12,643 research outputs found
General relativistic neutrino transport using spectral methods
We present a new code, Lorene's Ghost (for Lorene's gravitational handling of
spectral transport) developed to treat the problem of neutrino transport in
supernovae with the use of spectral methods. First, we derive the expression
for the nonrelativistic Liouville operator in doubly spherical coordinates (r,
theta, phi, epsilon, Theta, Phi)$, and further its general relativistic
counterpart. We use the 3 + 1 formalism with the conformally flat approximation
for the spatial metric, to express the Liouville operator in the Eulerian
frame. Our formulation does not use any approximations when dealing with the
angular arguments (theta, phi, Theta, Phi), and is fully energy-dependent. This
approach is implemented in a spherical shell, using either Chebyshev
polynomials or Fourier series as decomposition bases. It is here restricted to
simplified collision terms (isoenergetic scattering) and to the case of a
static fluid. We finish this paper by presenting test results using basic
configurations, including general relativistic ones in the Schwarzschild
metric, in order to demonstrate the convergence properties, the conservation of
particle number and correct treatment of some general-relativistic effects of
our code. The use of spectral methods enables to run our test cases in a
six-dimensional setting on a single processor.Comment: match published versio
Symmetric Regularization, Reduction and Blow-Up of the Planar Three-Body Problem
We carry out a sequence of coordinate changes for the planar three-body
problem which successively eliminate the translation and rotation symmetries,
regularize all three double collision singularities and blow-up the triple
collision. Parametrizing the configurations by the three relative position
vectors maintains the symmetry among the masses and simplifies the
regularization of binary collisions. Using size and shape coordinates
facilitates the reduction by rotations and the blow-up of triple collision
while emphasizing the role of the shape sphere. By using homogeneous
coordinates to describe Hamiltonian systems whose configurations spaces are
spheres or projective spaces, we are able to take a modern, global approach to
these familiar problems. We also show how to obtain the reduced and regularized
differential equations in several convenient local coordinates systems.Comment: 51 pages, 4 figure
Colliding String Waves and Duality
The collision of plane waves corresponding to massless states of closed
string is considered in -dimensional space-time. The reduced tree level
effective action is known to be manifestly invariant, being the
number of transverse spatial dimensions in the collision process. We adopt a
coset space reformulation of the effective two dimensional theory and discuss
the relation of this process with classical integrable systems in two
dimensions in the presence of gravity. We show how it is possible to generate
new backgrounds for the scattering process, from known background solutions to
the equations of motion, in the coset reformulation. We present explicit
calculations for the case of four space-time dimensions as an illustrative
example.Comment: 14 page
Initial Conditions and the Structure of the Singularity in Pre-Big-Bang Cosmology
We propose a picture, within the pre-big-bang approach, in which the universe
emerges from a bath of plane gravitational and dilatonic waves. The waves
interact gravitationally breaking the exact plane symmetry and lead generically
to gravitational collapse resulting in a singularity with the Kasner-like
structure. The analytic relations between the Kasner exponents and the initial
data are explicitly evaluated and it is shown that pre-big-bang inflation may
occur within a dense set of initial data. Finally, we argue that plane waves
carry zero gravitational entropy and thus are, from a thermodynamical point of
view, good candidates for the universe to emerge from.Comment: 18 pages, LaTeX, epsfig. 3 figures included. Minor changes; paragraph
added in the introduction, references added and typos corrected. Final
version published in Classical and Quantum Gravit
Robot graphic simulation testbed
The objective of this research was twofold. First, the basic capabilities of ROBOSIM (graphical simulation system) were improved and extended by taking advantage of advanced graphic workstation technology and artificial intelligence programming techniques. Second, the scope of the graphic simulation testbed was extended to include general problems of Space Station automation. Hardware support for 3-D graphics and high processing performance make high resolution solid modeling, collision detection, and simulation of structural dynamics computationally feasible. The Space Station is a complex system with many interacting subsystems. Design and testing of automation concepts demand modeling of the affected processes, their interactions, and that of the proposed control systems. The automation testbed was designed to facilitate studies in Space Station automation concepts
Characterization of initial fluctuations for the hydrodynamical description of heavy ion collisions
Event-by-event fluctuations in the initial conditions for a hydrodynamical
description of heavy-ion collisions are characterized. We propose a
Bessel-Fourier decomposition with respect to the azimuthal angle, the radius in
the transverse plane and rapidity. This allows for a complete characterization
of fluctuations in all hydrodynamical fields including energy density,
pressure, fluid velocity, shear stress and bulk viscous pressure. It has the
advantage that fluctuations can be ordered with respect to their wave length
and that they can be propagated mode-by-mode within the hydrodynamical
formalism. Event ensembles can then be characterized in terms of a functional
probability distribution. For the event ensemble of a Monte Carlo Glauber
model, we provide evidence that the latter is close to Gaussian form, thus
allowing for a particularly simple characterization of the event distribution.Comment: 40 pages, 16 figure
- …