5,886 research outputs found
Multiple steady states for characteristic initial value problems
The time dependent, isentropic, quasi-one-dimensional equations of gas dynamics and other model equations are considered under the constraint of characteristic boundary conditions. Analysis of the time evolution shows how different initial data may lead to different steady states and how seemingly anamolous behavior of the solution may be resolved. Numerical experimentation using time consistent explicit algorithms verifies the conclusions of the analysis. The use of implicit schemes with very large time steps leads to erroneous results
Diffeomorphisms, Noether Charges and Canonical Formalism in 2D Dilaton Gravity
We carry out a parallel study of the covariant phase space and the
conservation laws of local symmetries in two-dimensional dilaton gravity. Our
analysis is based on the fact that the Lagrangian can be brought to a form that
vanishes on-shell giving rise to a well-defined covariant potential for the
symplectic current. We explicitly compute the symplectic structure and its
potential and show that the requirement to be finite and independent of the
Cauchy surface restricts the asymptotic symmetries.Comment: 14 pages, latex with psfig macro, one figur
Exact Potts Model Partition Functions for Strips of the Honeycomb Lattice
We present exact calculations of the Potts model partition function
for arbitrary and temperature-like variable on -vertex
strip graphs of the honeycomb lattice for a variety of transverse widths
equal to vertices and for arbitrarily great length, with free
longitudinal boundary conditions and free and periodic transverse boundary
conditions. These partition functions have the form
, where
denotes the number of repeated subgraphs in the longitudinal direction. We give
general formulas for for arbitrary . We also present plots of
zeros of the partition function in the plane for various values of and
in the plane for various values of . Explicit results for partition
functions are given in the text for (free) and (cylindrical),
and plots of partition function zeros are given for up to 5 (free) and
(cylindrical). Plots of the internal energy and specific heat per site
for infinite-length strips are also presented.Comment: 39 pages, 34 eps figures, 3 sty file
Caracterizacion de las zonas de asentamiento de postlarvas de langosta, Panulirus argus, en la costa noreste de Yucatan
A Planck-like problem for quantum charged black holes
Motivated by the parallelism existing between the puzzles of classical
physics at the beginning of the XXth century and the current paradoxes in the
search of a quantum theory of gravity, we give, in analogy with Planck's black
body radiation problem, a solution for the exact Hawking flux of evaporating
Reissner-Nordstrom black holes. Our results show that when back-reaction
effects are fully taken into account the standard picture of black hole
evaporation is significantly altered, thus implying a possible resolution of
the information loss problem.Comment: 6 pages, LaTeX file, Awarded Fifth Prize in the Gravity Research
Foundation Essay Competition for 200
Particles and energy fluxes from a CFT perspective
We analyze the creation of particles in two dimensions under the action of
conformal transformations. We focus our attention on Mobius transformations and
compare the usual approach, based on the Bogolubov coefficients, with an
alternative but equivalent viewpoint based on correlation functions. In the
latter approach the absence of particle production under full Mobius
transformations is manifest. Moreover, we give examples, using the
moving-mirror analogy, to illustrate the close relation between the production
of quanta and energy.Comment: Revised version. To appear in Phys.Rev.
Inflation, Renormalization, and CMB Anisotropies
In single-field, slow-roll inflationary models, scalar and tensorial
(Gaussian) perturbations are both characterized by a zero mean and a non-zero
variance. In position space, the corresponding variance of those fields
diverges in the ultraviolet. The requirement of a finite variance in position
space forces its regularization via quantum field renormalization in an
expanding universe. This has an important impact on the predicted scalar and
tensorial power spectra for wavelengths that today are at observable scales. In
particular, we find a non-trivial change in the consistency condition that
relates the tensor-to-scalar ratio "r" to the spectral indices. For instance,
an exact scale-invariant tensorial power spectrum, n_t=0, is now compatible
with a non-zero ratio r= 0.12 +/- 0.06, which is forbidden by the standard
prediction (r=-8n_t). Forthcoming observations of the influence of relic
gravitational waves on the CMB will offer a non-trivial test of the new
predictions.Comment: 4 pages, jpconf.cls, to appear in the Proceedings of Spanish
Relativity Meeting 2009 (ERE 09), Bilbao (Spain
A Parallel Preconditioner for 2D Elliptic Boundary Value Problems
This work presents the implementation on a Linux Cluster of a
parallel preconditioner for the solution of the linear system resulting from
the finite element discretization of a 2D second order elliptic boundary value
problem. The numerical method, proposed by Bramble, Pasciak and Schatz, is
developed using Domain Decomposition techniques, which are based on the
splitting of the computational domain into subregions of smaller size,
enforcing suitable compatibility conditions. The Fortran code is implemented
using PETSc: a suite of data structures and routines devoted to the scientific
parallel computing and based on the MPI standard for all message-passing
communications. The main interest of the paper is to investigate how the
architectural aspects of the cluster influence the performance of the
considered algorithm. We provide an analysis of the execution times as well as
of the scalability, using as test case the classical Poisson equation with
Dirichlet boundary conditions
Semiclassical zero-temperature corrections to Schwarzschild spacetime and holography
Motivated by the quest for black holes in AdS braneworlds, and in particular
by the holographic conjecture relating 5D classical bulk solutions with 4D
quantum corrected ones, we numerically solve the semiclassical Einstein
equations (backreaction equations) with matter fields in the (zero temperature)
Boulware vacuum state. In the absence of an exact analytical expression for
in four dimensions we work within the s-wave approximation. Our
results show that the quantum corrected solution is very similar to
Schwarzschild till very close to the horizon, but then a bouncing surface for
the radial function appears which prevents the formation of an event horizon.
We also analyze the behavior of the geometry beyond the bounce, where a
curvature singularity arises. In the dual theory, this indicates that the
corresponding 5D static classical braneworld solution is not a black hole but
rather a naked singularity.Comment: 26 pages, 4 figures; revised version (title changed, conclusions
shortened), published as Phys. Rev. D73, 104023 (2006
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