Thepresent study deals with spatially homogeneous and anisotropic Bianchi-I cosmological models representing massive strings with variable G and decaying vacuum energy density $ \Lambda$ . The energy-momentum tensor, as formulated by Letelier (Phys. Rev. D 20, 1294 (1979); Phys. Rev. D 28, 2414 (1983)), has been used to construct massive string cosmological models for which we assume the expansion scalar in the models is proportional to one of the components of shear tensor and barotropic EoS. The Einstein field equations have been solved by considering the time-dependent deceleration parameter which yields a scale factor $ a(t) = (\sinh(\alpha t))^{\frac{1}{n}}$ , where n is a positive constant. For n > 1 , this generates a transition of the Universe from the early decelerating phase to the recent accelerating phase and the transition redshift zt has been calculated. The study reveals that massive strings dominate the early Universe evolving with deceleration and in the later phase they disappear, which is in good agreement with current astronomical observations. The cosmological constant $ \Lambda$ is found to be a positive decreasing function of time which is corroborated by results from recent Supernovae Ia observations. The physical and geometric properties of the models have been also discussed in detail.

String cosmological models from early deceleration to current acceleration phase with varying G and $ \Lambda$

The present study deals with spatially homogeneous and totally anisotropic locally rotationally symmetric (LRS) Bianchi type I cosmological model with variable $G$ and $\Lambda$ in presence of imperfect fluid. To get the deterministic model of Universe, we assume that the expansion $(\theta)$ in the model is proportional to shear $(\sigma)$. This condition leads to $A=\ell B^{n}$, where $A$,\;$B$ are metric potential. The cosmological constant $\Lambda$ is found to be decreasing function of time and it approaches a small positive value at late time which is supported by recent Supernovae Ia (SN Ia) observations. Also it is evident that the distance modulus curve of derived model matches with observations perfectly.

Bulk Viscous LRS Biachi-I Universe with variable $G$ and decaying $\Lambda$

The present study deals with spatially homogeneous and totally anisotropic locally rotationally symmetric (LRS) Bianchi type I cosmological model with variable G and Λ in presence of imperfect fluid. To get the deterministic model of Universe, we assume that the expansion (θ) in the model is proportional to shear (σ). This condition leads to A=lBn, where A, B are metric potential. The cosmological constant Λ is found to be decreasing function of time and it approaches a small positive value at late time which is supported by recent Supernovae Ia (SN Ia) observations. Also it is evident that the distance modulus curve of derived model matches with observations perfectly.

Bulk viscous LRS Bianchi-I Universe with variable G and decaying Λ

This paper is devoted to the study of role of particle creation and bulk viscosity on evolution of homogeneous and anisotropic model of the universe represented by Bianchi type I space time metric. Particle creation and bulk viscosity have been considered as separated irreversible processes. In order to discuss physical and geometrical behaviour of the model, a new set of exact solutions of Einstein’s field equation have been obtained in non-causal, truncated and full causal theories. Dynamical behaviours of models have also been discussed.

Anisotropic bulk viscous cosmological models with particle creation

The thermodynamical effect of particle creation on the early evolution of Friedmann-Robertson-Walker universe is analyzed in the context of open thermodynamical systems within the framework of Brans-Dicke theory. The field equations are solved exactly using the “gamma-law” equation of state p=(γ−1)ρ, where the adiabatic parameter γ depends on the scale factor of the universe. We assume the functional form of γ in such a way that it describes a unified description of two early phases of the evolution of the universe, viz. inflationary phase and radiation-dominated phase. The role of Brans-Dicke scalar and creation of matter particles is investigated during the expansion of the universe. It is found that the expansion of the universe is driven by the particle creation in each phase. The physical and geometrical behaviors of the solutions are discussed in detail.

https://www.tpbin.com/Uploads/Subjects/74a5d6eb-ab56-4a87-bf1a-af0414e74d3c.pdf

FRW models with particle creation in Brans-Dicke theory

The efiect of bulk viscosity on the evolution of the spatially ∞at Friedmann{Lemaitre{ Robertson{Walker (FLRW) models in the context of open thermodynamical systems, which allow for particle creation, is analysed within the framework of Brans{Dicke (BD) theory. The BD fleld equations are modifled with the incorporation of a creation pressure and bulk viscous stress. A class of physically plausible models has been taken into consideration. The behaviour of the particle number density and bulk viscosity is discussed with the evolution of the Brans{Dicke scalar fleld.

/pdf/bulk-viscosity-and-particle-creation-in-brans-dicke-theory-40xu9ntlij.pdf

G. P. Singh

Papers

Bulk viscosity and particle creation in Brans—Dicke theory