40,431 research outputs found
Newtonian Perturbations on Models with Matter Creation
Creation of Cold Dark Matter (CCDM) can macroscopically be described by a
negative pressure, and, therefore, the mechanism is capable to accelerate the
Universe, without the need of an additional dark energy component. In this
framework we discuss the evolution of perturbations by considering a
Neo-Newtonian approach where, unlike in the standard Newtonian cosmology, the
fluid pressure is taken into account even in the homogeneous and isotropic
background equations (Lima, Zanchin and Brandenberger, MNRAS {\bf 291}, L1,
1997). The evolution of the density contrast is calculated in the linear
approximation and compared to the one predicted by the CDM model. The
difference between the CCDM and CDM predictions at the perturbative
level is quantified by using three different statistical methods, namely: a
simple -analysis in the relevant space parameter, a Bayesian
statistical inference, and, finally, a Kolmogorov-Smirnov test. We find that
under certain circumstances the CCDM scenario analysed here predicts an overall
dynamics (including Hubble flow and matter fluctuation field) which fully
recovers that of the traditional cosmic concordance model. Our basic conclusion
is that such a reduction of the dark sector provides a viable alternative
description to the accelerating CDM cosmology.Comment: Physical Review D in press, 10 pages, 4 figure
Degree-dependent intervertex separation in complex networks
We study the mean length of the shortest paths between a vertex of
degree and other vertices in growing networks, where correlations are
essential. In a number of deterministic scale-free networks we observe a
power-law correction to a logarithmic dependence, in a wide range of network
sizes. Here is the number of vertices in the network, is the
degree distribution exponent, and the coefficients and depend on a
network. We compare this law with a corresponding dependence obtained
for random scale-free networks growing through the preferential attachment
mechanism. In stochastic and deterministic growing trees with an exponential
degree distribution, we observe a linear dependence on degree, . We compare our findings for growing networks with those for
uncorrelated graphs.Comment: 8 pages, 3 figure
Hamming distance and mobility behavior in generalized rock-paper-scissors models
This work reports on two related investigations of stochastic simulations
which are widely used to study biodiversity and other related issues. We first
deal with the behavior of the Hamming distance under the increase of the number
of species and the size of the lattice, and then investigate how the mobility
of the species contributes to jeopardize biodiversity. The investigations are
based on the standard rules of reproduction, mobility and predation or
competition, which are described by specific rules, guided by generalization of
the rock-paper-scissors game, valid in the case of three species. The results
on the Hamming distance indicate that it engenders universal behavior,
independently of the number of species and the size of the square lattice. The
results on the mobility confirm the prediction that it may destroy diversity,
if it is increased to higher and higher values.Comment: 7 pages, 9 figures. To appear in EP
Developing a site-conditions map for seismic hazard Assessment in Portugal
The evaluation of site effects on a broad scale is a critical issue for seismic hazard and risk assessment, land use planning and emergency planning. As characterization of site conditions based on the shear-wave velocity has become increasingly important, several methods have been proposed in the literature to estimate its average over the first thirty meters (Vs30) from more extensively available data. These methods include correlations with geologic-geographic defined units and topographic slope. In this paper we present the first steps towards the development of a site–conditions map for Portugal, based on a regional database of shear-wave velocity data, together with geological, geographic, and lithological information. We computed Vs30 for each database site and classified it according to the corresponding geological-lithological information using maps at the smallest scale available (usually 1:50000). We evaluated the consistency of Vs30 values within generalized-geological classes, and assessed the performance of expedient methodologies proposed in the literature
Simulations of a mortality plateau in the sexual Penna model for biological ageing
The Penna model is a strategy to simulate the genetic dynamics of
age-structured populations, in which the individuals genomes are represented by
bit-strings. It provides a simple metaphor for the evolutionary process in
terms of the mutation accumulation theory. In its original version, an
individual dies due to inherited diseases when its current number of
accumulated mutations, n, reaches a threshold value, T. Since the number of
accumulated diseases increases with age, the probability to die is zero for
very young ages (n = T). Here, instead
of using a step function to determine the genetic death age, we test several
other functions that may or may not slightly increase the death probability at
young ages (n < T), but that decreases this probability at old ones. Our
purpose is to study the oldest old effect, that is, a plateau in the mortality
curves at advanced ages. Imposing certain conditions, it has been possible to
obtain a clear plateau using the Penna model. However, a more realistic one
appears when a modified version, that keeps the population size fixed without
fluctuations, is used. We also find a relation between the birth rate, the
age-structure of the population and the death probability.Comment: submitted to Phys. Rev.
Kinetic modelling of epitaxial film growth with up- and downward step barriers
The formation of three-dimensional structures during the epitaxial growth of
films is associated to the reflection of diffusing particles in descending
terraces due to the presence of the so-called Ehrlich-Schwoebel (ES) barrier.
We generalize this concept in a solid-on-solid growth model, in which a barrier
dependent on the particle coordination (number of lateral bonds) exists
whenever the particle performs an interlayer diffusion. The rules do not
distinguish explicitly if the particle is executing a descending or an
ascending interlayer diffusion. We show that the usual model, with a step
barrier in descending steps, produces spurious, columnar, and highly unstable
morphologies if the growth temperature is varied in a usual range of mound
formation experiments. Our model generates well-behaved mounded morphologies
for the same ES barriers that produce anomalous morphologies in the standard
model. Moreover, mounds are also obtained when the step barrier has an equal
value for all particles independently if they are free or bonded. Kinetic
roughening is observed at long times, when the surface roughness w and the
characteristic length scale as and where
and , independently of the growth
temperature.Comment: 15 pages, 7 figure
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