5,166 research outputs found
The role of domain wall junctions in Carter's pentahedral model
The role of domain wall junctions in Carter's pentahedral model is
investigated both analytically and numerically. We perform, for the first time,
field theory simulations of such model with various initial conditions. We
confirm that there are very specific realizations of Carter's model
corresponding to square lattice configurations with X-type junctions which
could be stable. However, we show that more realistic realizations, consistent
with causality constraints, do lead to a scaling domain wall network with
Y-type junctions. We determine the network properties and discuss the
corresponding cosmological implications, in particular for dark energy.Comment: 6 pages, 6 figure
Scaling of cosmological domain wall networks with junctions
We describe the results of the largest and most accurate three-dimensional
field theory simulations of domain wall networks with junctions. We consider a
previously introduced class of models which, in the limit of large number
of coupled scalar fields, approaches the so-called `ideal' model (in terms of
its potential to lead to network frustration). We consider values of
between N=2 and N=20. In all cases we find compelling evidence for a gradual
approach to scaling, with the quantitative scaling parameters having only a
mild dependence on . These results strongly support our no-frustration
conjecture.Comment: 4 pages, 2 figure
The growth of linear perturbations in generic defect models for structure formation
We study the growth of linear perturbations induced by a generic causal
scaling source as a function of the cosmological parameters ,
and . We show that for wavenumbers k \gsim 0.01 h/Mpc the
spectrum of density and velocity perturbations scale in a similar way to that
found in inflationary models with primordial perturbations. We show that this
result is independent of the more or less incoherent nature of the source, the
small scale power spectrum of the source and of deviations from scaling which
naturally occur at late times if .Comment: 4 pages, 2 figure
The cluster abundance in cosmic string models for structure formation
We use the present observed number density of large X-ray clusters to
constrain the amplitude of matter density perturbations induced by cosmic
strings on the scale of Mpc (), in both open cosmologies
and flat models with a non-zero cosmological constant. We find a slightly lower
value of than that obtained in the context of primordial Gaussian
fluctuations generated during inflation. This lower normalization of
results from the mild non-Gaussianity on cluster scales, where the one point
probability distribution function is well approximated by a
distribution. We use our estimate of to constrain the string linear
energy density and show that it is consistent with the COBE
normalization.Comment: 5 pages, 2 figure
Cosmic Numbers: A Physical Classification for Cosmological Models
We introduce the notion of the cosmic numbers of a cosmological model, and
discuss how they can be used to naturally classify models according to their
ability to solve some of the problems of the standard cosmological model.Comment: 3 pages, no figures. v2: Two references added, cosmetic changes.
Version to appear in Phys. Rev. D (Brief reports
Expert system application education project
Artificial intelligence (AI) technology, and in particular expert systems, has shown potential applicability in many areas of operation at the Kennedy Space Center (KSC). In an era of limited resources, the early identification of good expert system applications, and their segregation from inappropriate ones can result in a more efficient use of available NASA resources. On the other hand, the education of students in a highly technical area such as AI requires an extensive hands-on effort. The nature of expert systems is such that proper sample applications for the educational process are difficult to find. A pilot project between NASA-KSC and the University of Central Florida which was designed to simultaneously address the needs of both institutions at a minimum cost. This project, referred to as Expert Systems Prototype Training Project (ESPTP), provided NASA with relatively inexpensive development of initial prototype versions of certain applications. University students likewise benefit by having expertise on a non-trivial problem accessible to them at no cost. Such expertise is indispensible in a hands-on training approach to developing expert systems
Automated knowledge generation
The general objectives of the NASA/UCF Automated Knowledge Generation Project were the development of an intelligent software system that could access CAD design data bases, interpret them, and generate a diagnostic knowledge base in the form of a system model. The initial area of concentration is in the diagnosis of the process control system using the Knowledge-based Autonomous Test Engineer (KATE) diagnostic system. A secondary objective was the study of general problems of automated knowledge generation. A prototype was developed, based on object-oriented language (Flavors)
A Supernova Brane Scan
We consider a `brane-world scenario' recently introduced by Dvali, Gabadadze
and Porrati, and subsequently proposed as an alternative to a cosmological
constant in explaining the current acceleration of the universe. We show that,
contrary to these claims, this proposal is already strongly disfavoured by the
available Type Ia Supernovae, Cosmic Microwave Background and cluster data.Comment: Further cosmetic changes; to appear in The Astrophysical Journal,
v56
String Imprints from a Pre-inflationary Era
We derive the equations governing the dynamics of cosmic strings in a flat
anisotropic universe of Bianchi type I and study the evolution of simple cosmic
string loop solutions. We show that the anisotropy of the background can have a
characteristic effect in the loop motion. We discuss some cosmological
consequences of these findings and, by extrapolating our results to cosmic
string networks, we comment on their ability to survive an inflationary epoch,
and hence be a possible fossil remnant (still visible today) of an anisotropic
phase in the very early universe.Comment: 5 pages, 3 figure
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