Emergent universe with scalar (or tachyonic) field in higher derivative theory

We consider a spatially homogeneous and isotropic flat Robertson-Walker model filled with a scalar (or tachyonic) field minimally coupled to gravity in the framework of higher derivative theory. We discuss the possibility of the emergent universe with normal and phantom scalar fields (or normal and phantom tachynoic fields) in higher derivative theory. We find the exact solution of field equations in normal and phantom scalar fields and observe that the emergent universe is not possible in normal scalar field as the kinetic term is negative. However, the emergent universe exists in phantom scalar field in which the model has no time-like singularity at infinite past. The model evolves into an inflationary stage and finally admits an accelerating phase at late time. The equation of state parameter is found to be less than −1 in early time and tends to −1 in late time of the evolution. The scalar potential increases from zero at infinite past to a flat potential in late time. More precisely, we discuss the particular case for phantom field in detail. We also carry out a similar analysis in case of normal and phantom tachyonic field and observe that only phantom tachyonic field solution represents an emergent universe. We find that the coupling parameter of higher order correction affects the evolution of the emergent universe. The stability of solutions and their physical behaviors are discussed in detail.     

Variable relation between string densities

In this work we have proposed a variable relation between densities of cosmic string. Bianchi type II,VIII and IX space time is explored in the context of general relativity. The field equations are solved by assuming that the shear scalar is proportional to expansion scalar. Three different cases are investigated. It is observed that strings contribute significantly in isotropy and acceleration of the universe.     

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.     

On real-time evolution in cosmology

Real-time evolution plays an important role to understand the dynamics of the early Universe. It would be of importance to be able to investigate such typical time dependent processes like particle production, reheating, creation and evolution of fluctuations, etc. In this paper we derive the one-loop renormalized coupled einstein field equations of a scalar field with λϕ⁴ interaction in a classical curved space-time of Friedmann-Robertson-Walker type. These equations can be used to calculate quantum corrections for the dynamics in the early Universe.     

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.     

Interacting holographic dark energy with variable deceleration parameter and tachyon scalar field dark energy model in LRS Bianchi type-II universe

In this work, we have considered the spatially homogeneous and anisotropic Bianchi type-II universe filled with two interacting fluids; dark matter and holographic dark energy components. Assuming the proportionality relation between one of the components of shear scalar and expansion scalar which yields time dependent deceleration parameter, an exact solution to Einstein’s field equations in Bianchi type-II line element is obtained. We have investigated geometric and kinematics properties of the model and the behaviour of the holographic dark energy. It is observed that the mean anisotropic parameter is uniform through the whole evolution of the universe and the coincidence parameter increases with increasing time. The solutions are also found to be in good agreement with the results of recent observations. We have applied the statefinder diagnostics method to study the behaviour of different stages of the universe and to differentiate the proposed dark energy model from the ΛCDM model. We have also established a correspondence between the holographic dark energy model and the tachyon scalar field dark energy model. We have reconstructed the potential and the dynamics of the tachyon scalar field, which describes accelerated expansion of the universe.     

Massive scalar field in the Bianchi Type I space time

A class of exact solutions of Eistein's field equations with attractive massive scalar field in LRS Bianchi type I space time is obtained. It is shown that how the dynamical importance of the scalar field and the shear change in the course of evolution. This revised version was published online in July 2006 with corrections to the Cover Date.     

Massless scalar field in Robertson-Walker space-time

Recently, exact spatially-homogeneous solutions have been found for a conformally invariant massless scalar fieldS(t) coupled to a Robertson-Walker geometry. We investigate extending these solutions to inhomogeneous scalar fieldsS(t, x ^l ). For an isotropic scalar fieldS(t, r) we find a solution satisfying three of the five field equations. If we use REDUCE, we show that the remaining equations forceS=S(t).     

Torsion,massive electrodynamics and gravitation induced by scalar fields

The equivalence of Lagrangian containing gravitational, electromagnetic, scalar, and torsion fields is discussed. It is shown that the equation for the variation of the scalar field leads to a torsion wave equation generated by electromagnetic field leads to a torsion wave equation generated by electromagnetic fields. The system is proved to be equivalent to a Proca field coupling torsion non-minimally to a massive photon and having the scalar Higgs field as a strength of this photon-torsion coupling. The generalized Maxwell equations containing the scalar fields are obtained. The torsion potential around the Sun or a more massive collapsing star in the weak field limit is estimated.     

Open and closed Robertson-Walker-type Lyttleton-Bondi universes

The exterior field of the Robertson-Walker-type metric in the Lyttleton-Bondi universe is studied and exact solutions are obtained for closed and open universes. Only the flat space solution was previously known.     

The plane symmetric vacuum solutions of modified field equations in metric f(R) gravity

The f(R) theories of gravity have been interested in recent years. A considerable amount of work has been devoted to the study of modified field equations with the assumption of constant Ricci scalar which may be zero or nonzero. In this paper, the exact vacuum solutions of plane symmetric spacetime are analyzed in f(R) theory of gravity. The modified field equations are studied not only for R=constant but also for general case R≠constant. In particular, we show that the Novotný-Horský and anti-de Sitter spacetimes are the exact solutions of the field equations with the non-zero constant Ricci scalar. Finally, the family of solutions with R≠constant is obtained explicitly which includes the Novotný-Horský, Kottler-Whittaker, Taub and conformally flat spacetimes.     

Birkhoff's theorem in a conformally-invariant scalar field theory

A result having formal similarity to Birkhoff's theorem in general relativity is proved, both in vacuum as well as in the presence of electromagnetic fields, in a conformally-invariant scalar field theory with trace-free energy-momentum tensor for the special case when the scalar field is independent of time.     

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.     

Scalar field instability in multi-dimensional cosmology

The scalar field theory on the background of cosmological models with n(n ≥ 1) spaces of constant curvature is considered. We take the integrable case of Ricci flat internal spaces. The coupling between the scalar and the gravitational fields includes the minimal coupling as well as the conformal case. In the ground state of the scalar field we find the conditions for vacuum instability realized for most of the possible solutions to Einstein's equations if the coupling parameter takes appropriate values. For the excited states of the scalar field we show the induction of massive modes and discuss their properties.     

Stability of viscous fluid in Bianchi type-VI model with cosmological constant

In this paper, we investigate Bianchi type-VI cosmological model for the universe filled with dark energy and viscous fluid in the presence of cosmological constant. Also, we show accelerating expansion of the universe by drawing volume scale, pressure and energy density versus cosmic time. In order to solve the Einstein’s field equations, we assume the expansion scalar is proportional to a component of the shear tensor. Therefore, we obtain the directional scale factors and show the EOS parameter crosses over phantom divided-line.     

Conformal analogs of the Jordan-Brans-Dicke theory. II. (models including vacuum effects)

This paper is part of a series based on a modified Jordan tensor-scalar theory of gravitation. Given the current importance of research on vacuum phenomena in cosmic evolution, we examine several standard cosmological models with a scalar field and a physical vacuum, including models that have a dominant scalar field with the vacuum energy taken into account in various conformal representations of the Jordan theory, as well as models in which ordinary matter that obeys the conventional equations of state is present. Some noteworthy results are obtained which are, to a certain extent, consistent with currently available observational data.     

Axial perturbations of Wyman's solution

Wyman's solution is the most general solution to the static spherically-symmetric Einstein massless scalar field equations. It is shown that it has no axial perturbation in which the scalar field is incremented, except in the case where the initial scalar field and the cross-metric increments are negligible. The one dimensional Schrödinger equation which governs axial metrix perturbations is produced.     

Cosmic shear in inflationary models of Bianchi types VIII and IX

Two exact solutions of Einstein's field equations of vacuum are presented and investigated. We will regard the term vacuum fluid as the limiting case of scalar field with an almost constant potential. Considering the four velocity of this fluid we find, that in both solutions there is an anisotropic expansion of the cosmic fluid, but the fluid has vanishing vorticity.We investigate whether shear could prevent the transition into an inflationary era in these models, and the effect of shear on a scalar field is also considered. It is found that shear will speed up the rollover of the scalar field in some Bianchi type-VIII models.Possible initial conditions are discussed in light of the group structures of the models.     

On static spherical symmetric solutions of the Bach-Einstein gravitational field equations

For field equations of 4th order, follwing from a Lagrangian “Ricci scalar plus Weyl scalar”, it is shown (using methods of non-standard analysis) that in a neighbourhood of Minkowski space there do not exist regular static spherically symmetric solutions. With that (besides the known local expansions about r = o nad r = ∞ resp.) for the first time a global statement on the existence of such solutions is given. Finally, this result will be discussed in connection with Einstein's particle programme.     

Dynamics of warm inflation with gauge fields in Bianchi type I universe model

In this paper, we study the dynamics of warm inflation in which slow-roll inflation is driven by non-Abelian gauge fields. To this end, we use the geometry of locally rotationally symmetric Bianchi type I universe model. We construct dynamical equations, i.e., first model field equation, energy conservation equations and equation of motion under slow-roll approximation. In order to discuss inflationary perturbations, we evaluate parameters like scalar and tensor power spectra as well as scalar and tensor spectral indices. We also evaluate inflaton, directional Hubble parameter, slow-roll and perturbation parameters as well as tensor-scalar ratio as a function of inflaton during intermediate and logamediate inflationary eras. It is concluded that anisotropic inflationary universe model with non-Abelian gauge fields remains compatible with WMAP7.