593 research outputs found
What does the Letelier-Gal'tsov metric describe?
Recently the structure of the Letelier-Gal'tsov spacetime has become a matter
of some controversy. I show that the metric proposed in \cite{letgal} is
defined only on a dense subset of the whole manifold. In the case when it can
be defined on the remainder by continuity, the resulting spacetime corresponds
to a system of parallel cosmic strings at rest w.r.t. each other.Comment: 4pages, 1 figure. v2 A few words are changed in accordance with the
published versio
Geodesics around Weyl-Bach's Ring Solution
We explore some of the gravitational features of a uniform ring both in the
Newtonian potential theory and in General Relativity. We use a spacetime
associated to a Weyl static solution of the vacuum Einstein's equations with
ring like singularity. The Newtonian motion for a test particle in the
gravitational field of the ring is studied and compared with the corresponding
geodesic motion in the given spacetime. We have found a relativistic peculiar
attraction: free falling particle geodesics are lead to the inner rim but never
hit the ring.Comment: 8 figures, 14 pages. LaTeX w/ subfigure, graphic
Spacetime Defects: von K\'arm\'an vortex street like configurations
A special arrangement of spinning strings with dislocations similar to a von
K\'arm\'an vortex street is studied. We numerically solve the geodesic
equations for the special case of a test particle moving along twoinfinite rows
of pure dislocations and also discuss the case of pure spinning defects.Comment: 9 pages, 2figures, CQG in pres
On Accelerated Black Holes
The static and stationary C-metric are revisited in a generic framework and
their interpretations studied in some detail. Specially those with two event
horizons, one for the black hole and another for the acceleration. We found
that: i) The spacetime of an accelerated static black hole is plagued by either
conical singularities or lack of smoothness and compactness of the black hole
horizon; ii) By using standard black hole thermodynamics we show that
accelerated black holes have higher Hawking temperature than Unruh temperature
of the accelerated frame; iii) The usual upper bound on the product of the mass
and acceleration parameters <1/sqrt(27) is just a coordinate artifact. The main
results are extended to accelerated rotating black holes with no significant
changes.Comment: Substantial revision after referee's comments. 21 pages, 3 figures, 2
tables. (amsmath and graphicx packages). Accepted to Phys. Rev.
Numeric simulation of relativistic stellar core collapse and the formation of Reissner-Nordstrom space-times
The time evolution of a set of 22 Mo unstable charged stars that collapse is
computed integrating the Einstein-Maxwell equations. The model simulate the
collapse of an spherical star that had exhausted its nuclear fuel and have or
acquires a net electric charge in its core while collapsing. When the charge to
mass ratio is Q/M >= 1 the star do not collapse and spreads. On the other hand,
it is observed a different physical behavior with a charge to mass ratio 1 > Q/
M > 0.1. In this case, the collapsing matter forms a bubble enclosing a lower
density core. We discuss an immediate astrophysical consequence of these
results that is a more efficient neutrino trapping during the stellar collapse
and an alternative mechanism for powerful supernova explosions. The outer
space-time of the star is the Reissner-Nordstrom solution that match smoothly
with our interior numerical solution, thus the collapsing models forms
Reissner-Nordstrom black holes.Comment: 13 pages, 13 figures, paper accepte
Domain Wall Spacetimes: Instability of Cosmological Event and Cauchy Horizons
The stability of cosmological event and Cauchy horizons of spacetimes
associated with plane symmetric domain walls are studied. It is found that both
horizons are not stable against perturbations of null fluids and massless
scalar fields; they are turned into curvature singularities. These
singularities are light-like and strong in the sense that both the tidal forces
and distortions acting on test particles become unbounded when theses
singularities are approached.Comment: Latex, 3 figures not included in the text but available upon reques
Acceleration, streamlines and potential flows in general relativity: analytical and numerical results
Analytical and numerical solutions for the integral curves of the velocity
field (streamlines) of a steady-state flow of an ideal fluid with
equation of state are presented. The streamlines associated with an accelerate
black hole and a rigid sphere are studied in some detail, as well as, the
velocity fields of a black hole and a rigid sphere in an external dipolar field
(constant acceleration field). In the latter case the dipole field is produced
by an axially symmetric halo or shell of matter. For each case the fluid
density is studied using contour lines. We found that the presence of
acceleration is detected by these contour lines. As far as we know this is the
first time that the integral curves of the velocity field for accelerate
objects and related spacetimes are studied in general relativity.Comment: RevTex, 14 pages, 7 eps figs, CQG to appea
Exact General Relativistic Disks with Magnetic Fields
The well-known ``displace, cut, and reflect'' method used to generate cold
disks from given solutions of Einstein equations is extended to solutions of
Einstein-Maxwell equations. Four exact solutions of the these last equations
are used to construct models of hot disks with surface density, azimuthal
pressure, and azimuthal current. The solutions are closely related to Kerr,
Taub-NUT, Lynden-Bell-Pinault and to a one-soliton solution. We find that the
presence of the magnetic field can change in a nontrivial way the different
properties of the disks. In particular, the pure general relativistic
instability studied by Bicak, Lynden-Bell and Katz [Phys. Rev. D47, 4334, 1993]
can be enhanced or cured by different distributions of currents inside the
disk. These currents, outside the disk, generate a variety of axial symmetric
magnetic fields. As far as we know these are the first models of hot disks
studied in the context of general relativity.Comment: 21 pages, 11 figures, uses package graphics, accepted in PR
A Dynamical Systems Approach to Schwarzschild Null Geodesics
The null geodesics of a Schwarzschild black hole are studied from a dynamical
systems perspective. Written in terms of Kerr-Schild coordinates, the null
geodesic equation takes on the simple form of a particle moving under the
influence of a Newtonian central force with an inverse-cubic potential. We
apply a McGehee transformation to these equations, which clearly elucidates the
full phase space of solutions. All the null geodesics belong to one of four
families of invariant manifolds and their limiting cases, further characterized
by the angular momentum L of the orbit: for |L|>|L_c|, (1) the set that flow
outward from the white hole, turn around, then fall into the black hole, (2)
the set that fall inward from past null infinity, turn around outside the black
hole to continue to future null infinity, and for |L|<|L_c|, (3) the set that
flow outward from the white hole and continue to future null infinity, (4) the
set that flow inward from past null infinity and into the black hole. The
critical angular momentum Lc corresponds to the unstable circular orbit at
r=3M, and the homoclinic orbits associated with it. There are two additional
critical points of the flow at the singularity at r=0. Though the solutions of
geodesic motion and Hamiltonian flow we describe here are well known, what we
believe is a novel aspect of this work is the mapping between the two
equivalent descriptions, and the different insights each approach can give to
the problem. For example, the McGehee picture points to a particularly
interesting limiting case of the class (1) that move from the white to black
hole: in the limit as L goes to infinity, as described in Schwarzschild
coordinates, these geodesics begin at r=0, flow along t=constant lines, turn
around at r=2M, then continue to r=0. During this motion they circle in azimuth
exactly once, and complete the journey in zero affine time.Comment: 14 pages, 3 Figure
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