International Journal of Modern Physics D

Circular orbits are examined in static spacetimes belonging to the Weyl class of vacuum solutions which represent (nonlinear) superposition of the gravitational fields generated by certain collinear distributions of matter. In particular, solutions representing two and three Chazy–Curzon particles — all of them endowed with conical singularities — are considered. Conditions for geodesic motion in certain symmetry planes are discussed and results are summarized in a number of graphics too. All the discussion is developed in the framework of observer-dependent analysis of motion.

The present day observations favour a universe which is flat, accelerated and composed of ~ 1/3 matter (baryonic + dark) and ~ 2/3 of a negative pressure component, usually referred to as dark energy or quintessence. The Cosmic Lens All Sky Survey (CLASS), the largest radio-selected galactic mass scale gravitational lens search project to date, has resulted in the largest sample suitable for statistical analysis. In the work presented here, we exploit observed image separations of the multiply-imaged lensed radio sources in the sample. We use two different tests: (1) image separation distribution function n(Δθ) of the lensed radio sources and (2) Δθ_pred versus Δθ_obs as observational tools to constrain the cosmological parameters w and Ω_m. The results are in concordance with the bounds imposed by other cosmological tests.

Using Einstein and Papapetrou energy–momentum complexes, we explicitly calculate the energy and momentum distribution associated with spacetime homogeneous Gödel-type metrics. We obtain that the two definitions of energy-momentum complexes do not provide the same result for these type of metrics. However, it is shown that the results obtained are reduced to the energy–momentum densities of Gödel metric already available in the literature.

We discuss the inhomogeneous multidimensional mixmaster model in view of the appearing, near the cosmological singularity, of a scenario for the dimensional compactification in correspondence to an 11-dimensional space–time. Our analysis candidates such a collapsing picture toward the singularity to describe the actual expanding 3-dimensional Universe and an associated collapsed 7-dimensional space. To this end, a conformal factor is determined in front of the 4-dimensional metric to remove the 4-curvature divergences and the resulting Universe expands with a power-law inflation. Thus we provide an additional peculiarity of the eleven space-time dimensions in view of implementing a geometrical theory of unification.

It is argued that the "problem of time" in quantum gravity necessitates a refinement of the local inertial structure of the world, demanding a replacement of the usual Minkowski line element by a (4+2n)-dimensional pseudo-Euclidean line element, with the extra 2n being the number of internal phase space dimensions of the observed system. In the refined structure, the inverse of the Planck time takes over the role of observer-independent conversion factor usually played by the speed of light, which now emerges as an invariant but derivative quantity. In the relativistic theory based on the refined structure, energies and momenta turn out to be invariantly bounded from above, and lengths and durations similarly bounded from below, by their respective Planck scale values. Along the external timelike world-lines, the theory naturally captures the "flow of time" as a genuinely structural attribute of the world. The theory also predicts expected deviations — suppressed quadratically by the Planck energy — from the dispersion relations for free fields in the vacuum. The deviations from the special relativistic Doppler shifts predicted by the theory are also suppressed quadratically by the Planck energy. Nonetheless, in order to estimate the precision required to distinguish the theory from special relativity, an experiment with a binary pulsar emitting TeV range γ-rays is considered in the context of the predicted deviations from the second-order shifts.

The generalized Szekeres family of solution for quasi-spherical space–time of higher dimensions are obtained in the scalar tensor theory of gravitation. Brans–Dicke field equations expressed in Dicke's revised units are exhaustively solved for all the subfamilies of the said family. A particular group of solutions may also be interpreted as due to the presence of the so-called C-field of Hoyle and Narlikar and for a chosen sign of the coupling parameter. The models show either expansion from a big bang type of singularity or a collapse with the turning point at a lower bound. There is one particular case which starts from the big bang, reaches a maximum and collapses with the in course of time to a crunch.

In this paper we present a class of exact inhomogeneous solutions to Einstein's equations for higher dimensional Szekeres metric with perfect fluid and a cosmological constant. We also show particular solutions depending on the choices of various parameters involved and for dust case. Finally, we examine the asymptotic behaviour of some of these solutions.

We investigate the possibility of obtaining non-singular black-hole solutions in the brane world model by solving the effective field equations for the induced metric on the brane. The Reissner–Nordstrom solution on the brane was obtained by Dadhich et al. by imposing the null energy condition on the 3-brane for a bulk having non-zero Weyl curvature. In this work, we relax the condition of vanishing scalar curvature R, however, retaining the null condition. We have shown that it is possible to obtain a class of static non-singular spherically symmetric brane space–times which admits horizon. We obtain one such class of solution which is a regular version of the Reissner–Nordstrom solution in the standard general relativity.

To find what influence the charge of the black hole Q will bring to the evolution of the quasinormal modes, we calculate the quasinormal frequencies of the neutrino field (charge e=0) perturbations and those of the massless Dirac field (e≠0) perturbations in the RN metric. The influences of Q, e, the momentum quantum number l, and the mode number n are discussed. Among the conclusions, the most important one is that, at the stage of quasinormal ringing, when the black hole and the field have the same kind of charge (eQ>0), the quasinormal modes of the massless charged Dirac field decay faster than those of the neutral ones, and when eQ<0, the massless charged Dirac field decays slower, which may be helpful in the astronomical observation. In addition, we compare the influence from the charge of the black hole to the spin 1/2 field and scalar field perturbations including the extremal limit (M=Q) and find a turning point of Q exists in both cases. The explanation of this fact is unclear with some suggestions that may be helpful are given.

We analyze cylindrical gravitational waves in vacuo with general polarization and develop a viewpoint complementary to that presented recently by Niedermaier showing that the auxiliary sigma model associated with this family of waves is not renormalizable in the standard perturbative sense.

In this paper, it is shown that the so-called Hartle criterion to select correlated regions in configuration space of dynamical variables, is directly connected to the presence of conservation laws, that is the existence of conservation laws in minisuperspace quantum cosmology constitutes a selection rule to recover observable universes. In this context, we develop the quantum cosmology for a scalar–tensor theory of gravity, induced from the reduction procedure of a 5D-unification theory. A class of exact solutions for Wheeler–DeWitt equation is derived and their connection to classical observable universes is discussed.

It is shown explicitly using MIT bag model that in presence of ultra-strong magnetic fields, a nucleon either flattens or collapses in the direction transverse to the external magnetic field in the classical or quantum mechanical picture respectively. Which gives rise to some kind of mechanical instability. Alternatively, it is argued that the bag model of confinement may not be applicable in this strange situation.