13,199 research outputs found
The growth of matter perturbations in f(R) models
We consider the linear growth of matter perturbations on low redshifts in
some dark energy (DE) models. We discuss the definition of dark energy
(DE) in these models and show the differences with scalar-tensor DE models. For
the model recently proposed by Starobinsky we show that the growth
parameter takes the value for
and for , allowing
for a clear distinction from CDM. Though a scale-dependence appears in
the growth of perturbations on higher redshifts, we find no dispersion for
on low redshifts up to , is also
quasi-linear in this interval. At redshift , the dispersion is still
small with . As for some scalar-tensor models, we
find here too a large value for ,
for and for
. These values are largely outside the range found for DE
models in General Relativity (GR). This clear signature provides a powerful
constraint on these models.Comment: 14 pages, 7 figures, improved presentation, references added, results
unchanged, final version to be published in JCA
Axiomatization and Models of Scientific Theories
In this paper we discuss two approaches to the axiomatization of scien- tific theories in the context of the so called semantic approach, according to which (roughly) a theory can be seen as a class of models. The two approaches are associated respectively to Suppes’ and to da Costa and Chuaqui’s works. We argue that theories can be developed both in a way more akin to the usual mathematical practice (Suppes), in an informal set theoretical environment, writing the set theoretical predicate in the language of set theory itself or, more rigorously (da Costa and Chuaqui), by employing formal languages that help us in writing the postulates to define a class of structures. Both approaches are called internal, for we work within a mathematical framework, here taken to be first-order ZFC. We contrast these approaches with an external one, here discussed briefly. We argue that each one has its strong and weak points, whose discussion is relevant for the philosophical foundations of science
An exact master equation for the system-reservoir dynamics under the strong coupling regime and non-Markovian dynamics
In this paper we present a method to derive an exact master equation for a
bosonic system coupled to a set of other bosonic systems, which plays the role
of the reservoir, under the strong coupling regime, i.e., without resorting to
either the rotating-wave or secular approximations. Working with phase-space
distribution functions, we verify that the dynamics are separated in the
evolution of its center, which follows classical mechanics, and its shape,
which becomes distorted. This is the generalization of a result by Glauber, who
stated that coherent states remain coherent under certain circumstances,
specifically when the rotating-wave approximation and a zero-temperature
reservoir are used. We show that the counter-rotating terms generate
fluctuations that distort the vacuum state, much the same as thermal
fluctuations.Finally, we discuss conditions for non-Markovian dynamics
Causal Stability Ranking
Genotypic causes of a phenotypic trait are typically determined via randomized controlled intervention experiments. Such experiments are often prohibitive with respect to durations and costs. We therefore consider inferring stable rankings of genes, according to their causal effects on a phenotype, from observational data only. Our method allows for efficient design and prioritization of future experiments, and due to its generality it is useable for a broad spectrum of applications
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