1,036 research outputs found
Warped embeddings between Einstein manifolds
Warped embeddings from a lower dimensional Einstein manifold into a higher
dimensional one are analyzed. Explicit solutions for the embedding metrics are
obtained for all cases of codimension 1 embeddings and some of the codimension
n>1 cases. Some of the interesting features of the embedding metrics are
pointed out and potential applications of the embeddings are discussed.Comment: 12 pages, to appear in Mod. Phys. Lett.
Mixed perturbative expansion: the validity of a model for the cascading
A new type of perturbative expansion is built in order to give a rigorous
derivation and to clarify the range of validity of some commonly used model
equations.
This model describes the evolution of the modulation of two short and
localized pulses, fundamental and second harmonic, propagating together in a
bulk uniaxial crystal with non-vanishing second order susceptibility
and interacting through the nonlinear effect known as ``cascading'' in
nonlinear optics.
The perturbative method mixes a multi-scale expansion with a power series
expansion of the susceptibility, and must be carefully adapted to the physical
situation. It allows the determination of the physical conditions under which
the model is valid: the order of magnitude of the walk-off, phase-mismatch,and
anisotropy must have determined values.Comment: arxiv version is already officia
Finite-distance singularities in the tearing of thin sheets
We investigate the interaction between two cracks propagating in a thin
sheet. Two different experimental geometries allow us to tear sheets by
imposing an out-of-plane shear loading. We find that two tears converge along
self-similar paths and annihilate each other. These finite-distance
singularities display geometry-dependent similarity exponents, which we
retrieve using scaling arguments based on a balance between the stretching and
the bending of the sheet close to the tips of the cracks.Comment: 4 pages, 4 figure
The (p,q) String Tension in a Warped Deformed Conifold
We find the tension spectrum of the bound states of p fundamental strings and
q D-strings at the bottom of a warped deformed conifold. We show that it can be
obtained from a D3-brane wrapping a 2-cycle that is stabilized by both electric
and magnetic fluxes. Because the F-strings are Z_M-charged with non-zero
binding energy, binding can take place even if (p,q) are not coprime.
Implications for cosmic strings are briefly discussed.Comment: 17 pages, 1 figur
Codimension Two Compactifications and the Cosmological Constant Problem
We consider solutions of six dimensional Einstein equations with two compact
dimensions. It is shown that one can introduce 3-branes in this background in
such a way that the effective four dimensional cosmological constant is
completely independent of the brane tensions. These tensions are completely
arbitrary, without requiring any fine tuning. We must, however, fine tune bulk
parameters in order to obtain a sufficiently small value for the observable
cosmological constant. We comment in the effective four dimensional description
of this effect at energies below the compactification scale.Comment: 4 pages, rextex
Stable topological modes in two-dimensional Ginzburg-Landau models with trapping potentials
Complex Ginzburg-Landau (CGL) models of laser media (with the cubic-quintic
nonlinearity) do not contain an effective diffusion term, which makes all
vortex solitons unstable in these models. Recently, it has been demonstrated
that the addition of a two-dimensional periodic potential, which may be induced
by a transverse grating in the laser cavity, to the CGL equation stabilizes
compound (four-peak) vortices, but the most fundamental "crater-shaped"
vortices (CSVs), alias vortex rings, which are, essentially, squeezed into a
single cell of the potential, have not been found before in a stable form. In
this work we report families of stable compact CSVs with vorticity S=1 in the
CGL model with the external potential of two different types: an axisymmetric
parabolic trap, and the periodic potential. In both cases, we identify
stability region for the CSVs and for the fundamental solitons (S=0). Those
CSVs which are unstable in the axisymmetric potential break up into robust
dipoles. All the vortices with S=2 are unstable, splitting into tripoles.
Stability regions for the dipoles and tripoles are identified too. The periodic
potential cannot stabilize CSVs with S>=2 either; instead, families of stable
compact square-shaped quadrupoles are found
Cosmological Evolution of a Purely Conical Codimension-2 Brane World
We study the cosmological evolution of isotropic matter on an infinitely thin
conical codimension-two brane-world. Our analysis is based on the boundary
dynamics of a six-dimensional model in the presence of an induced gravity term
on the brane and a Gauss-Bonnet term in the bulk. With the assumption that the
bulk contains only a cosmological constant Lambda_B, we find that the isotropic
evolution of the brane-universe imposes a tuned relation between the energy
density and the brane equation of state. The evolution of the system has fixed
points (attractors), which correspond to a final state of radiation for
Lambda_B=0 and to de Sitter state for Lambda_B>0. Furthermore, considering
anisotropic matter on the brane, the tuning of the parameters is lifted, and
new regions of the parametric space are available for the cosmological
evolution of the brane-universe. The analysis of the dynamics of the system
shows that, the isotropic fixed points remain attractors of the system, and for
values of Lambda_B which give acceptable cosmological evolution of the equation
of state, the line of isotropic tuning is a very weak attractor. The initial
conditions, in this case, need to be fine tuned to have an evolution with
acceptably small anisotropy.Comment: 20 pages, 4 figures, typo correcte
Invasive pulmonary aspergillosis in chronic obstructive pulmonary disease: an emerging fungal pathogen
ABSTRACTAcute invasive pulmonary aspergillosis occurs predominantly in immunocompromised hosts, with increasing numbers of cases of invasive aspergillosis among patients with chronic obstructive pulmonary disease (COPD) being reported. Among 13 cases of invasive aspergillosis diagnosed in COPD patients admitted to the intensive care unit with acute respiratory distress, the only risk factor for invasive fungal infection was corticosteroid treatment. Invasive aspergillosis should be suspected in COPD patients receiving steroid treatment who have extensive pulmonary infiltrates. Survival depends on rapid diagnosis and early appropriate treatment. A decrease or interruption of steroid treatment should be considered as part of the overall therapeutic strategy
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