745 research outputs found
On the Stability of Circular Orbits of Particles Moving around Black Holes Surrounded by Axially Symmetric Structures
The Rayleigh criterion is used to study the stability of circular orbits of
particles moving around static black holes surrounded by different axially
symmetric structures with reflection symmetry, like disks, rings and halos. We
consider three models of disks one of infinite extension and two finite, and
one model of rings. The halos are represented by external quadrupole moments
(either oblate or prolate). Internal quadrupole perturbation (oblate and
prolate) are also considered. For this class of disks the counter-rotation
hypothesis implies that the stability of the disks is equivalent to the
stability of test particles. The stability of Newtonian systems is also
considered and compared with the equivalent relativistic situation. We find
that the general relativistic dynamics favors the formation of rings.Comment: RevTex, 9 pages, 14 eps figs. Phys. Rev. D to appea
Riemann-Christoffel flows
A geometric flow based in the Riemann-Christoffel curvature tensor that in
two dimensions has some common features with the usual Ricci flow is presented.
For dimensional spaces this new flow takes into account all the components
of the intrinsic curvature. For four dimensional Lorentzian manifolds it is
found that the solutions of the Einstein equations associated to a "detonant"
sphere of matter, as well, as a Friedman-Roberson-Walker cosmological model are
examples of Riemann-Christoffel flows. Possible generalizations are mentioned.Comment: 3 pages, RevTex,small changes, Int. J. Theor. Phys. (in press
Spacetime Defects: Torsion Loops
Spacetimes with everywhere vanishing curvature tensor, but with torsion
different from zero only on world sheets that represent closed loops in
ordinary space are presented, also defects along open curves with end points at
infinity are studied. The case of defects along timelike loops is also
considered and the geodesics in these spaces are briefly discussed.Comment: 8 pages, LATE
Simple pairs potential-density for flat rings
Pairs potential-density in terms of elementary functions that represents flat
rings structures are presented. We study structures representing one or several
concentric flat rings. Also disks surrounded by concentric flat rings are
exhibited. The stability of concentrically circular orbits of particles moving
on a flat ring structure is analyzed for radial perturbations.Comment: 5 pages, 5 figures, RevTex, to appear in MNRA
Conformal Collineations and Ricci Inheritance Symmetry in String Cloud and String Fluids
Conformal collineations (a generalization of conformal motion) and Ricci
inheritance collineations, defined by , for
string cloud and string fluids in general relativity are studied. By
investigating the kinematical and dynamical properties of such fluids and using
the field equations, some recent studies on the restrictions imposed by
conformal collineations are extended, and new results are found.Comment: 12 pages, LaTeX, no figures, to appear in Int. J. Mod. Phys.
A General Relativistic Model for the Gravitational Field of Active Galactic Nuclei Surrounded by a Disk
An exact but simple general relativistic model for the gravitational field of
active galactic nuclei is constructed, based on the superposition in Weyl
coordinates of a black hole, a Chazy-Curzon disk and two rods, which represent
matter jets. The influence of the rods on the matter properties of the disk and
on its stability is examined. We find that in general they contribute to
destabilize the disk. Also the oscillation frequencies for perturbed circular
geodesics on the disk are computed, and some geodesic orbits for the superposed
metric are numerically calculated.Comment: 24 pages, LATEX. Phys. Rev. D (in press
- …