3,678 research outputs found
Tensorial perturbations in the bulk of inflating brane worlds
In this paper we consider the stability of some inflating brane-world models
in quantum cosmology. It is shown that whereas the singular model based on the
construction of inflating branes from Euclidean five-dimensional anti-de Sitter
space is unstable to tensorial cosmological perturbations in the bulk, the
nonsingular model which uses a five-dimensional asymptotically anti-de Sitter
wormhole to construct the inflating branes is stable to these perturbations.Comment: 4 pages, RevTex, to appear in Phys. Rev.
Spectropolarimetric analysis of an active region filament. I. Magnetic and dynamical properties from single component inversions
The determination of the magnetic filed vector in solar filaments is possible
by interpreting the Hanle and Zeeman effects in suitable chromospheric spectral
lines like those of the He I multiplet at 10830 A. We study the vector magnetic
field of an active region filament (NOAA 12087). Spectropolarimetric data of
this active region was acquired with the GRIS instrument at the GREGOR
telescope and studied simultaneously in the chromosphere with the He I 10830 A
multiplet and in the photosphere with the Si I 10827 A line. As it is usual
from previous studies, only a single component model is used to infer the
magnetic properties of the filament. The results are put into a solar context
with the help of the Solar Dynamic Observatory images. Some results clearly
point out that a more complex inversion had to be done. Firstly, the Stokes
map of He I does not show any clear signature of the presence of the filament.
Secondly, the local azimuth map follows the same pattern than Stokes as if
the polarity of Stokes were conditioning the inference to very different
magnetic field even with similar linear polarization signals. This indication
suggests that the Stokes could be dominated by the below magnetic field
coming from the active region, and not, from the filament itself. Those and
more evidences will be analyzed in depth and a more complex inversion will be
attempted in the second part of this series.Comment: 18 pages, 19 figures, accepted for publication in A&
Wormholes and Ringholes in a Dark-Energy Universe
The effects that the present accelerating expansion of the universe has on
the size and shape of Lorentzian wormholes and ringholes are considered. It is
shown that, quite similarly to how it occurs for inflating wormholes, relative
to the initial embedding-space coordinate system, whereas the shape of the
considered holes is always preserved with time, their size is driven by the
expansion to increase by a factor which is proportional to the scale factor of
the universe. In the case that dark energy is phantom energy, which is not
excluded by present constraints on the dark-energy equation of state, that size
increase with time becomes quite more remarkable, and a rather speculative
scenario is here presented where the big rip can be circumvented by future
advanced civilizations by utilizing sufficiently grown up wormholes and
ringholes as time machines that shortcut the big-rip singularity.Comment: 11 pages, RevTex, to appear in Phys. Rev.
On the warp drive space-time
In this paper the problem of the quantum stability of the two-dimensional
warp drive spacetime moving with an apparent faster than light velocity is
considered. We regard as a maximum extension beyond the event horizon of that
spacetime its embedding in a three-dimensional Minkowskian space with the
topology of the corresponding Misner space. It is obtained that the interior of
the spaceship bubble becomes then a multiply connected nonchronal region with
closed timelike curves and that the most natural vacuum allows quantum
fluctuations which do not induce any divergent behaviour of the re-normalized
stress-energy tensor, even on the event (Cauchy) chronology horizon. In such a
case, the horizon encloses closed timelike curves only at scales close to the
Planck length, so that the warp drive satisfies the Ford's negative energy-time
inequality. Also found is a connection between the superluminal two-dimensional
warp drive space and two-dimensional gravitational kinks. This connection
allows us to generalize the considered Alcubierre metric to a standard,
nonstatic metric which is only describable on two different coordinate patchesComment: 7 pages, minor comment on chronology protection added, RevTex, to
appear in Phys. Rev.
Cosmological predictions from the Misner brane
Within the spirit of five-dimensional gravity in the Randall-Sundrum
scenario, in this paper we consider cosmological and gravitational implications
induced by forcing the spacetime metric to satisfy a Misner-like symmetry. We
first show that in the resulting Misner-brane framework the Friedmann metric
for a radiation dominated flat universe and the Schwarzschild or anti-de Sitter
black holes metrics are exact solutions on the branes, but the model cannot
accommodate any inflationary solution. The horizon and flatness problems can
however be solved in Misner-brane cosmology by causal and noncausal
communications through the extra dimension between distant regions which are
outside the horizon. Based on a semiclassical approximation to the
path-integral approach, we have calculated the quantum state of the
Misner-brane universe and the quantum perturbations induced on its metric by
brane propagation along the fifth direction. We have then considered testable
predictions from our model. These include a scale-invariant spectrum of density
perturbations whose amplitude can be naturally accommodated to the required
value 10, and a power spectrum of CMB anisotropies whose
acoustic peaks are at the same sky angles as those predicted by inflationary
models, but having much smaller secondary-peak intensities. These predictions
seem to be compatible with COBE and recent Boomerang and Maxima measurementsComment: 16 pages, RevTe
Decoherence in an accelerated universe
In this paper we study the decoherence processes of the semiclassical
branches of an accelerated universe due to their interaction with a scalar
field with given mass. We use a third quantization formalism to analyze the
decoherence between two branches of a parent universe caused by their
interaction with the vaccum fluctuations of the space-time, and with other
parent unverses in a multiverse scenario.Comment: 11 pages, 2 figure
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