1,002 research outputs found
Qualitative Analysis of Causal Cosmological Models
The Einstein's field equations of Friedmann-Robertson-Walker universes filled
with a dissipative fluid described by both the {\em truncated} and {\em
non-truncated} causal transport equations are analyzed using techniques from
dynamical systems theory. The equations of state, as well as the phase space,
are different from those used in the recent literature. In the de Sitter
expansion both the hydrodynamic approximation and the non-thermalizing
condition can be fulfilled simultaneously. For these expansions
turn out to be stable provided a certain parameter of the fluid is lower than
1/2. The more general case is studied in detail as well.Comment: RevTeX file, 23 pages. Accepted for publication in J. Math. Phy
Power Counting and Perturbative One Pion Exchange in Heavy Meson Molecules
We discuss the possible power counting schemes that can be applied in the
effective field theory description of heavy meson molecules, such as the
X(3872) or the recently discovered Zb(10610) and Zb(10650) states. We argue
that the effect of coupled channels is suppressed by at least two orders in the
effective field theory expansion, meaning that they can be safely ignored at
lowest order. The role of the one pion exchange potential between the heavy
mesons, and in particular the tensor force, is also analyzed. By using
techniques developed in atomic physics for handling power-law singular
potentials, which have been also successfully employed in nuclear physics, we
determine the range of center-of-mass momenta for which the tensor piece of the
one pion exchange potential is perturbative. In this momentum range, the one
pion exchange potential can be considered a subleading order correction,
leaving at lowest order a very simple effective field theory consisting only on
contact-range interactions.Comment: 21 pages, 1 figur
Magnetic relaxation in the Bianchi-I universe
Extended Einstein-Maxwell model and its application to the problem of
evolution of magnetized Bianchi-I Universe are considered. The evolution of
medium magnetization is governed by a relaxation type extended constitutive
equation. The series of exact solutions to the extended master equations is
obtained and discussed. The anisotropic expansion of the Bianchi-I Universe is
shown to become non-monotonic (accelerated/decelerated) in both principal
directions (along the magnetic field and orthogonal to it). A specific type of
expansion, the so-called evolution with hidden magnetic field, is shown to
appear when the magnetization effectively screens the magnetic field and the
latter disappears from the equations for gravitational field.Comment: 32 page
«It’s the context, stupid»: The European Union’s public diplomacy in times of ontological insecurity
Public diplomacy, despite its numerous and varied definitions, is essentially a communications process. By engaging the academic literature of public diplomacy with Lasswell’s model of communication and Braddock’s rearticulation of his model, this paper proposes an integrated framework that allows for the systematization of public diplomacy research. The framework is composed of the independent variable of context, which influences a set of dependent variables: the actors, publics, messages, objectives and tools of public diplomacy. Accordingly, this paper argues that public diplomacy research has been traditionally approached from an agent-centric perspective, and despite its obvious significance, the influence of context has been understudied. In order to test the utility of the model, the paper applies it to the case study of the European Union’s public diplomacy during two different settings. First, it will expose the main characteristics of the EU’s public diplomacy during times of globalization, where the EU’s public diplomacy was characterized by its normativity. Subsequently, the current context of deglobalization and de-europeanization will be introduced and analyzed through the following research question: what happens to the EU’s public diplomacy when the founding myth upon which it is constructed is under threat? By altering the context, one can easily see an emerging but clear transformation of the characteristics of the EU’s public diplomacy. By analyzing official, policy, and legal documents, and engaging with the academic literature on the topic, the paper concludes that the main objective of the EU’s public diplomacy in a changing world should be to provide for ontological security through (emotional) strategic metanarratives
Relaxation versus collision times in the cosmological radiative era
We consider the Lema\^{\i}tre-Tolman-Bondi metric with an inhomogeneous viscous fluid source satisfying the equation of state of an interactive mixture of radiation and matter. Assuming conditions prior to the decoupling era, we apply Extended Irreversible Thermodynamcs (EIT) to this mixture. Using the full transport equation of EIT we show that the relaxation time of shear viscosity can be several orders of magnitude larger than the Thomson collision time between photons and electrons. A comparison with the ``truncated'' transport equation for these models reveals that the latter cannot describe properly the decoupling of matter and radiatio
Stability of inflationary solutions driven by a changing dissipative fluid
In this paper the second Lyapunov method is used to study the stability of
the de Sitter phase of cosmic expansion when the source of the gravitational
field is a viscous fluid. Different inflationary scenarios related with
reheating and decay of mini-blackholes into radiation are investigated using an
effective fluid described by time--varying thermodynamical quantities.Comment: 17 pages, LaTeX 2.09, 2 figures. To be published in Classical and
Quantum Gravit
The Heavy Quark Spin Symmetry Partners of the X(3872)
We explore the consequences of heavy quark spin symmetry for the charmed
meson-antimeson system in a contact-range (or pionless) effective field theory.
As a trivial consequence, we theorize the existence of a heavy quark spin
symmetry partner of the X(3872), with , which we call X(4012) in
reference to its predicted mass. If we additionally assume that the X(3915) is
a heavy spin symmetry partner of the X(3872), we end up predicting a
total of six molecular states. We also discuss the error
induced by higher order effects such as finite heavy quark mass corrections,
pion exchanges and coupled channels, allowing us to estimate the expected
theoretical uncertainties in the position of these new states.Comment: 18 pages; final version accepted for publicatio
Gauge invariance in the presence of a cutoff
We use the method of gauging equations to construct the electromagnetic
current operator for the two-nucleon system in a theory with a finite cutoff.
The employed formulation ensures that the two-nucleon T-matrix and
corresponding five-point function, in the cutoff theory, are identical to the
ones formally defined by a reference theory without a cutoff. A feature of our
approach is that it effectively introduces a cutoff into the reference theory
in a way that maintains the long-range part of the exchange current operator;
for applications to effective field theory (EFT), this property is usually
sufficient to guarantee the predictive power of the resulting cutoff theory. In
addition, our approach leads to Ward-Takahashi (WT) identities that are linear
in the interactions.
From the point of view of EFT's where such a WT identity is satisfied in the
reference theory, this ensures that gauge invariance in the cutoff theory is
maintained order by order in the expansion.Comment: 15 pages, 2 figure
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
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