hubble parameter, which is related to average scale factor of the model that yields a constant value for the deceleration parameter. Exact solutions that correspond to singular and non-singular models are found with heat flow. The physical constraints on the solution and, in particular, the thermodynamical laws that govern such solutions are discussed in some detail.     

Whitrow-Randall's relation and Brans-Dicke cosmology re-visited

The homogeneous and isotropic Brans-Dicke cosmological solutions satisfying Whitrow-Randall's relation which have been discussed recently by Berman and Som are re-examined. After correcting an error in their work, we extend their results and present the most general solution by solving a differential equation completely. We show that Mach's assumption leads to power-law solutions in the Euclidean case.     

Variable-G cosmology and creation

It has recently been asserted that a universe with a time-varying gravitational constantG necessarily implies creation if the rest mass of matter particles is constant. It is shown that this is not necessarily true. An example of a cosmological model with variableG and is presented, in which there is no creation and in which the rest mass of matter particles is constant.     

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.     

Emergence and expansion of cosmic space in an accelerating BIon

The European Physical Journal C - In this paper, we calculate the $$B\rightarrow D$$ transition form factors (TFFs) within the light-cone sum rules (LCSRs) and predict the ratio $$\mathcal {R}(D)$$...     

Causal dissipative cosmology

The full version of the causal thermodynamics of non-equilibrium phenomena is discussed in the context of the flat Friedmann-Robertson-Walker cosmological model. Power law solutions for the scale factor are shown to exist. It is also shown that the temporal behaviour of the temperature depends on the functional dependence of the coefficient of bulk viscosity on density.