60 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
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
Hiding and Confining Charges via "Tube-like" Wormholes
We describe two interesting effects in wormhole physics. First, we find that
a genuinely charged matter source may appear neutral to an external observer -
a phenomenon opposite to the famous Misner-Wheeler "charge without charge"
effect. This phenomenon takes place when coupling a bulk
gravity/nonlinear-gauge-field system to a charged lightlike brane as a matter
source. The "charge-hiding" effect occurs in a wormhole solution which connects
a non-compact "universe", comprising the exterior region of
Schwarzschild-(anti-)de-Sitter (SdS) or purely Schwarzschild black hole beyond
the Schwarzschild horizon, to a Levi-Civita-Bertotti-Robinson-type (LCBR)
"tube-like" "universe" via a wormhole "throat" occupied by the brane. In this
solution the whole electric flux produced by the brane is expelled into the
"tube-like" "universe" and the brane is detected as neutral by an observer in
the non-compact "universe". Next, we find a truly charge-confining wormhole
solution when we couple the bulk gravity/nonlinear-gauge-field system to two
oppositely charged lightlike branes. The latter system possesses a "two-throat"
wormhole solution, where the "left-most" and the "right-most" "universes" are
two identical copies of the exterior region of SdS black hole beyond the
Schwarzschild horizon, whereas the "middle" "universe" is of LCBR "tube-like"
form with geometry dS_2 x S^2. It comprises the finite-extent intermediate
region of dS_2 between its two horizons. Both "throats" are occupied by the two
oppositely charged lightlike branes and the whole electric flux produced by the
latter is confined entirely within the middle "tube-like" "universe". A crucial
ingredient is the special form of the nonlinear gauge field action, which
contains both the standard Maxwell term as well as a square root of the latter.
This theory was previously shown to produce a QCD-like confining dynamics in
flat space-time.Comment: 26 pages, 2 figures; v.2 several references added, missing constant
factors in few equations inserted, acknowledgement added, results unchanged;
v.3 28 pages, several clarifying remarks, references and acknowledgements
added, version to appear in International Journal of Modern Physics
G1 Cosmologies with Gravitational and Scalar Waves
I present here a new algorithm to generate families of inhomogeneous massless
scalar field cosmologies. New spacetimes, having a single isometry, are
generated by breaking the homogeneity of massless scalar field models
along one direction. As an illustration of the technique I construct
cosmological models which in their late time limit represent perturbations in
the form of gravitational and scalar waves propagating on a non-static
inhomogeneous background. Several features of the obtained metrics are
discussed, such as their early and late time limits, structure of singularities
and physical interpretation.Comment: 24 pages, 2 figure
Axially symmetric solutions in f(R)-gravity
Axially symmetric solutions for f (R)-gravity can be derived starting from
exact spherically sym- metric solutions achieved by Noether symmetries. The
method takes advantage of a complex coordi- nate transformation previously
developed by Newman and Janis in General Relativity. An example is worked out
to show the general validity of the approach. The physical properties of the
solution are also considered.Comment: 13 pages, 1 figure, to appear in Classical and Quantum Gravity 201
Cascade Birth of Universes in Multidimensional Spaces
The formation mechanism of universes with distinctly different properties is
considered within the framework of pure gravity in a space of D > 4 dimensions.
The emergence of the Planck scale and its relationship to the inflaton mass are
discussed.Comment: 10 p., minor correction
General Relativity in Electrical Engineering
In electrical engineering metamaterials have been developed that offer
unprecedented control over electromagnetic fields. Here we show that general
relativity lends the theoretical tools for designing devices made of such
versatile materials. Given a desired device function, the theory describes the
electromagnetic properties that turn this function into fact. We consider media
that facilitate space-time transformations and include negative refraction. Our
theory unifies the concepts operating behind the scenes of perfect invisibility
devices, perfect lenses, the optical Aharonov-Bohm effect and electromagnetic
analogs of the event horizon, and may lead to further applications
Energy Release During Disk Accretion onto a Rapidly Rotating Neutron Star
The energy release L_s on the surface of a neutron star (NS) with a weak
magnetic field and the energy release L_d in the surrounding accretion disk
depend on two independent parameters that determine its state (for example,
mass M and cyclic rotation frequency f) and is proportional to the accretion
rate. We derive simple approximation formulas illustrating the dependence of
the efficiency of energy release in an extended disk and in a boundary layer
near the NS surface on the frequency and sense of rotation for various NS
equations of state. Such formulas are obtained for the quadrupole moment of a
NS, for a gap between its surface and a marginally stable orbit, for the
rotation frequency in an equatorial Keplerian orbit and in the marginally
stable circular orbit, and for the rate of NS spinup via disk accretion. In the
case of NS and disk counterrotation, the energy release during accretion can
reach . The sense of NS rotation is a factor that strongly
affects the observed ratio of nuclear energy release during bursts to
gravitational energy release between bursts in X-ray bursters. The possible
existence of binary systems with NS and disk counterrotation in the Galaxy is
discussed. Based on the static criterion for stability, we present a method of
constructing the dependence of gravitational mass M on Kerr rotation parameter
j and on total baryon mass (rest mass) m for a rigidly rotating neutron star.
We show that all global NS characteristics can be expressed in terms of the
function M(j, m) and its derivatives.Comment: 42 pages, 12 figures, to appear in Astronomy Letters, 2000, v.26,
p.69
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