95 research outputs found
Accretion process onto super-spinning objects
The accretion process onto spinning objects in Kerr spacetimes is studied
with numerical simulations. Our results show that accretion onto compact
objects with Kerr parameter (characterizing the spin)
is very different. In the super-spinning case, for moderately larger than
, the accretion onto the central object is extremely suppressed due to a
repulsive force at short distance. The accreting matter cannot reach the
central object, but instead is accumulated around it, forming a high density
cloud that continues to grow. The radiation emitted in the accretion process
will be harder and more intense than the one coming from standard black holes;
e.g. -rays could be produced as seen in some observations.
Gravitational collapse of this cloud might even give rise to violent bursts. As
increases, a larger amount of accreting matter reaches the central object
and the growth of the cloud becomes less efficient. Our simulations find that a
quasi-steady state of the accretion process exists for ,
independently of the mass accretion rate at large radii. For such high values
of the Kerr parameter, the accreting matter forms a thin disk at very small
radii. We provide some analytical arguments to strengthen the numerical
results; in particular, we estimate the radius where the gravitational force
changes from attractive to repulsive and the critical value
separating the two qualitatively different regimes of accretion. We briefly
discuss the observational signatures which could be used to look for such
exotic objects in the Galaxy and/or in the Universe.Comment: 11 pages, 5 figures. v2: with explanation of the origin of the
critical value |a|/M = 1.
Geometrical locus of massive test particle orbits in the space of physical parameters in Kerr space-time
Gravitational radiation of binary systems can be studied by using the
adiabatic approximation in General Relativity. In this approach a small
astrophysical object follows a trajectory consisting of a chained series of
bounded geodesics (orbits) in the outer region of a Kerr Black Hole,
representing the space time created by a bigger object. In our paper we study
the entire class of orbits, both of constant radius (spherical orbits), as well
as non-null eccentricity orbits, showing a number of properties on the physical
parameters and trajectories. The main result is the determination of the
geometrical locus of all the orbits in the space of physical parameters in Kerr
space-time. This becomes a powerful tool to know if different orbits can be
connected by a continuous change of their physical parameters. A discussion on
the influence of different values of the angular momentum of the hole is given.
Main results have been obtained by analytical methods.Comment: 26 pages, 12 figure
Pseudo-Newtonian gravitational potential for Schwarzschild-de Sitter spacetimes
Pseudo-Newtonian gravitational potential describing the gravitational field
of static and spherically symmetric black holes in the universe with a
repulsive cosmological constant is introduced. In order to demonstrate the
accuracy of the pseudo-Newtonian approach, the related effective potential for
test-particle motion is constructed and compared with its general relativistic
counterpart given by the Schwarzschild-de Sitter geometry. The results indicate
that such an approach could be useful in applications of developed Newtonian
theories of accretion discs in astrophysically interesting situations in large
galactic structures for the Schwarzschild-de Sitter spacetimes with the
cosmological parameter y=(1/3)\Lambda M^2<10^{-6}.Comment: 17 pages, 11 figures, accepted by International Journal of Modern
Physics D (2008
Exact inhomogeneous Einstein-Maxwell-Dilaton cosmologies
We present solution generating techniques which permit to construct exact
inhomogeneous and anisotropic cosmological solutions to a four-dimensional low
energy limit of string theory containing non-minimally interacting
electromagnetic and dilaton fields. Some explicit homogeneous and inhomogeneous
cosmological solutions are constructed. For example, inhomogeneous exact
solutions presenting Gowdy - type EMD universe are obtained. The asymptotic
behaviour of the solutions is investigated. The asymptotic form of the metric
near the initial singularity has a spatially varying Kasner form. The character
of the space-time singularities is discussed. The late evolution of the
solutions is described by a background homogeneous and anisotropic universe
filled with weakly interacting gravitational, dilatonic and electromagnetic
waves.Comment: 10 pages, latex; v2: English corrected, new comments and reference
added in section 1, acknowledgments added; v3: final version to be published
in Phys. Rev.
A Bound Quantum Particle in a Riemann-Cartan space with Topological Defects and Planar Potential
Starting from a continuum theory of defects, that is the analogous to
three-dimensional Einstein-Cartan-Sciama-Kibble gravity, we consider a charged
particle with spin 1/2 propagating in a uniform magnetic field coincident with
a wedge dispiration of finite extent. We assume the particle is bound in the
vicinity of the dispiration by long range attractive (harmonic) and short range
(inverse square) repulsive potentials. Moreover, we consider the effects of
spin-torsion and spin-magnetic field interactions. Exact expressions for the
energy eigenfunctions and eigenvalues are determined. The limit, in which the
defect region becomes singular, is considered and comparison with the
electromagnetic Aharonov-Bohm effect is made.Comment: 10 page
Relic gravitational waves and present accelerated expansion
We calculate the current power spectrum of the gravitational waves created at
the big bang (and later amplified by the different transitions during the
Universe expansion) taking into account the present stage of accelerated
expansion. Likewise, we determine the power spectrum in a hypothetical second
dust era that would follow the present one if at some future time the dark
energy, that supposedly drives the current accelerated expansion, evolved in
such a way that it became dynamically equivalent to cold dark matter. The
calculated power spectrum as well as the evolution of the density parameter of
the waves may serve to discriminate between phases of expansion and may help
ascertain the nature of dark energy.Comment: 20 pages, uses revtex4, 1 figure ps and 3 figures eps. To be
published in Physical Review
Spherically Symmetric and Rotating Wormholes Produced by Lightlike Branes
Lightlike p-branes (LL-branes) with dynamical (variable) tension allow simple
and elegant Polyakov-type and dual to it Nambu-Goto-like world-volume action
formulations. Here we first briefly describe the dynamics of LL-branes as test
objects in various physically interesting gravitational backgrounds of black
hole type, including rotating ones. Next we show that LL-branes are the
appropriate gravitational sources that provide proper matter energy momentum
tensors in the Einstein equations of motion needed to generate traversable
wormhole solutions, in particular, self-consistent cylindrical rotating
wormholes, with the LL-branes occupying their throats. Here a major role is
being played by the dynamical LL-brane tension which turns out to be negative
but may be of arbitrary small magnitude. As a particular solution we obtain
traversable wormhole with Schwarzschild geometry generated by a LL-brane
positioned at the wormhole throat, which represents the correct consistent
realization of the original Einstein-Rosen "bridge" manifold.Comment: 27 pages; important clarifications regarding the meaning of the
original Einstein-Rosen "bridge" construction; an important addition to the
Appendix; acknowledgments adde
Metric fluctuations and the Weak Equivalence Principle
We describe space--time fluctuations by means of small fluctuations of the
metric on a given background metric. From a minimally coupled Klein--Gordon
equation we obtain within a weak-field approximation up to second order and an
averaging procedure over a finite space--time scale given by the quantum
particle in the non--relativistic limit a modified Schr\"odinger equation. The
dominant modification consists in an anomalous inertial mass tensor which
depends on the type of particle and on the fluctuation scenario. The scenario
considered in this paper is a most simple picture of spacetime fluctuations and
gives an existence proof for an apparent violation of the weak equivalence
principle and, in general, for a violation of Lorentz invariance.Comment: 10 pages, to appear in Class. Quantum Grav. (2008
Is it possible to detect gravitational waves with atom interferometers?
We investigate the possibility to use atom interferometers to detect
gravitational waves. We discuss the interaction of gravitational waves with an
atom interferometer and analyze possible schemes
Fully Explorable Horned Particles Hiding Charge
The charge-hiding effect by a horned particle, which was studied for the case
where gravity/gauge-field system is self-consistently interacting with a
charged lightlike brane (LLB) as a matter source, is now studied for the case
of a time like brane. From the demand that no surfaces of infinite coordinate
time redshift (horizons) appear in the problem we are lead now to a completly
explorable horned particle space for traveller that goes through the horned
particle (as was the case for the LLB) but now also in addition to this, the
horned region is fully visible to a static external observer. This requires
negative surface energy density for the shell sitting at the throat. We study a
gauge field subsystem which is of a special non-linear form containing a
square-root of the Maxwell term and which previously has been shown to produce
a QCD-like confining gauge field dynamics in flat space-time. The condition of
finite energy of the system or asymptotic flatness on one side of the horned
particle implies that the charged object sitting at the throat expels all the
flux it produces into the other side of the horned particle, which turns out to
be of a "tube-like" nature. An outside observer in the asymptotically flat
universe detects, therefore, apparently neutral object. The hiding of the
electric flux behind the tube-like region of a horned particle is the only
possible way that a truly charged particle can still be of finite energy, in a
theory that in flat space describes confinement. This points to the physical
relevance of such solutions, even though there is the need of negative energy
density at the throat of the horned particle, which can be of quantum
mechanical origin.Comment: The new version has been accepted for publication in Classical and
Quantum Gravity. Title changed to "Fully Explorable Horned Particles Hiding
Charge". Horned Particles terminology is used now instead of "wormholes" to
dscribe the solutions here. arXiv admin note: text overlap with
arXiv:1108.373
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