Incompatibility of five dimensional LRS Bianchi type-V string and mesonic string cosmological models in general relativity

In this paper, we constructed some cosmological models in five dimensional LRS Bianchi type-V space time based on general theory of relativity. Further, it is shown that source density of the meson field does not survive either in massive scalar field or in mass less scalar field. Some physical and geometrical properties of the models are discussed.     

Plane-symmetric dark energy model with a massive scalar field

Investigation of dark energy models in the presence of scalar fields are attracting several kinds of research because they play a vital role in the discussion of a new scenario of accelerated expansion of the universe. In this paper, we obtain an exact plane-symmetric dark energy cosmological model in the presence of an attractive massive scalar field by solving Einstein field equations using some physically relevant conditions. We have obtained all the cosmological parameters corresponding to the model. We have also presented a physical discussion of our model using a graphical representation of these parameters. The results exhibit an expanding and accelerating dark energy model of the universe, which are consistent with modern cosmological observations.     

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.     

A Study of an Inhomogeneous Bianchi-I Model in Lyra Geometry

The inhomogeneous Bianchi-I model based on Lyra's geometry has been studied in the cosmological theory in presence of a massless inhomogeneous scalar field whose potential has a flat part. The field equations are solved using separation of variables and it is shown that one of the time part of the field equations are solvable for any arbitrary other cosmic scale function. Solutions for a particular form of cosmic scale (time part) is presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Teleparallel gravity with scalar and vector fields

We consider a cosmological model in which a scalar field is non-minimally coupled to scalar torsion and a vector field through two coupling functions in the framework of teleparallel gravity. The explicit forms of the coupling functions and the scalar field potential are explored, under the assumption that the Lagrangian admits the Noether symmetry in the Friedmann–Lemaître–Robertson–Walker (FLRW) space–time. The existence of such symmetry allows to solve the equations of motion and achieve exact solutions of the scale factor, scalar and vector fields. It is found that the vector field contributes significantly in the accelerating expansion of the universe in the early times, while the scalar field plays an essential role in the late times.     

Inhomogeneous cosmological models in the presence of massless scalar fields

Non-static inhomogeneous cosmological models are obtained in general relativity for the case of a plane symmetric massless scalar field with cosmological constant A,when the source of the gravitational field is a viscous fluid.Some physical and geometrical behaviors of the solutions are also discussed.     

String effective gravitation and extended inflation

The low-energy string gravitation is investigated for the case of reduction with a variable (in string units) inner space. A flat cosmological model of the corresponding four-dimensional theory is constructed. The model equations are analyzed qualitatively for a potential-dominated scalar field as the source. It is demonstrated that an extended inflation stage with one purely scalar dilaton field is possible here, in contrast with the case of a constant inner space. The pattern of cosmological evolution in various conformal representations is discussed.Translated from Astrofizika, Vol. 38, No. 1, pp. 99–119, January–March, 1995.     

Effect of dark energy on cosmological parameters with LVDP in lyra manifold

We have constructed a model in Lyra manifold and time varying cosmological constant with perfect fluid using LVDP (Linear Varying Deceleration Parameter). Bianchi type-III metric is used as source of investigation. To get a deterministic solution of the field equation the expansion scalar (θ) is considered as proportional to the shear scalar (σ). The cosmological constant is found to be positive which satisfies the result obtained by supernova Type-Ia Observations [1999]. Here we analyse the behaviour of pressure and deceleration parameter by using different form of dark energy(DE). In addition to it, some physical and geometrical properties of the solutions are studied.     

Cosmological dynamics of a non-minimally coupled bulk scalar field in DGP setup

We consider cosmological dynamics of a canonical bulk scalar field, which is coupled non-minimally to 5-dimensional Ricci scalar in a DGP setup. We show that presence of this non-minimally coupled bulk scalar field affects the jump conditions of the original DGP model significantly. Within a superpotential approach, we perform some numerical analysis of the model parameter space and consider bulk-brane energy exchange in this setup. Also we show that the normal, ghost-free branch of the DGP solutions in this case has the potential to realize a self-consistent phantom-like behavior and therefore explains late time acceleration of the universe in a consistent way.     

Searching for α variation and cosmic acceleration in the generalized BSBM theory with tachyonic potential

We consider the BSBM(Bekenstein, Sandvik, Barrow and Magueijo) cosmological model in the presence of tachyon potential with the aim of studying the stability of the model and test it against observations. The phase space analysis shows that from fourteen critical points that represent the state of the universe, only one is stable.With a small perturbation, the universe transits from a state of unstable deceleration to stable acceleration. The stability analysis combined with the best fitting process imposes constraints on the cosmological parameters that are in agreement with observation. In the BSBM theory, the variation of fundamental constants is driven from variation of a scalar field. The tachyonic scalar field, responsible for both variation of fundamental constants and universal acceleration, is reconstructed.     

Multidimensional Cosmology and Asymptotical AdS

A non-linear gravitational model with a multidimensional geometry and quadratic scalar curvature is considered. For certain parameter ranges, the extra dimensions are stabilized if the internal spaces have negative constant curvature. As a consequence, the 4-dimensional effective cosmological constant as well as the bulk cosmological constant become negative. The homogeneous and isotropic external space is asymptotically AdS₄. The connection between the D-dimensional and the 4-dimensional fundamental mass scales sets an additional restriction on the parameters of the considered non-linear models. This revised version was published online in July 2006 with corrections to the Cover Date.     

The inflationary Kantowski-Sachs cosmological model in general relativity

In this paper, we have investigated the inflationary Kantowski-Sachs cosmological model in the presence of mass less scalar field with a flat potential. To get an inflationary solution, we have considered a flat region of a constant potential V. Some physical and kinematical properties of the model are studied.     

Bianchi type–I dark energy cosmological model coupled with a massive scalar field

In the present investigation we are mainly concerned with a massive scalar field in an axially symmetric Bianchi type – I space-time. Einstein field equations are solved to obtain an exact cosmological model. We have used certain physically meaningful conditions for this purpose. Kinematical cosmological parameters are determined, and their dynamical aspects are discussed. It is observed that our model represents accelerated expansion of the Universe. It is observed that our model agrees with the scenario of accelerated expansion of the Universe confirmed by supernova 1a experimental data.     

Cosmological dynamics of a quintom field on the warped DGP brane

As a generalization of the Brans-Dicke type scalar-tensor gravity in a braneworld context, we study cosmological phase space of a braneworld model with induced gravity in the presence of a scalar field on the brane. We consider a quintom field minimally or non-minimally coupled to induced gravity on the warped DGP brane and we present a detailed analysis of the critical points, their stability and late-time cosmological viability of the solutions within a phase space approach. In particular, de Sitter solutions, different from the famous self-accelerated branch of the DGP model are found and the phase-space analysis for checking their attractor properties is performed. We analyze also the possibility of crossing of the phantom divide by the effective equation of state parameter of the model. We also focus on the classical stability of the solutions in w–w′ phase plane.     

A Class of higher dimensional spherically symmetric cosmological model in Lyra geometry

Exact analytical solutions are obtained for a higher dimensional spherically symmetric inhomogeneous metric in presence of a mass-less scalar field with a flat potential within the framework of Lyra geometry. Assuming a homogeneous scalar field, we have shown that the metric can be reduced to a generalized FRW type.     

Exact Kantowski–Sachs cosmological models in general theory of relativity

We have studied cosmological model generated by perfect fluid coupled with mass less scalar field for Kantowski–Sachs space–time in general theory of relativity. Two different physically viable models of the universe are obtained by using a special law of variation for Hubble’s parameter that yields a constant value of deceleration parameter. Some physical consequences of the models have been discussed in case of Zel’dovich fluid.     

Cosmological Massive Scalar Field Interacting with Viscous Fluid

Relativistic cosmological field equations are obtained for a Robertson-Walker space time interacting with viscous fluid and massive scalar field. The cosmological solutions to the field equations are obtained and the nature of the scalar field as well as the viscous fluid are studied. It is found that the solutions obtained are realistic only for a closed Universe. This revised version was published online in July 2006 with corrections to the Cover Date.     

De Sitter type of cosmological model with a self-gravitating nonlinear scalar field

Homogeneous isotropic cosmological solutions are obtained for a de Sitter type of metric in the presence of a self-gravitating scalar field with cubic nonlinearity. Unlike the usual de Sitter case which is indefinitely expanding it is here interestingly found that in the presence of a nonlinear scalar field the model gives a bounce from a maximum of spatial volume. The possibility of bounce from a maximum, however, disappears when a linear scalar field is considered instead.     

Inhomogeneous Mesonic Perfect Fluid Models in Modified Theory of General Relativity

In this paper, we have investigated plane symmetric inhomogeneous cosmological models in the presence of massless scalar field in modified theory of general relativity when source of the gravitational field is a perfect fluid. The physical and geometrical aspects of the models together with singularity involved in the models are also discussed. PACS Nos.: 04.50. + h•4.20.Cv• 4.20.Dw     

Quintessential inflation models with a nonminimally coupled scalar field

The evolution of a homogeneous, isotropic cosmological model driven by a nonminimally coupled scalar field is studied. The potential for the quintessential inflation model proposed by Peebles and Vilenkin is selected as a scalar potential. Possible scenarios for the cosmological dynamics are described in the conformal Einstein and Jordan representations. It is shown that, unlike in models with a minimal scalar field, here a class of solutions exists for which the scalar field is fixed at finite values during cosmological expansion. __________ Translated from Astrofizika, Vol. 49, No. 3, pp. 487–498 (August 2006).