4,840 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
Cosmological perturbations and the reionization epoch
We investigate the dependence of the epoch of reionization on the properties
of cosmological perturbations, in the context of cosmologies permitted by WMAP.
We compute the redshift of reionization using a simple model based on the
Press-Schechter approximation. For a power-law initial spectrum we estimate
that reionization is likely to occur at a redshift ,
consistent with the WMAP determination based on the temperature-polarization
cross power spectrum. We estimate the delay in reionization if there is a
negative running of the spectral index, as weakly indicated by WMAP. We then
investigate the dependence of the reionization redshift on the nature of the
initial perturbations. We consider chi-squared probability distribution
functions with various degrees of freedom, motivated both by non-standard
inflationary scenarios and by defect models. We find that in these models
reionization is likely occur much earlier, and to be a slower process, than in
the case of initial gaussian fluctuations. We also consider a hybrid model in
which cosmic strings make an important contribution to the seed fluctuations on
scales relevant for reionization. We find that in order for that model to agree
with the latest WMAP results, the string contribution to the matter power
spectrum on the standard scale is likely to be at most at the
level of one percent, which imposes tight constraints on the value of the
string mass per unit length.Comment: 6 pages LaTeX file with 3 figures incorporate
Cosmic reionization constraints on the nature of cosmological perturbations
We study the reionization history of the Universe in cosmological models with non-Gaussian density fluctuations, taking them to have a renormalized probability distribution function parametrized by the number of degrees of freedom, . We compute the ionization history using a simple semi-analytical model, considering various possibilities for the astrophysics of reionization. In all our models we require that reionization is completed prior to , as required by the measurement of the Gunn--Peterson optical depth from the spectra of high-redshift quasars. We confirm previous results demonstrating that such a non-Gaussian distribution leads to a slower reionization as compared to the Gaussian case. We further show that the recent WMAP three-year measurement of the optical depth due to electron scattering, , weakly constrains the allowed deviations from Gaussianity on the small scales relevant to reionization if a constant spectral index is assumed. We also confirm the need for a significant suppression of star formation in mini-halos, which increases dramatically as we decrease
Bouncing Eddington-inspired Born-Infeld cosmologies: an alternative to Inflation ?
We study the dynamics of a homogeneous and isotropic
Friedmann-Robertson-Walker universe in the context of the Eddington-inspired
Born-Infeld theory of gravity. We generalize earlier results, obtained in the
context of a radiation dominated universe, to account for the evolution of a
universe permeated by a perfect fluid with an arbitrary equation of state
parameter . We show that a bounce may occur for , if is
time-dependent, and we demonstrate that it is free from tensor singularities.
We argue that Eddington-inspired Born-Infeld cosmologies may be a viable
alternative to the inflationary paradigm as a solution to fundamental problems
of the standard cosmological model.Comment: 4 pages, 1 figur
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)
Cosmic strings, loops, and linear growth of matter perturbations
We describe the detailed study and results of high-resolution numerical
simulations of string-induced structure formation in open universes and those
with a non-zero cosmological constant. The effect from small loops generated
from the string network has also been investigated. We provide a
semi-analytical model which can reproduce these simulation results. A detailed
study of cosmic string network properties regarding structure formation is also
given, including the correlation time, the topological analysis of the source
spectrum, the correlation between long strings and loops, and the evolution of
long-string and loop energy densities. For models with 8 h^{-1}\sigma_8$, and an overall shape which are consistent within
uncertainties with those currently inferred from galaxy surveys. The cosmic
string scenario with hot dark matter requires a strongly scale-dependent bias
in order to agree with observations.Comment: 60 pages, 24 figure
Topological defects: A problem for cyclic universes?
We study the behaviour of cosmic string networks in contracting universes,
and discuss some of their possible consequences. We note that there is a
fundamental time asymmetry between defect network evolution for an expanding
universe and a contracting universe. A string network with negligible loop
production and small-scale structure will asymptotically behave during the
collapse phase as a radiation fluid. In realistic networks these two effects
are important, making this solution only approximate. We derive new scaling
solutions describing this effect, and test them against high-resolution
numerical simulations. A string network in a contracting universe, together
with the gravitational radiation background it has generated, can significantly
affect the dynamics of the universe both locally and globally. The network can
be an important source of radiation, entropy and inhomogeneity. We discuss the
possible implications of these findings for bouncing and cyclic cosmological
models.Comment: 11 RevTeX 4 pages, 6 figures; version to appear in Phys. Rev.
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