2,772 research outputs found
Generalized gravitational entropy without replica symmetry
We explore several extensions of the generalized entropy construction of
Lewkowycz and Maldacena, including a formulation that does not rely on
preserving replica symmetry in the bulk. We show that an appropriately general
ansatz for the analytically continued replica metric gives us the flexibility
needed to solve the gravitational field equations beyond general relativity. As
an application of this observation we study Einstein-Gauss-Bonnet gravity with
a small Gauss-Bonnet coupling and derive the condition that the holographic
entanglement entropy must be evaluated on a surface which extremizes the
Jacobson-Myers entropy. We find that in both general relativity and
Einstein-Gauss-Bonnet gravity replica symmetry breaking terms are permitted by
the field equations, suggesting that they do not generically vanish.Comment: 24 pages, 3 figures. v3: fixed some more typos, v2: fixed minor typo
Modelling ripples in Orion with coupled dust dynamics and radiative transfer
In light of the recent detection of direct evidence for the formation of
Kelvin-Helmholtz instabilities in the Orion nebula, we expand upon previous
modelling efforts by numerically simulating the shear-flow driven gas and dust
dynamics in locations where the H region and the molecular cloud
interact. We aim to directly confront the simulation results with the infrared
observations. Methods: To numerically model the onset and full nonlinear
development of the Kelvin-Helmholtz instability we take the setup proposed to
interpret the observations, and adjust it to a full 3D hydrodynamical
simulation that includes the dynamics of gas as well as dust. A dust grain
distribution with sizes between 5-250 nm is used, exploiting the gas+dust
module of the MPI-AMRVAC code, in which the dust species are represented by
several pressureless dust fluids. The evolution of the model is followed well
into the nonlinear phase. The output of these simulations is then used as input
for the SKIRT dust radiative transfer code to obtain infrared images at several
stages of the evolution, which can be compared to the observations. Results: We
confirm that a 3D Kelvin-Helmholtz instability is able to develop in the
proposed setup, and that the formation of the instability is not inhibited by
the addition of dust. Kelvin-Helmholtz billows form at the end of the linear
phase, and synthetic observations of the billows show striking similarities to
the infrared observations. It is pointed out that the high density dust regions
preferentially collect on the flanks of the billows. To get agreement with the
observed Kelvin-Helmholtz ripples, the assumed geometry between the background
radiation, the billows and the observer is seen to be of critical importance.Comment: 8 pages, 10 figure
Derivation of the blackfold effective theory
We study fluctuations and deformations of black branes over length scales
larger than the horizon radius. We prove that the Einstein equations for the
perturbed p-brane yield, as constraints, the equations of the effective
blackfold theory. We solve the Einstein equations for the perturbed geometry
and show that it remains regular on and outside the black brane horizon. This
study provides an ab initio derivation of the blackfold effective theory and
gives explicit expressions for the metrics near the new black holes and black
branes that result from it, to leading order in a derivative expansion.Comment: 20 pages. v4: Typo corrected in eq. (6.11) -- erratum in the
published versio
Polarization in Monte Carlo radiative transfer and dust scattering polarization signatures of spiral galaxies
Polarization is an important tool to further the understanding of interstellar dust and the sources behind it. In this paper we describe our implementation of polarization that is due to scattering of light by spherical grains and electrons in the dust Monte Carlo radiative transfer code SKIRT. In contrast to the implementations of other Monte Carlo radiative transfer codes, ours uses co-moving reference frames that rely solely on the scattering processes. It fully supports the peel-off mechanism that is crucial for the efficient calculation of images in 3D Monte Carlo codes. We develop reproducible test cases that push the limits of our code. The results of our program are validated by comparison with analytically calculated solutions. Additionally, we compare results of our code to previously published results. We apply our method to models of dusty spiral galaxies at near-infrared and optical wavelengths. We calculate polarization degree maps and show them to contain signatures that trace characteristics of the dust arms independent of the inclination or rotation of the galaxy
Models of Human Vascular Disease: Is There an Animal of La Mancha?
Cervantes understood that models–be they physical or moral lessons–are valid only in as much as they mirror that which they seek to mimic. This is the essential issue presented by Diego et al. in the article published in Revista Española de Cardiología. Drug-eluting stents have changed the practice of medicine and are perhaps the most common intervention used today. Millions of stents are placed each year and yet critical questions remain as to whether one design is better than another. The challenge in major part is that, though device designs may be significantly different one from another, detection of a clinical difference is difficult given the rarity of side effects. Human clinical trials are too small and too short to detect differences even in fatal events that occur in 1 in 100 patients per year. The natural fallback is to rely on animal model systems and yet it is unclear how best to use them. Diego et al. describe a study that compares the proliferative response elicited after deployment of paclitaxel-eluting and bare metal stents in porcine coronary arteries. They suggest that the ability of a stent platform to significantly impact late vascular healing depends upon the degree of injury that is created at the time of implantation. Such a result has profound impact on how we consider animal model systems for critical technologies, our view of vascular biology and vascular repair, and our appreciation of the history of work in this field. Moreover, the study shows how a difficult parameter rarely controlled in human interventions–the extent of injury–is such a powerful regulator of clinical effect and restenotic side effect.National Institutes of Health (U.S.) (Grant RO1/GM049039)National Institute of General Medical Sciences (U.S.) (Grant RO1/GM049039
Black Branes in a Box: Hydrodynamics, Stability, and Criticality
We study the effective hydrodynamics of neutral black branes enclosed in a
finite cylindrical cavity with Dirichlet boundary conditions. We focus on how
the Gregory-Laflamme instability changes as we vary the cavity radius R. Fixing
the metric at the cavity wall increases the rigidity of the black brane by
hindering gradients of the redshift on the wall. In the effective fluid, this
is reflected in the growth of the squared speed of sound. As a consequence,
when the cavity is smaller than a critical radius the black brane becomes
dynamically stable. The correlation with the change in thermodynamic stability
is transparent in our approach. We compute the bulk and shear viscosities of
the black brane and find that they do not run with R. We find mean-field theory
critical exponents near the critical point.Comment: 23 pages, 3 figures. v2: added comments on first-order phase
transitio
Fraction-like ratings from preferential voting
A method is given for resolving a matrix of preference scores into a well-specfied mixture of options. This is done in agreement with several desirable properties, including the continuity of the mixing proportions with respect to the preference scores and a condition of compatibility with the Condorcet-Smith majority principle. These properties are achieved by combining the classical rating method of Zermelo with a projection procedure introduced in previous papers of the same authors
Rhinoplasty. The Difficult Nasal Tip: Total Resection of the Alar Cartilages
There are many ways to reconstruct and make nasal tips more attractive. Sometimes we cannot find the best way unless we at least remove all surplus from the tip. This may occur in primary or secondary rhinoplasty. In principle, anything is possible when relocating and reconstructing. However, sometimes we face reality when we uncover the tip: broken or bulging cartilages that are difficult to put right. For this reason, in 1987 we thought of totally resectioning the alar cartilages in a case of secondary rhinoplasty with an unsightly appearance. After a year the result was seen to be correct from an aesthetic and a functional perspective and is still so today. Aesthetically, it kept its shape and did not collapse with nasal respiratory failure. We covered the end of the crus medialis with a small, temporary, one- to two-layered fascia patch. Except in exceptional cases, we now use this procedure: Total sectioning of the alar cartilages including the domes, or maintenance of them by preserving the fibroadipose tip tissue with a suture in the middle of the end of the crus medialis and by covering this with temporary fascia, which usually has two layers depending on the thickness of the skin of the tip. This procedure is indicated mainly in secondary rhinoplasty when the cartilages of the tip are completely destroyed, and in primary rhinoplasty when the tip is excessively wide and bulbous. Our philosophy is, therefore, elegance and beauty of the nasal tip with a solid and equilateral base without prejudices
Gregory-Laflamme instability of a slowly rotating black string
We study the Gregory-Laflamme instability of a 5-dimensional slowly rotating
black string in which the 4-dimensional section is described by the Kerr black
hole. We treat the rotation in a perturbative way introducing a small parameter
for the rotation. It is found that rotation makes the Gregory-Laflamme
instability stronger. Both the critical wavelength at the onset of instability
and the growth time-scale are found to decrease as the rotation increases.Comment: 26 pages, 1 figur
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