183 research outputs found
Quantum fluctuations and CMB anisotropies in one-bubble open inflation models
We first develop a method to calculate a complete set of mode functions which
describe the quantum fluctuations generated in one-bubble open inflation
models. We consider two classes of models. One is a single scalar field model
proposed by Bucher, Goldhaber and Turok and by us as an example of the open
inflation scinario, and the other is a two-field model such as the
``supernatural'' inflation proposed by Linde and Mezhlumian. In both cases we
assume the difference in the vacuum energy density between inside and outside
the bubble is negligible. There are two kinds of mode functions. One kind has
usual continuous spectrum and the other has discrete spectrum with
characteristic wavelengths exceeding the spatial curvature scale. The latter
can be further devided into two classes in terms of its origin. One is called
the de Sitter super-curvature mode, which arises due to the global spacetime
structure of de Sitter space, and the other is due to fluctuations of the
bubble wall. We calculate the spectrum of quantum fluctuations in these models
and evaluate the resulting large angular scale CMB anisotropies. We find there
are ranges of model parameters that are consistent with observed CMB
anisotropies.Comment: 22 pages revtex file, 12 postscript figures, tarred, gzippe
Can the string scale be related to the cosmic baryon asymmetry?
In a previous work, a mechanism was presented by which baryon asymmetry can
be generated during inflation from elliptically polarized gravitons.
Nonetheless, the mechanism only generated a realistic baryon asymmetry under
special circumstances which requires an enhancement of the lepton number from
an unspecified GUT. In this note we provide a stringy embedding of this
mechanism through the Green-Schwarz mechanism, demonstrating that if the
model-independent axion is the source of the gravitational waves responsible
for the lepton asymmetry, one can observationally constrain the string scale
and coupling.Comment: 12 Pages, typo corrected in the tex
Spherically symmetric loop quantum gravity: analysis of improved dynamics
We study the âimproved dynamicsâ for the treatment of spherically symmetric space-times in loop quantum gravity introduced by Chiou et al in analogy with the one that has been constructed by Ashtekar, Pawlowski and Singh for the homogeneous space-times. In this dynamics the polymerization parameter is a well motivated function of the dynamical variables, reflecting the fact that the quantum of area depends on them. Contrary to the homogeneous case, its implementation does not trigger undesirable physical properties. We identify semiclassical physical states in the quantum theory and derive the corresponding effective semiclassical metrics. We then discuss some of their properties. Concretely, the space-time approaches sufficiently fast the Schwarzschild geometry at low curvatures. Besides, regions where the singularity is in the classical theory get replaced by a regular but discrete effective geometry with finite and Planck order curvature, regardless of the mass of the black hole. This circumvents trans-Planckian curvatures that appeared for astrophysical black holes in the quantization scheme without the improvement. It makes the resolution of the singularity more in line with the one observed in models that use the isometry of the interior of a Schwarzschild black hole with the KantowskiâSachs loop quantum cosmologies. One can observe the emergence of effective violations of the null energy condition in the interior of the black hole as part of the mechanism of the elimination of the singularity
Spherically symmetric loop quantum gravity: analysis of improved dynamics
We study the "improved dynamics" for the treatment of spherically symmetric
space-times in loop quantum gravity introduced by Chiou {\em et al.} in analogy
with the one that has been constructed by Ashtekar, Pawlowski and Singh for the
homogeneous space-times. In this dynamics the polymerization parameter is a
well motivated function of the dynamical variables, reflecting the fact that
the quantum of area depends on them. Contrary to the homogeneous case, its
implementation does not trigger undesirable physical properties. We identify
semiclassical physical states in the quantum theory and derive the
corresponding effective semiclassical metrics. We then discuss some of their
properties. Concretely, the space-time approaches sufficiently fast the
Schwarzschild geometry at low curvatures. Besides, regions where the
singularity is in the classical theory get replaced by a regular but discrete
effective geometry with finite and Planck order curvature, regardless of the
mass of the black hole. This circumvents trans-Planckian curvatures that
appeared for astrophysical black holes in the quantization scheme without the
improvement. It makes the resolution of the singularity more in line with the
one observed in models that use the isometry of the interior of a Schwarzschild
black hole with the Kantowski--Sachs loop quantum cosmologies. One can observe
the emergence of effective violations of the null energy condition in the
interior of the black hole as part of the mechanism of the elimination of the
singularity.Comment: 26 pages, 8 figure
The dS/dS Correspondence
We present a holographic duality for the de Sitter static patch which
consolidates basic features of its geometry and the behavior of gravity and
brane probes, valid on timescales short compared to the decay or Poincare
recurrence times. Namely de Sitter spacetime in dimensions with
curvature radius is holographically dual to two conformal field theories on
, cut off at an energy scale 1/R where they couple to each other
and to dimensional gravity. As part of our analysis, we study brane
probes in de Sitter and thermal Anti de Sitter spaces, and interpret the terms
in the corresponding DBI action via strongly coupled thermal field theory. This
provides a dual field theoretic interpretation of the fact that probes take
forever to reach a horizon in general relativity.Comment: 29 pages, harvmac big; 3 figures; v2: references added, minor typo
fixe
On the logarithmic epiperimetric inequality for the obstacle problem
We give three different proofs of the log-epiperimetric inequality at singular points for the obstacle problem. In the first, direct proof, we write the competitor explicitly; the second proof is also constructive, but this time the competitor is given through the solution of an evolution problem on the sphere. We compare the competitors obtained in the different proofs and their relation to other similar results that appeared recently. Finally, in the appendix, we give a general theorem, which can be applied also in other contexts and in which the construction of the competitor is reduced to finding a flow satisfying two differential inequalities
Perturbations of Spatially Closed Bianchi III Spacetimes
Motivated by the recent interest in dynamical properties of topologically
nontrivial spacetimes, we study linear perturbations of spatially closed
Bianchi III vacuum spacetimes, whose spatial topology is the direct product of
a higher genus surface and the circle. We first develop necessary mode
functions, vectors, and tensors, and then perform separations of (perturbation)
variables. The perturbation equations decouple in a way that is similar to but
a generalization of those of the Regge--Wheeler spherically symmetric case. We
further achieve a decoupling of each set of perturbation equations into
gauge-dependent and independent parts, by which we obtain wave equations for
the gauge-invariant variables. We then discuss choices of gauge and stability
properties. Details of the compactification of Bianchi III manifolds and
spacetimes are presented in an appendix. In the other appendices we study
scalar field and electromagnetic equations on the same background to compare
asymptotic properties.Comment: 61 pages, 1 figure, final version with minor corrections, to appear
in Class. Quant. Gravi
Bulk Emission of Scalars by a Rotating Black Hole
We study in detail the scalar-field Hawking radiation emitted into the bulk by a higher-dimensional, rotating black hole. We numerically compute the angular eigenvalues, and solve the radial equation of motion in order to find transmission factors. The latter are found to be enhanced by the angular momentum of the black hole, and to exhibit the well-known effect of superradiance. The corresponding power spectra for scalar fields show an enhancement with the number of dimensions, as in the non-rotating case. On the other hand, the proportion of the total (i.e., bulk+brane) power that is emitted into the bulk decreases monotonically with the angular momentum. We compute the total mass loss rate of the black hole for a variety of black-hole angular momenta and bulk dimensions, and find that, in all cases, the bulk emission remains significantly smaller than the brane emission. The angular-momentum loss rate is also computed and found to have a smaller value in the bulk than on the brane
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