International Journal of Modern Physics D

The generality of inflation in closed FRW Universe is studied for the models with a scalar field on a brane and with a complex scalar field. The results obtained are compared with the previously known results for the model with a scalar field and a perfect fluid. The influence of the measure chosen in the initial condition space on the ratio of inflationary solution is described.

The effect of strong quantizing magnetic field on low density stellar matter is investigated using Thomas–Fermi–Dirac (TFD) model. The Wigner–Seitz cell structure is assumed for the low density matter. The significant changes in the properties of such low density matter in presence of strong magnetic fields are discussed. It is seen that the decay time scale for magnetic field decreases by at least two orders of magnitude in such a model calculation.

We discuss the Kretschmann, Chern–Pontryagin and Euler invariants among the second order scalar invariants of the Riemann tensor in any spacetime in the Newman–Penrose formalism and in the framework of gravitoelectromagnetism, using the Kerr–Newman geometry as an example. An analogy with electromagnetic invariants leads to the definition of regions of gravitoelectric or gravitomagnetic dominance.

An expression for the electrical conductivity at the core of a magnetar is derived using Boltzmann kinetic equation with the relaxation time approximation. The rates for the relevant scattering processes, e.g., electron–electron and electron–proton scattering processes are evaluated in presence of strong quantizing magnetic fields using tree level diagrams. It is found that in the presence of a strong quantizing magnetic field, electrical conductivity behaves like a second rank tensor. However, if the zeroth Landau levels are only level occupied by the charged particles, it again behaves like a scalar of a one-dimensional system.

We consider a magnetic cylindrically symmetric universe with a plane-symmetric inhomogeneous viscous fluid cosmological model with electromagnetic field. The viscosity coefficient of bulk viscous fluid is assumed to be a power function of mass density. To get a determinate solution, we assume the free gravitational field to be Petrov type-II non-degenerate. Without assuming any ad hoc law, we obtain a cosmological constant as a decreasing function of time. The behaviour of the electromagnetic field tensor together with some physical aspects of the model are also discussed.

If the universe is multiply connected and small the sky shows multiple images of cosmic objects, correlated by the covering group of the 3-manifold used to model it. These correlations were originally thought to manifest as spikes in pair separation histograms (PSH) built from suitable catalogues. Using probability theory we derive an expression for the expected pair separation histogram (EPSH) in a rather general topological-geometrical-observational setting. As a major consequence we show that the spikes of topological origin in PSH's are due to translations, whereas other isometries manifest as tiny deformations of the PSH corresponding to the simply connected case. This result holds for all Robertson–Walker spacetimes and gives rise to two basic corollaries: (i) that PSH's of Euclidean manifolds that have the same translations in their covering groups exhibit identical spike spectra of topological origin, making clear that even if the universe is flat the topological spikes alone are not sufficient for determining its topology; and (ii) that PSH's of hyperbolic 3-manifolds exhibit no spikes of topological origin. These corollaries ensure that cosmic crystallography, as originally formulated, is not a conclusive method for unveiling the shape of the universe. We also present a method that reduces the statistical fluctuations in PSH's built from simulated catalogues.

The Einstein field equations with bulk viscosity and variable G and Λ for Bianchi-type universes are studied under the assumption of a power-law time variation of the expansion factor, achieved via a suitable power-law assumption for the Hubble parameter suggested by M. S. Berman. All the models have a power-law variation of pressure and density and are singular at the epoch t = 0. The variation of G(t) as and Λ(t) as is consistent with these models.

A configuration with four attracting masses in the determination of Newtonian gravitational constant G by the time-of-swing method is proposed. One couple of attracting masses is put in the outer space of the two attracted masses and another couple one is put in the inner space. Because the signals of the two couples of attracted masses will be superposed while the noises due to the uncertainty of the attracted masses separation will be compensated. There exists an optimum separation between the attracted masses, in which the requirement of measuring this separation can be reduced by a factor of about 400.

In this paper, we have studied Brans–Dicke (BD) Cosmology in an anisotropic Kantowski–Sachs space–time model; considering variation of the velocity of light. We have addressed the flatness problem considering both cases namely (i) when the Brans–Dicke scalar field φ is constant (ii) when φ varies, specially for radiation dominated era perturbatively and non-perturbatively and asymptotic behaviour have been studied.

We present a systematic study of collisions of homogeneous, plane- fronted, impulsive light-like signals which do not interact after head-on collision. For the head-on collision of two such signals, six real parameters are involved, three from each of the incoming signals. We find two necessary conditions to be satisfied by these six parameters for the signals to be noninteracting after collision. We then solve the collision problem in the general case when these necessary conditions hold. After collision the two signals focus each other at Weyl curvature singularities on each others signal front. Our family of solutions contains some known collision solutions as special cases.