4,226 research outputs found
Slow roll inflation in the presence of a dark energy coupling
In models of coupled dark energy, in which a dark energy scalar field couples to other matter components, it is natural to expect a coupling to the inflaton as well. We explore the consequences of such a coupling in the context of single-field slow-roll inflation. Assuming an exponential potential for the quintessence field we show that the coupling to the inflaton causes the quintessence field to be attracted toward the minimum of the effective potential. If the coupling is large enough, the field is heavy and is located at the minimum. We show how this affects the expansion rate and the slow-roll of the inflaton field, and therefore the primordial perturbations generated during inflation. We further show that the coupling has an important impact on the processes of reheating and preheating
Random transition-rate matrices for the master equation
Random-matrix theory is applied to transition-rate matrices in the Pauli
master equation. We study the distribution and correlations of eigenvalues,
which govern the dynamics of complex stochastic systems. Both the cases of
identical and of independent rates of forward and backward transitions are
considered. The first case leads to symmetric transition-rate matrices, whereas
the second corresponds to general, asymmetric matrices. The resulting matrix
ensembles are different from the standard ensembles and show different
eigenvalue distributions. For example, the fraction of real eigenvalues scales
anomalously with matrix dimension in the asymmetric case.Comment: 15 pages, 12 figure
Global associations between UVR exposure and current eczema prevalence in children from ISAAC Phase Three
We sought to examine the relationship globally between UV dose exposure and current eczema prevalences.
ISAAC Phase Three provided data on eczema prevalence for 13-14 year-olds in 214 centres in 87 countries and for 6-7 year-olds in 132 centres in 57 countries. Linear and non-linear associations between (natural log transformed) eczema prevalence and the mean, maximum, minimum, standard deviation and range of monthly UV dose exposures were assessed using linear mixed-effects regression models.
For the 13-14 year olds, the country-level eczema prevalence was positively and linearly associated with country-level monthly mean (prevalence ratio: 1.31, 95% confidence interval: [1.05, 1.63] per kJ/m2) and minimum (1.25 [1.06, 1.47] per kJ/m2) UV dose exposure. Linear and
non-linear associations were also observed for other metrics of UV. Results were similar in trend, but non-significant, for the fewer centres with 6-7 year-olds (e.g. 1.24 [0.96, 1.59] per kJ/m2 for country-level monthly mean UV). No consistent within-country associations were observed (e.g. 1.05 [0.89, 1.23] and 0.92 [0.71, 1.18] per kJ/m2 for center-level monthly mean UV, for the 13-14 and 6-7 year-olds, respectively).
These ecological results support a role for UV exposure in explaining some of the variation in global childhood eczema prevalence
Disorder-Induced Resistive Anomaly Near Ferromagnetic Phase Transitions
We show that the resistivity rho(T) of disordered ferromagnets near, and
above, the Curie temperature T_c generically exhibits a stronger anomaly than
the scaling-based Fisher-Langer prediction. Treating transport beyond the
Boltzmann description, we find that within mean-field theory, d\rho/dT exhibits
a |T-T_c|^{-1/2} singularity near T_c. Our results, being solely due to
impurities, are relevant to ferromagnets with low T_c, such as SrRuO3 or
diluted magnetic semiconductors, whose mobility near T_c is limited by
disorder.Comment: 5 pages, 3 figures; V2: with a few clarifications, as publishe
Late-Time Behavior of Stellar Collapse and Explosions: II. Nonlinear Evolution
Problem with the figures should be corrected. Apparently a broken uuencoder
was the cause.Comment: 23pp, RevTex, 15 figures (included), NSF-ITP-93-8
A Variational Approach for Minimizing Lennard-Jones Energies
A variational method for computing conformational properties of molecules
with Lennard-Jones potentials for the monomer-monomer interactions is
presented. The approach is tailored to deal with angular degrees of freedom,
{\it rotors}, and consists in the iterative solution of a set of deterministic
equations with annealing in temperature. The singular short-distance behaviour
of the Lennard-Jones potential is adiabatically switched on in order to obtain
stable convergence. As testbeds for the approach two distinct ensembles of
molecules are used, characterized by a roughly dense-packed ore a more
elongated ground state. For the latter, problems are generated from natural
frequencies of occurrence of amino acids and phenomenologically determined
potential parameters; they seem to represent less disorder than was previously
assumed in synthetic protein studies. For the dense-packed problems in
particular, the variational algorithm clearly outperforms a gradient descent
method in terms of minimal energies. Although it cannot compete with a careful
simulating annealing algorithm, the variational approach requires only a tiny
fraction of the computer time. Issues and results when applying the method to
polyelectrolytes at a finite temperature are also briefly discussed.Comment: 14 pages, uuencoded compressed postscript fil
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