International Journal of Modern Physics D

The stable version of the anomaly-induced inflation does not need a fine tuning to induce sufficient expansion of the Universe. The non-stable version (Starobinsky model) provides the graceful exit to the FRW phase. Here, we indicate the possibility of the inflation which is stable at the beginning and unstable at the end. The effect is due to the soft supersymmetry breaking and the decoupling of the massive sparticles at low energy.

We consider the dynamics of a Bianchi type I spacetime in the presence of dilaton and magnetic fields. The general solution of the Einstein–Maxwell dilaton field equations can be obtained in an exact parametric form. Depending on the numerical values of the parameters of the model there are three distinct classes of solutions. The time evolution of the mean anisotropy, shear and deceleration parameters is considered in detail and it is shown that a magnetic-dilaton anisotropic Bianchi type I geometry does not isotropize, the initial anisotropy being present in the universe for all times.

Based on the criticism of existing models of intermittency in magnetohydrodynamic (MHD) turbulence,^2,4,5 a new model for the description of intermittency corrections in MHD turbulence is presented and compared with the other models, and is consistent with observational data in the solar wind.

Using semi-analytic approach, we present an estimate of the properties of the largest virialized dark halos in the present universe for three different scenarios of structure formation: SCDM, ΛCDM and OCDM models. The resulting virial mass and temperature increase from the lowest values of 1.6 × 10¹⁵ h^-1 M_⊙ and 9.8 keV in OCDM, the mid-range values of 9.0 × 10¹⁵ h^-1 M_⊙ and 31 keV in ΛCDM, to the highest values of 20.9 × 10¹⁵h^-1M_⊙, 65 keV in SCDM. As compared with the largest virialized object seen in the universe, the richest clusters of galaxies, we can safely rule out the OCDM model. In addition, the SCDM model is very unlikely because of the unreasonably high virial mass and temperature. Our computation favors the prevailing ΛCDM model in which superclusters may be marginally regarded as dynamically-virialized systems.

Barber's second self creation theory with perfect fluid source for an LRS Bianchi type-I metric is considered using deceleration parameter to be constant where the metric potentials are taken as functions of x and t. In particular, some exact solutions have also been obtained for the vacuum universe, Zel'dovich universe and radiation universe. Some physical properties of the models are also discussed.

A gauge invariant theory of gravitational fluctuations of brane-world model is presented. Without resorting to any specific gauge choice, a general method is presented in order to disentangle the fluctuations of the energy–momentum tensor of the brane from the fluctuations of the metric. The employed procedure is gauge-invariant at every step. As an application of the formalism we address the localization of metric fluctuations on scalar branes breaking spontaneously five-dimensional Poincaré invariance is addressed. Assuming that the four-dimensional Planck mass is finite and that the geometry is regular, it is demonstrated that the vector and scalar fluctuations of the metric are not localized on the brane.

We study the propagation of electromagnetic waves in a spacetime devoid of a metric but equipped with a linear electromagnetic spacetime relation H ~ χ · F. Here H is the electromagnetic excitation and F the field strength (E,B), whereas χ (having 36 independent components) characterizes the electromagnetic permittivity/permeability of spacetime. We derive analytically the corresponding Fresnel equation and show that it is always quartic in the wave covectors. We study the "Fresnel tensor density" as (cubic) function of χ and identify the leading part of χ (20 components) as indispensable for light propagation. Upon requiring electric-magnetic reciprocity of the spacetime relation, the leading part of χ induces the light cone structure of spacetime (9 components), i.e., the spacetime metric up to a function. The possible existence of an Abelian axion field (1 component of χ) and/or of a skewon field (15 components) and their effect on light propagation is discussed in detail. The newly introduced skewon field is expected to be T-odd and related to dissipation.

A new concept of data analysis has been attempted for studying the long/short term daily variations in cosmic ray (CR) intensity recorded with neutron monitors/meson telescopes. Fourier Technique has been applied on four different types of groups of days chosen according to their different geomagnetic conditions. The selected groups are 60 quietest days (60 QD), 120 quiet days (120 QD), continuous quiet days (CQD) and All Days (AD) in a year. CQD is a new set of days selected on the basis of Ap and Kp values. These are the days when transient magnetic variations are supposed to be regular and smooth continuously for a span of at least three days. The criteria of selection of CQD is based upon the mathematical manipulation with Ap index. The data of Deep River neutron monitoring station is used for the period 1985–1995, to investigate for a com-parative study of diurnal anisotropy in CR intensity on 60 QD, 120 QD, CQD and AD. It is observed that 60 QD are most suitable for the anisotropic studies on short/long term basis. The time/spatial variations in the amplitude and phase of the diurnal anisotropy become more pronounced for 60 QD for the period under investigation.

We derive the Teixeira, Wolk and Som method,¹ for obtaining electrostatic solutions from some given vacuum solutions, in its inverse form. Then we use it to obtain the geometrical mass M_S in the Schwarzschild spacetime, and we find , where M and Q are, respectively, the mass and charge parameters of the Reissner–Nordström spacetime. We compare M_S to the corresponding active gravitational mass and mass function.

We consider the evolution of a flat Friedmann–Robertson–Walker Universe, filled with a causal bulk viscous cosmological fluid, in the presence of variable gravitational and cosmological constants. The basic equation for the Hubble parameter, generalizing the evolution equation in the case of constant gravitational coupling and cosmological term is derived, under the supplementary assumption that the total energy of the Universe is conserved. By assuming that the cosmological constant is proportional to the square of the Hubble parameter and a power law dependence of the bulk viscosity coefficient, temperature and relaxation time on the energy density of the cosmological fluid, two classes of exact solutions of the field equations are obtained. In the first class of solutions the Universe ends in an inflationary era, while in the second class of solutions the expan-sion of the Universe is noninflationary for all times. In both models the cosmological "constant" is a decreasing function of time, while the gravitational "constant" increases in the early period of evolution of the Universe and tends in the large time limit to a constant value.

Using two different pseudo-Schwarzschild potentials proposed by Artemova et al.,¹ we formulate and solve the equations governing spherically symmetric transonic inflow and outflow in presence of a relativistic hadronic-pressure-mediated steady, standing, spherical shock around the central compact object. Then we self-consistently connect the accretion-wind solutions to calculate the mass outflow rate in terms of a minimum number of flow parameters. We also study the dependence of this rate on various boundary conditions governing the flow.

Here we study the accretion process around neutron stars, especially for the cases where shock does form in the accretion disk. In case of accretion flows around a black hole, close to the horizon the matter is supersonic. On the other hand for the case of neutron stars and white dwarfs, matter must be subsonic close to the inner boundary. So the nature of the inflowing matter around neutron stars and white dwarfs are strictly different from that around black holes in the inner region of the disk. Here we discuss a few phenomena and the corresponding solutions of hydrodynamic equations of matter in an accretion disk around slowly rotating neutron stars without magnetic field.

We provide a Hamiltonian analysis of the Mixmaster Universe dynamics showing the covariant nature of its chaotic behavior with respect to any choice of time variable. Asymptotically to the cosmological singularity, we construct the appropriate invariant measure for the system (which relies on the appearance of an "energy-like" constant of motion) in such a way that its existence is independent of fixing the time gauge, i.e. the corresponding lapse function. The key point in our analysis consists of introducing generic Misner–Chitré-like variables containing an arbitrary function, whose specification allows us to set up the same statistical scheme in any time gauge.

We describe the Kerr black hole in the ingoing and outgoing Kerr–Schild horizon penetrating coordinates. Starting from the null vector naturally defined in these coordinates, we construct the null tetrad for each case, as well as the corresponding geometrical quantities allowing us to explicitly derive the field equations for the perturbed scalar projections Ψ₀⁽¹⁾ and Ψ₄⁽¹⁾ of the Weyl tensor, including arbitrary source terms. This perturbative description, including arbitrary sources, described in horizon penetrating coordinates is desirable in several lines of research on black holes, and contributes to the implementation of a formalism aimed to study the evolution of the spacetime in the region where two black holes are close together.