536 research outputs found
Nonequilibrium Precursor Model for the Onset of Percolation in a Two-Phase System
Using a Boltzmann equation, we investigate the nonequilibrium dynamics of
nonperturbative fluctuations within the context of Ginzburg-Landau models. As
an illustration, we examine how a two-phase system initially prepared in a
homogeneous, low-temperature phase becomes populated by precursors of the
opposite phase as the temperature is increased. We compute the critical value
of the order parameter for the onset of percolation, which signals the
breakdown of the conventional dilute gas approximation.Comment: 4 pages, 4 eps figures (uses epsf), Revtex. Replaced with version in
press Physical Review
The collision of boosted black holes: second order close limit calculations
We study the head-on collision of black holes starting from unsymmetrized,
Brill--Lindquist type data for black holes with non-vanishing initial linear
momentum. Evolution of the initial data is carried out with the ``close limit
approximation,'' in which small initial separation and momentum are assumed,
and second-order perturbation theory is used. We find agreement that is
remarkably good, and that in some ways improves with increasing momentum. This
work extends a previous study in which second order perturbation calculations
were used for momentarily stationary initial data, and another study in which
linearized perturbation theory was used for initially moving holes. In addition
to supplying answers about the collisions, the present work has revealed
several subtle points about the use of higher order perturbation theory, points
that did not arise in the previous studies. These points include issues of
normalization, and of comparison with numerical simulations, and will be
important to subsequent applications of approximation methods for collisions.Comment: 20 pages, RevTeX, 6 figures included with psfi
Perturbative evolution of conformally flat initial data for a single boosted black hole
The conformally flat families of initial data typically used in numerical
relativity to represent boosted black holes are not those of a boosted slice of
the Schwarzschild spacetime. If such data are used for each black hole in a
collision, the emitted radiation will be partially due to the ``relaxation'' of
the individual holes to ``boosted Schwarzschild'' form. We attempt to compute
this radiation by treating the geometry for a single boosted conformally flat
hole as a perturbation of a Schwarzschild black hole, which requires the use of
second order perturbation theory. In this we attempt to mimic a previous
calculation we did for the conformally flat initial data for spinning holes. We
find that the boosted black hole case presents additional subtleties, and
although one can evolve perturbatively and compute radiated energies, it is
much less clear than in the spinning case how useful for the study of
collisions are the radiation estimates for the ``spurious energy'' in each
hole. In addition to this we draw some lessons on which frame of reference
appears as more favorable for computing black hole collisions in the close
limit approximation.Comment: 11 pages, RevTex, 4 figures included with psfig, to appear in PR
The initial value problem for linearized gravitational perturbations of the Schwarzschild naked singularity
The coupled equations for the scalar modes of the linearized Einstein
equations around Schwarzschild's spacetime were reduced by Zerilli to a 1+1
wave equation with a potential , on a field . For smooth metric
perturbations is singular at , the
mode harmonic number, and has a second order pole at . This is
irrelevant to the black hole exterior stability problem, where , and
, but it introduces a non trivial problem in the naked singular case
where , and the singularity appears in the relevant range of
. We solve this problem by developing a new approach to the evolution of the
even mode, based on a {\em new gauge invariant function}, -related
to by an intertwiner operator- that is a regular function of the
metric perturbation {\em for any value of }. This allows to address the
issue of evolution of gravitational perturbations in this non globally
hyperbolic background, and to complete the proof of the linear instability of
the Schwarzschild naked singularity, by showing that a previously found
unstable mode is excitable by generic initial data. This is further illustrated
by numerically solving the linearized equations for suitably chosen initial
data.Comment: typos corrected, references adde
Drake Equation for the Multiverse: From the String Landscape to Complex Life
It is argued that selection criteria usually referred to as "anthropic
conditions" for the existence of intelligent (typical) observers widely adopted
in cosmology amount only to preconditions for primitive life. The existence of
life does not imply in the existence of intelligent life. On the contrary, the
transition from single-celled to complex, multi-cellular organisms is far from
trivial, requiring stringent additional conditions on planetary platforms. An
attempt is made to disentangle the necessary steps leading from a selection of
universes out of a hypothetical multiverse to the existence of life and of
complex life. It is suggested that what is currently called the "anthropic
principle" should instead be named the "prebiotic principle."Comment: 6 pages, RevTeX, in press, Int. J. Mod. Phys.
Linear stability of Einstein-Gauss-Bonnet static spacetimes. Part II: vector and scalar perturbations
We study the stability under linear perturbations of a class of static
solutions of Einstein-Gauss-Bonnet gravity in dimensions with spatial
slices of the form \Sigma_{\k}^n \times {\mathbb R}^+, \Sigma_{\k}^n an
manifold of constant curvature \k. Linear perturbations for this class of
space-times can be generally classified into tensor, vector and scalar types.
In a previous paper, tensor perturbations were analyzed. In this paper we study
vector and scalar perturbations. We show that vector perturbations can be
analyzed in general using an S-deformation approach and do not introduce
instabilities. On the other hand, we show by analyzing an explicit example
that, contrary to what happens in Einstein gravity, scalar perturbations may
lead to instabilities in black holes with spherical horizons when the
Gauss-Bonnet string corrections are taken into account.Comment: 16 pages, 6 figure
Information-Entropic Measure of Energy-Degenerate Kinks in Two-Field Models
We investigate the existence and properties of kink-like solitons in a class
of models with two interacting scalar fields. In particular, we focus on models
that display both double and single-kink solutions, treatable analytically
using the Bogomol'nyi--Prasad--Sommerfield bound (BPS). Such models are of
interest in applications that include Skyrmions and various
superstring-motivated theories. Exploring a region of parameter space where the
energy for very different spatially-bound configurations is degenerate, we show
that a newly-proposed momentum-space entropic measure called Configurational
Entropy (CE) can distinguish between such energy-degenerate spatial profiles.
This information-theoretic measure of spatial complexity provides a
complementary perspective to situations where strictly energy-based arguments
are inconclusive
On low energy quantum gravity induced effects on the propagation of light
Present models describing the interaction of quantum Maxwell and
gravitational fields predict a breakdown of Lorentz invariance and a non
standard dispersion relation in the semiclassical approximation. Comparison
with observational data however, does not support their predictions. In this
work we introduce a different set of ab initio assumptions in the canonical
approach, namely that the homogeneous Maxwell equations are valid in the
semiclassical approximation, and find that the resulting field equations are
Lorentz invariant in the semiclassical limit. We also include a
phenomenological analysis of possible effects on the propagation of light, and
their dependence on energy, in a cosmological context.Comment: 12 page
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