473 research outputs found
The Dynamics of Multi-Scalar Field Cosmological Models and Assisted Inflation
We investigate the dynamical properties of a class of spatially homogeneous
and isotropic cosmological models containing a barotropic perfect fluid and
multiple scalar fields with independent exponential potentials. We show that
the assisted inflationary scaling solution is the global late-time attractor
for the parameter values for which the model is inflationary, even when
curvature and barotropic matter are included. For all other parameter values
the multi-field curvature scaling solution is the global late-time attractor
(in these solutions asymptotically the curvature is not dynamically
negligible). Consequently, we find that in general all of the scalar fields in
multi-field models with exponential potentials are non-negligible in late-time
behaviour, contrary to what is commonly believed. The early-time and
intermediate behaviour of the models is also studied. In particular, n-scalar
field models are investigated and the structure of the saddle equilibrium
points corresponding to inflationary m-field scaling solutions and
non-inflationary m-field matter scaling solutions are also studied (where m<n),
leading to interesting transient dynamical behaviour with new physical
scenarios of potential importance.Comment: 27 pages, uses REVTeX Added an appendix illustrating some of the
details needed to compute the stability of the assisted inflationary solutio
Brane-world Cosmologies with non-local bulk effects
It is very common to ignore the non-local bulk effects in the study of
brane-world cosmologies using the brane-world approach. However, we shall
illustrate through the use of three different scenarios, that the non-local
bulk-effect does indeed have significant impact on both the
initial and future behaviour of brane-world cosmologies.Comment: 17 pages, no figures, iopart.cls, submitted to CQ
Antigenic and Genetic Variability of Human Metapneumoviruses
Human metapneumovirus (HMPV) is a member of the subfamily Pneumovirinae within the family Paramyxoviridae. Other members of this subfamily, respiratory syncytial virus and avian pneumovirus, can be divided into subgroups based on genetic or antigenic differences or both. For HMPV, the existence of different genetic lineages has been described on the basis of variation in a limited set of available sequences. We address the antigenic relationship between genetic lineages in virus neutralization assays. In addition, we analyzed the genetic diversity of HMPV by phylogenetic analysis of sequences obtained for part of the fusion protein (n = 84) and the complete attachment protein open reading frames (n = 35). On the basis of sequence diversity between attachment protein genes and the differences in virus neutralization titers, two HMPV serotypes were defined. Each serotype could be divided into two genetic lineages, but these did not reflect major antigenic differences
Qualitative Analysis of Viscous Fluid Cosmological Models satisfying the Israel-Stewart theory of Irreversible Thermodynamics
Isotropic and spatially homogeneous viscous fluid cosmological models are
investigated using the truncated Israel-Stewart theory of irreversible
thermodynamics to model the bulk viscous pressure. The governing system of
differential equations is written in terms of dimensionless variables and a set
of dimensionless equations of state is then utilized to complete the system.
The resulting dynamical system is analyzed using geometric techniques from
dynamical systems theory to find the qualitative behaviour of the
Friedmann-Robertson-Walker models with bulk viscosity. In these models there
exists a free parameter such that the qualitative behaviour of the models can
be quite different (for certain ranges of values of this parameter) from that
found in models satisfying the Eckart theory studied previously. In addition,
the conditions under which the models inflate are investigated.Comment: 29 pages, 8 Encapsulated PostScript Figures, uses the IOP style file
On subsequential spaces
AbstractSimple generators for the coreflective category of subsequential spaces, one of them countable, are constructed. Every such must have subsequential order ω1. Subsequentialness is a local property and a countable property, both in a strong sense. A T2-subsequential space may be pseudocompact without being sequential, in contrast to T2-subsequential compact (countably compact, sequentially compact) spaces all being sequential. A compact subsequential space need not be sequential
Anisotropic brane cosmologies with exponential potentials
We study Bianchi I type brane cosmologies with scalar matter self-interacting
through combinations of exponential potentials. Such models correspond in some
cases to inflationary universes. We discuss the conditions for accelerated
expansion to occur, and pay particular attention to the influence of extra
dimensions and anisotropy. Our results show that the associated effects evolve
in such a way that they become negligible in the late time limit, those related
to the anisotropy disappearing earlier. This study focuses mainly on single
field models, but we also consider a generalization yielding models with
multiple non-interacting fields and examine its features briefly. We conclude
that in the brane scenario, as happens in general relativity, an increase in
the number of fields assists inflation.Comment: 11 pages, 1 figur
Interacting viscous ghost tachyon, K-essence and dilaton scalar field models of dark energy
We study the correspondence between the interacting viscous ghost dark energy
model with the tachyon, K-essence and dilaton scalar field models in the
framework of Einstein gravity. We consider a spatially non-flat FRW universe
filled with interacting viscous ghost dark energy and dark matter. We
reconstruct both the dynamics and potential of these scalar field models
according to the evolutionary behavior of the interacting viscous ghost dark
energy model, which can describe the accelerated expansion of the universe. Our
numerical results show that the interaction and viscosity have opposite effects
on the evolutionary properties of the ghost scalar filed models.Comment: 16 pages, 17 figure
Respiratory Infections by HMPV and RSV Are Clinically Indistinguishable but Induce Different Host Response in Aged Individuals
Background: Human metapneumovirus and respiratory syncytial virus can cause severe respiratory diseases, especially in infants, young children, and the elderly. So far it remains unclear why infections in the elderly become life threatening despite the presence of neutralizing antibodies in the serum, and to which extent double infections worsen the clinical course. Methods: Young and aged BALB/c-mice were infected with RSV or/and HMPV. Appearance of the mice was observed during course of infection. On day 5 p.i. animals were dispatched by cervical dislocation and levels of TNF-a and NF-kB were determined. Results: The observation of activity, weight and appearance of the different mice showed no differences among the tested groups. Despite this, the immunologic response depends on the animals ’ age and the virus they were infected with. In young animals, NF-kB levels were elevated if infected with HMPV and HMPV/RSV but remained low in RSV infections, whereas in aged animals the opposite was observed: solely RSV-infected animals showed elevated levels of NF-kB. TNF-a was slightly elevated in HMPV-infected young and old animals, but only in young animals this elevation was significant. Conclusions: Contrary to other studies, no weight loss or change in activity despite productive lung infection with the different viruses were observed. This may be due to the weaker anaesthesia or the lesser volume of virus solution used
Self-similar Bianchi models: II. Class B models
In a companion article (referred hearafter as paper I) a detailed study of
the simply transitive Spatially Homogeneous (SH) models of class A concerning
the existence of a simply transitive similarity group has been given. The
present work (paper II) continues and completes the above study by considering
the remaining set of class B models. Following the procedure of paper I we find
all SH models of class B subjected only to the minimal geometric assumption to
admit a proper Homothetic Vector Field (HVF). The physical implications of the
obtained geometric results are studied by specialising our considerations to
the case of vacuum and law perfect fluid models. As a result we
regain all the known exact solutions regarding vacuum and non-tilted perfect
fluid models. In the case of tilted fluids we find the \emph{general
}self-similar solution for the exceptional type VI model and we
identify it as equilibrium point in the corresponding dynamical state space. It
is found that this \emph{new} exact solution belongs to the subclass of models
, is defined for and
although has a five dimensional stable manifold there exist always two unstable
modes in the restricted state space. Furthermore the analysis of the remaining
types, guarantees that tilted perfect fluid models of types III, IV, V and
VII cannot admit a proper HVF strongly suggesting that these models either
may not be asymptotically self-similar (type V) or may be extreme tilted at
late times. Finally for each Bianchi type, we give the extreme tilted
equilibrium points of their state space.Comment: Latex, 15 pages, no figures; to appear in Classical Quantum Gravity
(uses iopart style/class files); (v2) minor corrections to match published
versio
Phase Space Analysis of Quintessence Cosmologies with a Double Exponential Potential
We use phase space methods to investigate closed, flat, and open
Friedmann-Robertson-Walker cosmologies with a scalar potential given by the sum
of two exponential terms. The form of the potential is motivated by the
dimensional reduction of M-theory with non-trivial four-form flux on a
maximally symmetric internal space. To describe the asymptotic features of
run-away solutions we introduce the concept of a `quasi fixed point.' We give
the complete classification of solutions according to their late-time behavior
(accelerating, decelerating, crunch) and the number of periods of accelerated
expansion.Comment: 46 pages, 5 figures; v2: minor changes, references added; v3: title
changed, refined classification of solutions, 3 references added, version
which appeared in JCA
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