714 research outputs found
The Universe With Bulk Viscosity
Exact solutions for a model with variable , and bulk viscosity
are obtained. Inflationary solutions with constant (de Sitter-type) and
variable energy density are found. An expanding anisotropic universe is found
to isotropize during its course of expansion but a static universe is not. The
gravitational constant is found to increase with time and the cosmological
constant decreases with time as .Comment: 7 LateX pages, no figure
On the generalized continuity equation
A generalized continuity equation extending the ordinary continuity equation
has been found using quanternions. It is shown to be compatible with Dirac,
Schrodinger, Klein-Gordon and diffusion equations. This generalized equation is
Lorentz invariant. The transport properties of electrons are found to be
governed by Schrodinger-like equation and not by the diffusion equation.Comment: 9 Latex pages, no figure
Cosmic Acceleration With A Positive Cosmological Constant
We have considered a cosmological model with a phenomenological model for the
cosmological constant of the form \Lambda=\bt\fr{\ddot R}{R}, \bt is a
constant. For age parameter consistent with observational data the Universe
must be accelerating in the presence of a positive cosmological constant. The
minimum age of the Universe is , where is the present Hubble
constant. The cosmological constant is found to decrease as . Allowing
the gravitational constant to change with time leads to an ever increasing
gravitational constant at the present epoch. In the presence of a viscous fluid
this decay law for is equivalent to the one with () provided \alpha=\fr{\bt}{3(\bt-2)}. The
inflationary solution obtained from this model is that of the de-Sitter type.Comment: a more revised versio
Cosmological Models with Variable Gravitational and Cosmological constants in Gravity
We consider the evolution of a flat Friedmann-Roberstson-Walker Universe in a
higher derivative theories, including terms to the
Einstein-Hilbert action in the presence of a variable gravitational and
cosmological constants. We study here the evolution of the gravitational and
cosmological constants in the presence of radiation and matter domination era
of the universe. We present here new cosmological solutions which are
physically interesting for model building.Comment: 14 pages, no figure. to be published in Int. J. Mod. Phys.
Decoupled execution of synchronous coordination models via behavioural automata
Synchronous coordination systems allow the exchange of data by logically
indivisible actions involving all coordinated entities. This paper introduces
behavioural automata, a logically synchronous coordination model based on the
Reo coordination language, which focuses on relevant aspects for the concurrent
evolution of these systems. We show how our automata model encodes the Reo and
Linda coordination models and how it introduces an explicit predicate that
captures the concurrent evolution, distinguishing local from global actions,
and lifting the need of most synchronous models to involve all entities at each
coordination step, paving the way to more scalable implementations.Comment: In Proceedings FOCLASA 2011, arXiv:1107.584
Integrated Structure and Semantics for Reo Connectors and Petri Nets
In this paper, we present an integrated structural and behavioral model of
Reo connectors and Petri nets, allowing a direct comparison of the two
concurrency models. For this purpose, we introduce a notion of connectors which
consist of a number of interconnected, user-defined primitives with fixed
behavior. While the structure of connectors resembles hypergraphs, their
semantics is given in terms of so-called port automata. We define both models
in a categorical setting where composition operations can be elegantly defined
and integrated. Specifically, we formalize structural gluings of connectors as
pushouts, and joins of port automata as pullbacks. We then define a semantical
functor from the connector to the port automata category which preserves this
composition. We further show how to encode Reo connectors and Petri nets into
this model and indicate applications to dynamic reconfigurations modeled using
double pushout graph transformation
A note on reactive transitions and Reo connectors
The structure of a reactive transition system can to be modi
ed on the
y by e.g. removing, reversing or adding new transitions. The
topic has been studied by D. Gabbay and his collaborators in di erent
contexts. In this paper we take their work a step further, introducing a
suitable notion of bisimulation and obtaining a Hennessy-Milner theorem
with respect to a hybrid logic in which transition properties can be
expressed. Our motivation is to provide a characterisation of equivalence
for such systems in order to exploit their possible roles in the formal
description of software connectors in Reo, either from a behavioural (semantic)
or spatial (syntactic) point of view.âSmartEGOV/NORTE-01-0145-FEDER-000037â, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (EFDR). Additional support was provided by the European Regional Development Fund through the Operational Programme for Competitiveness and Internationalisation - COMPETE 2020 and by National Funds through the Portuguese funding agency, FCT - Fundação para a CiĂȘncia e a Tecnologia within project UID/MAT/04106/2013 at CIDMA. The first author is also supported by an Individual Doctoral Grant (reference number PD/BD/114186/2016
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