Cosmological viscous fluid models in the presence of electromagnetic field and zero-mass scalar field

The problem of electromagnetic field interacting with viscous fluid without and with zero-mass scalar field has been studied. It has been shown that electromagnetic field cannot interact with viscous fluid for spherically-symmetric Robertson-Walker metric. Exact solutions corresponding to the problem of electromagnetic field interactions in presence of viscous fluid and zero-mass scalar field have been obtained subject to various physical conditions. It presents a scope for the study of imperfect fluid FRW models showing the existence of the electromagnetic field due to the presence of zero-mass scalar field.     

Non-Existence Of <Emphasis Type="Italic">N</Emphasis>-Dimensional Static Plane Symmetric Solutions In Bimetric Relativity Theory

A problem of static plane symmetric metric in the perfect fluid, the mesonic massive scalar field and in their coupling is studied in Rosen’s (1973) bimetric theory of relativity. It was found that the matter field like either perfect fluid or mesonic massive scalar field or their coupling does not survive in bimetric theory of gravitation when the space–time is governed by n-dimensional static plane symmetric metric.     

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.     

Conformally symmetric Abelian Higgs sunspot and non-metricity of the Sun's spacetime

A U(1)-symmetric Yang-Mills-Higgs (i.e., an Abelian Higgs) sunspot's model is recognized to originate from a massless, complex-valued scalar field coupled minimally to electromagnetic gauge potentials in the background of a (globally)conformally symmetric semi-metric spacetime, whose metric structure is described by the generalized Einstein equations with nonvanishing (positive-valued) cosmological constant. It is shown, in particular, that non-linearity (selfcoupling) of the scalar field appears due to a non-zeroness of the cosmological term, whereas its non-zero vacuum amplitude is induced by the (Ricci scalar) curvature of the Sun's spacetime manifold.     

Robertson-Walker type model with conformally invariant scalar field with trace-free energy momentum tensor

A non-static exact solution of the Einstein equations corresponding to a conformally invariant scalar field with trace-free energy momentum tensor is obtained for the Robertson-Walker type metric. Some physical properties of the solution are discussed.     

Corrected Hawking temperature of (2+1) dimensional BTZ (Banados-Teitelboim-Zanelli) rotating Black Hole by using tunneling method

We study the (2+1) dimensional BTZ (Banados-Teitelboim-Zanelli) rotating Black Hole. Along with the scalar field it obeys the Klein-Gordon equation of motion. We use the dragging coordinate system to isolate the r–t sector from the metric. By considering the massless particle and scalar field, we calculate the corrected Hawking temperature with the help of tunneling method.     

Exact Scalar Field Cosmology with Causal Viscous Fluid

An exact cosmological solution for Friedman Robertson Walker (FRW)metric is obtained with a classical scalar field φ along with a potential in the presence of a causal viscous fluid. Assuming the scale factor to be a function of the scalar field we have obtained solutions in both the truncated and the full causal theory. It is shown that an inflationary model can be obtained by imposing certain constraints on some constants. The radiative bulk viscosity during the decoupling era has also been studied as a special case. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Some Bianchi type cosmological models in f(R) gravity

The modified theories of gravity, especially the f(R) gravity, have attracted much attention in the last decade. In this context, we study the exact vacuum solutions of Bianchi type I, III and Kantowski-Sachs spacetimes in the metric version of f(R) gravity. The field equations are solved by taking expansion scalar θ proportional to shear scalar σ which gives A=B ⁿ , where A and B are the metric coefficients. The physical behavior of the solutions has been discussed using some physical quantities. Also, the function of the Ricci scalar is evaluated in each case.     

Inhomogeneous Plane Symmetric Models in a Scalar-Tensor Theory

For the general plane symmetric metric, some exact solutions of Einstein field equations in the scalar tensor theory developed by Saez and Ballestev are presented in vaccum and in the presence of stiff fluid. The physical and kinematical features of the models are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gravitational perfect fluid collapse in f(R) gravity

In this paper, we investigate spherically symmetric perfect fluid gravitational collapse in metric f(R) gravity. We take non-static spherically symmetric metric in the interior region and static spherically symmetric metric in the exterior region of a star. The junction conditions between interior and exterior spacetimes are derived. The field equations in f(R) theory are solved using the assumption of constant Ricci scalar. Inserting their solution into junction conditions, the gravitational mass is found. Further, the apparent horizons and their time of formation is discussed. We conclude that the constant scalar curvature term f(R ₀) acts as a source of repulsive force and thus slows down the collapse of matter. The comparison with the corresponding results available in general relativity indicates that f(R ₀) plays the role of the cosmological constant.     

On Einstein–Rosen Cosmic Strings in a Scalar Tensor Theory of Gravitation

Explicit field equations of a scalar tensor theory of gravitation proposed by Saez and Ballester are obtained with the aid of Einstein–Rosen cylindrically symmetric metric in the presence of cosmic string source. The field equations being highly non–linear static and non–static cases have been considered separately. It is observed that in the static case the geometric strings do not exist while in the non–static case cosmological model does not exist in this theory.     

Einstein-scalar-Yang-Mills black holes: a thermodynamical approach

We try to find out the nature of different thermodynamical parameters for a black hole solution drawn for a special case in Einstein-Scalar-Yang-Mills gravity. Whether a phase transition occurs for the solution or not is a matter of interest. The nature of the phase transition is tried to understand. Ruppeiner metric and the corresponding Ricci scalar is constructed. It is noticed that the metric is not of positive Ricci for all the parametric values and there exist(s) point(s) in thermodynamic space where the Ricci scalar becomes zero.     

Gravitational stability of dark energy in galaxies and clusters of galaxies

We analyze the behavior of the scalar field as dark energy of the Universe in a static world of galaxies and clusters of galaxies. We find the analytical solutions of evolution equations of the density and velocity perturbations of dark matter and dark energy, which interact only gravitationally, along with the perturbations of metric in a static world with background Minkowski metric. It was shown that quintessential and phantom dark energy in the static world of galaxies and clusters of galaxies is gravitationally stable and can only oscillate by the influence of self-gravity. In the gravitational field of dark matter perturbations, it is able to condense monotonically, but the amplitude of density and velocity perturbations on all scales remains small. It was also illustrated that the “accretion” of phantom dark energy in the region of dark matter overdensities causes formation of dark energy underdensities-the regions with negative amplitude of density perturbations of dark energy.     

Plane symmetric solutions to the Einstein’s field equations with dark energy

In this paper, we solve the Einstein’s field equations for the space-time described by a special plane symmetric metric with dark energy, and the exact solutions which offer an alternative and complementary approach to study cosmological models are obtained. The dark energy is given by either the quintessence or the modified Chaplygin gas. We show the models are isotropic and analyze the expansion scalar and the deceleration parameter of the models.     

The scalar, vector and tensor contributions of a stochastic background of magnetic fields to cosmic microwave background anisotropies

We study the contribution of a stochastic background (SB) of primordial magnetic fields (PMFs) on the anisotropies in temperature and polarization of the cosmic microwave background (CMB) radiation. A SB of PMF modelled as a fully inhomogeneous component induces non-Gaussian scalar, vector and tensor metric linear perturbations. We give the exact expressions for the Fourier spectra of the relevant energy–momentum components of such a SB, given a power-law dependence parametrized by a spectral index   n_B   for the magnetic field power spectrum cut at a damping scale k _D. For all the values of   n_B   considered here, the contribution to the CMB temperature pattern by such a SB is dominated by the scalar contribution and then by the vector one at higher multipoles. We also give an analytic estimate of the scalar contribution to the CMB temperature pattern.     

Bianchi VI0 cosmological model in Saez and Ballester theory

The problem of perfect fluid distribution in spatially homogeneous and anisotropic Bianchi type VI₀ space-time is considered in a scalar tensor theory of gravitation proposed by Saez and Ballester (1985). Exact solutions of the field equations are derived when the metric potentials are functions of cosmic time only. Some physical and geometrical properties of the solutions are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gravitational singularities via acceleration: The case of the Schwarzschild solution and Bach's gamma metric

The so called gamma metric corresponds to a two‐parameter family of axially symmetric, static solutions of Einstein's equations found by Bach. It contains the Schwarzschild solution for a particular value of one of the parameters, that rules a deviation from spherical symmetry. It is shown that there is invariantly definable singular behaviour beyond the one displayed by the Kretschmann scalar when a unique, hypersurface orthogonal, timelike Killing vector exists. In this case, a particle can be defined to be at rest when its world‐line is a corresponding Killing orbit. The norm of the acceleration on such an orbit proves to be singular not only for metrics that deviate from Schwarzschild's metric, but also on approaching the horizon of Schwarzschild metric itself, in contrast to the discontinuous behaviour of the curvature scalar.