7,449 research outputs found
Constraints On Dark Energy Models From Galaxy Clusters and Gravitational Lensing Data
The Sunyaev--Zel'dovich (SZ) effect is a global distortion of the Cosmic
Microwave Background (CMB) spectrum as a result of its interaction with a hot
electron plasma in the intracluster medium of large structures gravitationally
viralized such as galaxy clusters (GC). Furthermore, this~hot gas of electrons
emits X-rays due to its fall in the gravitational potential well of the GC.
The~analysis of SZ and X-ray data provides a method for calculating distances
to GC at high redshifts. On the other hand, many galaxies and GC produce a
Strong Gravitational Lens (SGL) effect, which has become a useful astrophysical
tool for cosmology. We use these cosmological tests in addition to more
traditional ones to constrain some alternative dark energy (DE) models,
including the study of the history of cosmological expansion through the
cosmographic parameters. Using Akaike and Bayesian Information Criterion, we
find that the and models are the most favoured by the
observational data. In addition, we found at low redshift a peculiar behavior
of slowdown of the universe, which occurs in dynamical DE models when we use
data from GC.Comment: 21 page, 5 figures, 8 tables. Published: 22 January 2018, Universe,
MDPI, Special Issue "Progress in Cosmology in the Centenary of the 1917
Einstein Paper
Self-sustained current oscillations in the kinetic theory of semiconductor superlattices
We present the first numerical solutions of a kinetic theory description of
self-sustained current oscillations in n-doped semiconductor superlattices. The
governing equation is a single-miniband Boltzmann-Poisson transport equation
with a BGK (Bhatnagar-Gross-Krook) collision term. Appropriate boundary
conditions for the distribution function describe electron injection in the
contact regions. These conditions seamlessly become Ohm's law at the injecting
contact and the zero charge boundary condition at the receiving contact when
integrated over the wave vector. The time-dependent model is numerically solved
for the distribution function by using the deterministic Weighted Particle
Method. Numerical simulations are used to ascertain the convergence of the
method. The numerical results confirm the validity of the Chapman-Enskog
perturbation method used previously to derive generalized drift-diffusion
equations for high electric fields because they agree very well with numerical
solutions thereof.Comment: 26 pages, 16 figures, to appear in J. Comput. Phy
The Political Economy of Exchange Rate Policies in Argentina
This paper identifies three periods in Argentine exchange rate policy, corresponding with different development strategies and political coalitions. The first, periodic adjustments to a pegged rate until the mid-1970s, accompanied an Import Substitution Industrialization approach and a redistribution of income from agrarian to industrial production during an era of strong labor union influence. A second period, from the mid-1970s, marked by accelerated devaluations, represented efforts by both military and civilian governments to meet the demands of their respective constituencies while maintaining capital flows from abroad; these efforts culminated in hyperinflation. A third period, beginning in the early 1990s, was notable for exchange rate stability, based on the Convertibility Plan, as a broad consensus emerged that the exchange rate should be used for containing inflation rather than engaging in redistribution among domestic interest groups. The authors conclude that additional innovations in Argentina`s monetary and exchange rate policies may eventually be necessary.
Chaos in resonant-tunneling superlattices
Spatio-temporal chaos is predicted to occur in n-doped semiconductor
superlattices with sequential resonant tunneling as their main charge transport
mechanism. Under dc voltage bias, undamped time-dependent oscillations of the
current (due to the motion and recycling of electric field domain walls) have
been observed in recent experiments. Chaos is the result of forcing this
natural oscillation by means of an appropriate external microwave signal.Comment: 3 pages, LaTex, RevTex, 3 uuencoded figures (1.2M) are available upon
request from [email protected], to appear in Phys.Rev.
Chaotic motion of space charge wavefronts in semiconductors under time-independent voltage bias
A standard drift-diffusion model of space charge wave propagation in
semiconductors has been studied numerically and analytically under dc voltage
bias. For sufficiently long samples, appropriate contact resistivity and
applied voltage - such that the sample is biased in a regime of negative
differential resistance - we find chaos in the propagation of nonlinear fronts
(charge monopoles of alternating sign) of electric field. The chaos is always
low-dimensional, but has a complex spatial structure; this behavior can be
interpreted using a finite dimensional asymptotic model in which the front
(charge monopole) positions and the electrical current are the only dynamical
variables.Comment: 12 pages, 8 figure
Perfiles de los visitantes en el turismo de naturaleza: análisis exploratorio en el entorno de Doñana
El turismo de naturaleza tiene una demanda cada vez mayor en los últimos años. En el presente trabajo se han estudiado los perfiles de los visitantes en el turismo de naturaleza. Para ello, este estudio se ha planteado en el entorno de Doñana. Este espacio natural es uno de los más importantes de España y de Europa. La muestra de estudio se ha compuesto de 130 turistas que visitaron o que tenían intención de visitar el Parque Nacional de Doñana. Como resultado, entre otros aspectos, se observa que el perfil dominante del turista está más próximo a un tipo de visitante ocasional de la naturaleza.Nature tourism has an increasing demand in last years. In this work it was studied the profiles of visitors to nature tourism. To this end, this study has arisen in Doñana. This natural area is one of the most important in Spain and Europe. The sample was composed of 130 tourists who had visited or intended to visit the National Park of Doñana. As a result, inter alia, it observed that the dominant profile of the tourist is closer to a kind of casual visitor of nature
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