945 research outputs found
Reflection and Transmission at the Apparent Horizon during Gravitational Collapse
We examine the wave-functionals describing the collapse of a self-gravitating
dust ball in an exact quantization of the gravity-dust system. We show that
ingoing (collapsing) dust shell modes outside the apparent horizon must
necessarily be accompanied by outgoing modes inside the apparent horizon, whose
amplitude is suppressed by the square root of the Boltzmann factor at the
Hawking temperature. Likewise, ingoing modes in the interior must be
accompanied by outgoing modes in the exterior, again with an amplitude
suppressed by the same factor. A suitable superposition of the two solutions is
necessary to conserve the dust probability flux across the apparent horizon,
thus each region contains both ingoing and outgoing dust modes. If one
restricts oneself to considering only the modes outside the apparent horizon
then one should think of the apparent horizon as a partial reflector, the
probability for a shell to reflect being given by the Boltzmann factor at the
Hawking temperature determined by the mass contained within it. However, if one
considers the entire wave function, the outgoing wave in the exterior is seen
to be the transmission through the horizon of the interior outgoing wave that
accompanies the collapsing shells. This transmission could allow information
from the interior to be transferred to the exterior.Comment: 19 pages, no figures. To appear in Phys. Rev.
Topological Quintessence
A global monopole (or other topological defect) formed during a recent phase
transition with core size comparable to the present Hubble scale, could induce
the observed accelerating expansion of the universe. In such a model,
topological considerations trap the scalar field close to a local maximum of
its potential in a cosmologically large region of space. We perform detailed
numerical simulations of such an inhomogeneous dark energy system (topological
quintessence) minimally coupled to gravity, in a flat background of initially
homogeneous matter. We find that when the energy density of the field in the
monopole core starts dominating the background density, the spacetime in the
core starts to accelerate its expansion in accordance to a \Lambda CDM model
with an effective inhomogeneous spherical dark energy density parameter
\Omega_\Lambda(r). The matter density profile is found to respond to the global
monopole profile via an anti-correlation (matter underdensity in the monopole
core). Away from the monopole core, the spacetime is effectively
Einstein-deSitter (\Omega_\Lambda(r_{out}) -> 0) while at the center
\Omega_\Lambda(r ~ 0) is maximum. We fit the numerically obtained expansion
rate at the monopole core to the Union2 data and show that the quality of fit
is almost identical to that of \Lambda CDM. Finally, we discuss potential
observational signatures of this class of inhomogeneous dark energy models.Comment: Accepted in Phys. Rev. D (to appear). Added observational bounds on
parameters. 10 pages (two column revtex), 6 figures. The Mathematica files
used to produce the figures of this study may be downloaded from
http://leandros.physics.uoi.gr/topquin
Optical alignment and polarization conversion of neutral exciton spin in individual InAs/GaAs quantum dots
We investigate exciton spin memory in individual InAs/GaAs self-assembled
quantum dots via optical alignment and conversion of exciton polarization in a
magnetic field. Quasiresonant phonon-assisted excitation is successfully
employed to define the initial spin polarization of neutral excitons. The
conservation of the linear polarization generated along the bright exciton
eigenaxes of up to 90% and the conversion from circular- to linear polarization
of up to 47% both demonstrate a very long spin relaxation time with respect to
the radiative lifetime. Results are quantitatively compared with a model of
pseudo-spin 1/2 including heavy-to-light hole mixing.Comment: 5 pages, 3 figure
Rearrangements and Dilatancy for Sheared Dense Materials
Constitutive equations are proposed for dense materials, based on the
identification of two types of free-volume activated rearrangements associated
to shear and compaction. Two situations are studied: the case of an amorphous
solid in a stress-strain test, and the case of a lubricant in tribology test.
Varying parameters, strain softening, shear thinning, and stick-slip motion can
be observed.Comment: 4 pages, 3 figure
Alternative Methods of Describing Structure Formation in the Lemaitre-Tolman Model
We describe several new ways of specifying the behaviour of Lemaitre-Tolman
(LT) models, in each case presenting the method for obtaining the LT arbitrary
functions from the given data, and the conditions for existence of such
solutions. In addition to our previously considered `boundary conditions', the
new ones include: a simultaneous big bang, a homogeneous density or velocity
distribution in the asymptotic future, a simultaneous big crunch, a
simultaneous time of maximal expansion, a chosen density or velocity
distribution in the asymptotic future, only growing or only decaying
fluctuations. Since these conditions are combined in pairs to specify a
particular model, this considerably increases the possible ways of designing LT
models with desired properties.Comment: Accepted by Phys Rev D. RevTeX 4, 13 pages, no figures. Part of a
series: gr-qc/0106096, gr-qc/0303016, gr-qc/0309119. Replacement contains
very minor correction
Expansion-Free Evolving Spheres Must Have Inhomogeneous Energy Density Distributions
In a recent paper a systematic study on shearing expansion-free spherically
symmetric distributions was presented. As a particular case of such systems,
the Skripkin model was mentioned, which corresponds to a nondissipative perfect
fluid with a constant energy density. Here we show that such a model is
inconsistent with junction conditions. It is shown that in general for any
nondissipative fluid distribution, the expansion-free condition requires the
energy density to be inhomogeneous. As an example we consider the case of dust,
which allows for a complete integration.Comment: 8 pages, Latex. To appear in Phys. Rev.D. Typos correcte
CMB anisotropies seen by an off-center observer in a spherically symmetric inhomogeneous universe
The current authors have previously shown that inhomogeneous, but spherically
symmetric universe models containing only matter can yield a very good fit to
the SNIa data and the position of the first CMB peak. In this work we examine
how far away from the center of inhomogeneity the observer can be located in
these models and still fit the data well. Furthermore, we investigate whether
such an off-center location can explain the observed alignment of the lowest
multipoles of the CMB map. We find that the observer has to be located within a
radius of 15 Mpc from the center for the induced dipole to be less than that
observed by the COBE satellite. But for such small displacements from the
center, the induced quadru- and octopoles turn out to be insufficiently large
to explain the alignment.Comment: 8 pages (REVTeX4), 7 figures; v2: minor changes, matches published
versio
Classical and quantum LTB model for the non-marginal case
We extend the classical and quantum treatment of the Lemaitre-Tolman-Bondi
(LTB) model to the non-marginal case (defined by the fact that the shells of
the dust cloud start with a non-vanishing velocity at infinity). We present the
classical canonical formalism and address with particular care the boundary
terms in the action. We give the general relation between dust time and Killing
time. Employing a lattice regularization, we then derive and discuss for
particular factor orderings exact solutions to all quantum constraints.Comment: 23 pages, no figures, typos correcte
Gravitational radiation from collapsing magnetized dust
In this article we study the influence of magnetic fields on the axial
gravitational waves emitted during the collapse of a homogeneous dust sphere.
We found that while the energy emitted depends weakly on the initial matter
perturbations it has strong dependence on the strength and the distribution of
the magnetic field perturbations. The gravitational wave output of such a
collapse can be up to an order of magnitude larger or smaller calling for
detailed numerical 3D studies of collapsing magnetized configurations
A Lemaitre-Tolman-Bondi cosmological wormhole
We present a new analytical solution of the Einstein field equations
describing a wormhole shell of zero thickness joining two
Lema{\i}tre-Tolman-Bondi universes, with no radial accretion. The material on
the shell satisfies the energy conditions and, at late times, the shell becomes
comoving with the dust-dominated cosmic substratum.Comment: 5 pages, latex, no figures, to appear in Phys. Rev.
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