3,504 research outputs found
Quantum Harmonic Black Holes
Inspired by the recent conjecture that black holes are condensates of
gravitons, we investigate a simple model for the black hole degrees of freedom
that is consistent both from the point of view of Quantum mechanics and of
General Relativity. Since the two perspectives should "converge" into a unified
picture for small, Planck size, objects, we expect our construction is a useful
step for understanding the physics of microscopic, quantum black holes. In
particular, we show that a harmonically trapped condensate gives rise to two
horizons, whereas the extremal case (corresponding to a remnant with vanishing
Hawking temperature) naturally falls out of its spectrum.Comment: 7 pages, no figures. Clarifications and comments adde
Effects of non-unity Lewis numbers in diffusion Flames
The purpose of this work is to carry out direct numerícal simulations of diffusion controlled combustión with non-unity Lewis numbers for the reactants and producís,
thus accounting for the düferential diífusion effects of the temperature and concentration fields. We use a formulation (Liñán (1991a)) based on combining the conservation equations in a way to elimínate the reaction terms similar to the method used by Burke and Schumann (1928) for unity Lewis numbers. We present calculations for an axisymmetric fuel jet and for a planar, time evolving mixing layer, leaving out the effects of thermal expansión and variations of the transport coefficients due to the heat reisase. Our results show that the front of the fíame sbifts toward the fuel or oxygen sides owing to the effect of the düferential diífusion and that the location of máximum temperature may not coincide with the fíame. The dependence of the distríbution of the reaction products on their Lewis number has been investigated
Bubble dynamics: (nucleating) radiation inside dust
We consider two spatially flat Friedmann-Robertson-Walker spacetimes divided
by a time-like thin shell in the nontrivial case in which the inner region of
finite extension contains radiation and the outer region is filled with dust.
We will then show that, while the evolution is determined by a large set of
constraints, an analytical description for the evolution of the bubble radius
can be obtained by formally expanding for short times after the shell attains
its minimum size. In particular, we will find that a bubble of radiation,
starting out with vanishing expansion speed, can be matched with an expanding
dust exterior, but not with a collapsing dust exterior, regardless of the dust
energy density. The former case can then be used to describe the nucleation of
a bubble of radiation inside an expanding dust cloud, although the final
configuration contains more energy than the initial dust, and the reverse
process, with collapsing radiation transforming into collapsing dust, is
therefore energetically favored. We however speculate a (small) decaying vacuum
energy or cosmological constant inside dust could still trigger nucleation.
Finally, our perturbative (yet analytical) approach can be easily adapted to
different combinations of matter inside and outside the shell, as well as to
more general surface density, of relevance for cosmology and studies of defect
formation during phase transitions.Comment: 8 pages, 4 figures. Revised version for PR
On the Unruh effect in de Sitter space
We give an interpretation of the temperature in de Sitter universe in terms
of a dynamical Unruh effect associated with the Hubble sphere. As with the
quantum noise perceived by a uniformly accelerated observer in static
space-times, observers endowed with a proper motion can in principle detect the
effect. In particular, we study a "Kodama observer" as a two-field Unruh
detector for which we show the effect is approximately thermal. We also
estimate the back-reaction of the emitted radiation and find trajectories
associated with the Kodama vector fields are stable.Comment: 8 pages; corrected typos; sections structure revise
Typical Gibbs configurations for the 1d Random Field Ising Model with long range interaction
We study a one--dimensional Ising spin systems with ferromagnetic,
long--range interaction decaying as n^{-2+\a}, \a \in [0,\frac 12], in the
presence of external random fields. We assume that the random fields are given
by a collection of symmetric, independent, identically distributed real random
variables, gaussian or subgaussian with variance . We show that for
temperature and variance of the randomness small enough, with an overwhelming
probability with respect to the random fields, the typical configurations,
within volumes centered at the origin whose size grow faster than any power of
, % {\bf around the origin} are intervals of spins followed by
intervals of spins whose typical length is \simeq
\th^{-\frac{2}{(1-2\a)}} for 0\le \a<1/2 and
for \a=1/2
A finite-difference scheme for three-dimensional incompressible flows in spherical coordinates
In this study we have developed a flexible and efficient numerical scheme for
the simulation of three-dimensional incompressible flows in spherical
coordinates. The main idea, inspired by a similar strategy as (Verzicco, R.,
Orlandi, P., 1996, A Finite-Difference Scheme for Three-Dimensional
Incompressible Flows in Cylindrical Coordinates) for cylindrical coordinates,
consists of a change of variables combined with a discretization on a staggered
mesh and the special treatment of few discrete terms that remove the
singularities of the Navier-Stokes equations at the sphere centre and along the
polar axis. This new method alleviates also the time step restrictions
introduced by the discretization around the polar axis while the sphere centre
still yields strong limitations, although only in very unfavourable flow
configurations.
The scheme is second-order accurate in space and is verified and validated by
computing numerical examples that are compared with similar results produced by
other codes or available from the literature.
The method can cope with flows evolving in the whole sphere, in a spherical
shell and in a sector without any change and, thanks to the flexibility of
finite-differences, it can employ generic mesh stretching (in two of the three
directions) and complex boundary conditions
The key role of miRNA in syndromic and sporadic forms of ascending aortic aneurysms as biomarkers and targets of novel therapeutic strategies
Increasing evidence shows that epigenetics also plays a key role in regulating the pathogenetic mechanism of all types of aortic aneurysms. It is well-known that epigenetic factors modulate gene expression. This mechanism appears to be of interest especially knowing the relevance of genetic susceptibility and genetic factors in the complex pathophysiology of aortic aneurysms, and of sporadic forms; in fact, the latter are the result of a close interaction between genetic and modifiable lifestyle factors (i.e., nutrition, smoking, infections, use of drugs, alcohol, sedentary lifestyle, etc.). Epigenetic factors include DNA methylation, post-translational histone modifications, and non-coding RNA. Here, our attention is focused on the role of miRNA in syndromic and sporadic forms of thoracic aortic aneurysms. They could be both biomarkers and targets of novel therapeutic strategies
Colourgrams GUI: A graphical user-friendly interface for the analysis of large datasets of RGB images
Colourgrams GUI is a graphical user-friendly interface developed in order to facilitate the analysis of large datasets of RGB images through the colourgrams approach. Briefly, the colourgrams approach consists in converting a dataset of RGB images into a matrix of one-dimensional signals, the colourgrams, each one codifying the colour content of the corresponding original image. This matrix of signals can be in turn analysed by means of common multivariate statistical methods, such as Principal Component Analysis (PCA) for exploratory analysis of the image dataset, or Partial Least Squares (PLS) regression for the quantification of colour-related properties of interest. Colourgrams GUI allows to easily convert the dataset of RGB images into the colourgrams matrix, to interactively visualize the signals coloured according to qualitative and/or quantitative properties of the corresponding samples and to visualize the colour features corresponding to selected colourgram regions into the image domain. In addition, the software also allows to analyse the colourgrams matrix by means of PCA and PLS
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