1,804 research outputs found
Bouncing Loop Quantum Cosmology from gravity
The big bang singularity could be understood as a breakdown of Einstein's
General Relativity at very high energies. Adopting this viewpoint, other
theories, that implement Einstein Cosmology at high energies, might solve the
problem of the primeval singularity. One of them is Loop Quantum Cosmology
(LQC) with a small cosmological constant that models a universe moving along an
ellipse, which prevents singularities like the big bang or the big rip, in the
phase space , where is the Hubble parameter and the energy
density of the universe. Using LQC when one considers a model of universe
filled by radiation and matter where, due to the cosmological constant, there
are a de Sitter and an anti de Sitter solution. This means that one obtains a
bouncing non-singular universe which is in the contracting phase at early
times. After leaving this phase, i.e., after bouncing, it passes trough a
radiation and matter dominated phase and finally at late times it expands in an
accelerated way (current cosmic acceleration). This model does not suffer from
the horizon and flatness problems as in big bang cosmology, where a period of
inflation that increases the size of our universe in more than 60 e-folds is
needed in order to solve both problems. The model has two mechanisms to avoid
these problems: The evolution of the universe through a contracting phase and a
period of super-inflation ()
On the number of limit cycles in discontinuous piecewise linear differential systems with two pieces separated by a straight line
In this paper we study the maximum number of limit cycles that can
exhibit a planar piecewise linear differential system formed by two pieces
separated by a straight line. More precisely, we prove that this maximum number
satisfies if one of the two linear differential systems has
its equilibrium point on the straight line of discontinuity
The Transition. Convergence and discrepancy in the international and national press coverage of Spain’s major postwar international news export
The role of the national and foreign press in the news coverage of the Spanish transition to democracy (1975-1978) has been a constant reference in the historical study of the period of political change after the end of the Francoist dictatorship. In this article we present the general results of three research projects concerning the role of the foreign press, of the Spanish daily press and the magazine marketin which we can observe both convergence and discrepance in the news narrative, editorial behaviour and political standpoints. The greater independence and informative freedom of the foreign press contrasts with the proximity of the Spanish press to both King and government with the exception of the critical support to reform expressed in both the new political magazines and newspapers during the first few months of the process of political change.El papel de la prensa nacional y extranjera en la cobertura informativa de la Transición española a la democracia (1975-1978) ha sido una referencia constante en la historiografía del período de cambio político en España tras el final de la dictadura de Franco, así como en la cultura periodística. En este artículo presentamos los resultados generales de tres proyectos de investigación sobre el papel de la prensa extranjera, de la prensa diaria española y de la prensa no diaria enlos que se pueden comprobar convergencias y discrepancias en el relato informativo, las valoraciones editoriales y los posicionamientos políticos. La mayor independencia y libertad informativa de la prensa extranjera contrasta con la proximidad de la prensa española al rey y al gobierno, con la excepción del apoyo crítico a la reforma de las nuevas revistas políticas y los diarios surgidos en los primeros meses del proceso de cambio político
Persistence of periodic solutions for higher order perturbed differential systems via Lyapunov-Schmidt reduction
In this work we first provide sufficient conditions to assure the persistence
of some zeros of functions having the form
for sufficiently
small. Here , for , are
smooth functions being an open bounded set.
Then we use this result to compute the bifurcation functions which controls the
periodic solutions of the following -periodic smooth differential system It is assumed that the
unperturbed differential system has a sub-manifold of periodic solutions
, . We also study the case when
the bifurcation functions have a continuum of zeros. Finally we provide the
explicit expressions of the bifurcation functions up to order 5
Bifurcations from families of periodic solutions in piecewise differential systems
Consider a differential system of the form where
and are piecewise
functions and -periodic in the variable . Assuming that the unperturbed
system has a -dimensional submanifold of periodic solutions
with , we use the Lyapunov-Schmidt reduction and the averaging theory to
study the existence of isolated -periodic solutions of the above
differential system
Viable Inflationary Evolution from Loop Quantum Cosmology Scalar-Tensor Theory
In this work we construct a bottom-up reconstruction technique for Loop
Quantum Cosmology scalar-tensor theories, from the observational indices.
Particularly, the reconstruction technique is based on fixing the functional
form of the scalar-to-tensor ratio as a function of the -foldings number.
The aim of the technique is to realize viable inflationary scenarios, and the
only assumption that must hold true in order for the reconstruction technique
to work is that the dynamical evolution of the scalar field obeys the slow-roll
conditions. We shall use two functional forms for the scalar-to-tensor ratio,
one of which corresponds to a popular inflationary class of models, the
-attractors. For the latter, we shall calculate the leading order
behavior of the spectral index and we shall demonstrate that the resulting
inflationary theory is viable and compatible with the latest Planck and
BICEP2/Keck-Array data. In addition, we shall find the classical limit of the
theory, and as we demonstrate, the Loop Quantum Cosmology corrected theory and
the classical theory are identical at leading order in the perturbative
expansion quantified by the parameter , which is the critical density
of the quantum theory. Finally, by using the formalism of slow-roll
scalar-tensor Loop Quantum Cosmology, we shall investigate how several
inflationary potentials can be realized by the quantum theory, and we shall
calculate directly the slow-roll indices and the corresponding observational
indices. In addition, the gravity frame picture is presented.Comment: PRD Accepte
Uniqueness of Petrov type D spatially inhomogeneous irrotational silent models
The consistency of the constraint with the evolution equations for spatially
inhomogeneous and irrotational silent (SIIS) models of Petrov type I, demands
that the former are preserved along the timelike congruence represented by the
velocity of the dust fluid, leading to \emph{new} non-trivial constraints. This
fact has been used to conjecture that the resulting models correspond to the
spatially homogeneous (SH) models of Bianchi type I, at least for the case
where the cosmological constant vanish. By exploiting the full set of the
constraint equations as expressed in the 1+3 covariant formalism and using
elements from the theory of the spacelike congruences, we provide a direct and
simple proof of this conjecture for vacuum and dust fluid models, which shows
that the Szekeres family of solutions represents the most general class of SIIS
models. The suggested procedure also shows that, the uniqueness of the SIIS of
the Petrov type D is not, in general, affected by the presence of a non-zero
pressure fluid. Therefore, in order to allow a broader class of Petrov type I
solutions apart from the SH models of Bianchi type I, one should consider more
general ``silent'' configurations by relaxing the vanishing of the vorticity
and the magnetic part of the Weyl tensor but maintaining their ``silence''
properties i.e. the vanishing of the curls of and the pressure
.Comment: Latex, 19 pages, no figures;(v2) some clarification remarks and an
appendix are added; (v3) minor changes to match published versio
Simultaneous occurrence of sliding and crossing limit cycles in piecewise linear planar vector fields
In the present study we consider planar piecewise linear vector fields with
two zones separated by the straight line . Our goal is to study the
existence of simultaneous crossing and sliding limit cycles for such a class of
vector fields. First, we provide a canonical form for these systems assuming
that each linear system has center, a real one for and a virtual one for
, and such that the real center is a global center. Then, working with a
first order piecewise linear perturbation we obtain piecewise linear
differential systems with three crossing limit cycles. Second, we see that a
sliding cycle can be detected after a second order piecewise linear
perturbation. Finally, imposing the existence of a sliding limit cycle we prove
that only one adittional crossing limit cycle can appear. Furthermore, we also
characterize the stability of the higher amplitude limit cycle and of the
infinity. The main techniques used in our proofs are the Melnikov method, the
Extended Chebyshev systems with positive accuracy, and the Bendixson
transformation.Comment: 24 pages, 7 figure
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