International Journal of Modern Physics D

We put together many observational data of SGRs and AXPs and analyze them with the main purpose to remove contradiction between the real age of these objects and their characteristic times of period change. This work indicates that SGRs and AXPs are neutron stars with magnetic fields up to ~3×10¹⁴G at birth, which is less than the possible value in the existing magnetar model. These neutron stars undergo star-quakes and reconnection of magnetic field occurs from time to time. As a result of these processes plasma is ejected from the NS and propeller mechanism starts to work. Due to the propeller effect increases and τ decreases. Indeed, high values are observed in SGRs and in half of the AXPs. Then, for a long time NS looses its activity, its decreases, τ increases and rapid cooling begins. Each NS stage (AXP, SGR, dim) may occur once or several times until the spin period of the neutron star becomes P>10–12 s. At the transition period to the SGR and AXP stages, the magnetic field component perpendicular to the rotation axis may increase up to 2–3 times. Observational data and mainly the data of AXP1E1048-5937 and DRQNS RX J1308.8+2127 support this idea.

The de Rham Laplacian Δ_(dR) for differential forms is a geometric generalization of the usual covariant Laplacian Δ, and it may be extended naturally to tensor-valued p-forms using the exterior covariant derivative associated with a metric connection. Using it the wave equation satisfied by the curvature tensors in general relativity takes its most compact form. This wave equation leads to the Teukolsky equations describing integral spin perturbations of black hole spacetimes.

We investigate gravitational properties of thin planar wall solutions of the Einstein's equations in the weak field approximation. We find the general metric solutions and discuss the behavior of a particle placed initially at rest to one side of the plane. Moreover we study the case of non-reflection-symmetric solutions.

In the current paper, we further improved the model for the migration of planets introduced and extended to time-dependent planetesimal accretion disks by Del Popolo. In the current study, the assumption of Del Popolo, that the surface density in planetesimals is proportional to that of gas, is relaxed. In order to obtain the evolution of planetesimal density, we use a method developed by Stepinski and Valageas which is able to simultaneously follow the evolution of gas and solid particles for up to 10⁷ years. Then, the disk model is coupled to migration model introduced by Del Popolo in order to obtain the migration rate of the planet in the planetesimal. We find that the properties of solids known to exist in protoplanetary systems, together with reasonable density profiles for the disk, lead to a characteristic radius in the range 0.03–0.2 AU for the final semi-major axis of the giant planet.Hence our model can explain the properties of discovered extrasolar giant planets.

The obstruction for a perturbative reconstruction of the five-dimensional bulk metric starting from the four-dimensional metric at the boundary, that is, the Dirichlet problem, is computed in dimensions 6≤d≤10 and some comments are made on its general structure and, in particular, on its relationship with the conformal anomaly, which we compute in dimension d=8.

Dirac's constraint analysis and the symplectic structure of geodesic equations are obtained for the general cylindrically symmetric stationary spacetime. For this metric, using the obtained first order Lagrangian, the geodesic equations of motion are integrated, and found some solutions for Lewis, Levi-Civita, and Van Stockum spacetimes.

In a recent work¹ we presented a reformulation of the canonical quantum gravity, based on adding the so-called kinematical term to the gravity-matter action. This revised approach leads to a self-consistent canonical quantization of the 3-geometries, which referred to the external time as provided via the added term.

Here, we show how the kinematical term can be interpreted in terms of a non-relativistic dust fluid which plies the role of a "real clock" for the quantum gravity theory, and, in the WKB limit of a cosmological problem, makes account for a dark matter component which, at present time, could play a dynamical role.

We study the head-on collision of linearly polarized, high frequency plane gravitational waves and their electromagnetic counterparts in the Einstein–Maxwell theory. The post-collision space-times are obtained by solving the vacuum Einstein and Einstein–Maxwell field equations in the geometrical optics approximation. The head-on collisions of all possible pairs of these systems of waves is described and the results are then generalized to nonlinearly polarized waves which exhibit the maximum two degrees of freedom of polarization.

Starting from the apparently periodic structures of the universe, revealed in a series of recent observations, we suppose that Dark Matter is composed of a quantum particle of very low mass and is clustered around the luminous matter of galaxies. We reduce the cosmological Friedman–Einstein dynamical system to a sort of Schrödinger equation for this quantum particle with a simple first quantization scheme. Comparing the eigen-solutions of this Cosmological Schrödinger Equation with the experimental periodic large scale structure, we predict the possible value of the mass of the dark quantum particle in two remarkable cases.

Solutions of gravitational equations of gauge theories of gravity in homogeneous isotropic world with massive scalar field are investigated in the case of flat cosmological models. Special attention is dedicated to behavior of solutions on contraction stage. It is shown, that on expansion stage inflationary solutions are generic feature of the model. At the same time on contraction stage an effective equation of state can be similar to the case of massless scalar field p=ρ at the beginning of evolution and tends to the equation of ultrarelativistic gas p=ρ/3. The Hubble parameter tends to some negative value on contraction stage, depending on the mass of the scalar field. Nonsingular solutions in this model are unstable.

We ask whether the problem on the initial conditions in Cosmology can be solved if an efficient elimination mechanism of the anisotropy was present at earlier epochs. We explore the consequences of the existence of an epoch in which the material content of the Universe was represented by an stress-energy tensor possessing anisotropic pressure which depends nonlinearly on the expansion parameter. We show that for selected candidate behaviors the associated shear pass through a maximum and vanishes asymptotically. We use the Hamiltonian formalism to define a constant of motion which enables us to classify the cosmological world-models.