386 research outputs found
Extended Quintessence with non-minimally coupled phantom scalar field
We investigate evolutional paths of an extended quintessence with a
non-minimally coupled phantom scalar field to the Ricci curvature. The
dynamical system methods are used to investigate typical regimes of dynamics at
the late time. We demonstrate that there are two generic types of evolutional
scenarios which approach the attractor (a focus or a node type critical point)
in the phase space: the quasi-oscillatory and monotonic trajectories approach
to the attractor which represents the FRW model with the cosmological constant.
We demonstrate that dynamical system admits invariant two-dimensional
submanifold and discussion that which cosmological scenario is realized depends
on behavior of the system on the phase plane . We formulate
simple conditions on the value of coupling constant for which
trajectories tend to the focus in the phase plane and hence damping
oscillations around the mysterious value . We describe this condition in
terms of slow-roll parameters calculated at the critical point. We discover
that the generic trajectories in the focus-attractor scenario come from the
unstable node. It is also investigated the exact form of the parametrization of
the equation of state parameter (directly determined from dynamics)
which assumes a different form for both scenarios.Comment: revtex4, 15 pages, 9 figures; (v2) published versio
Non-minimally coupled scalar field cosmology on the phase plane
In this publication we investigate dynamics of a flat FRW cosmological model
with a non-minimally coupled scalar field with the coupling term in the scalar field action. The quadratic potential function
is assumed. All the evolutional paths are visualized
and classified in the phase plane, at which the parameter of non-minimal
coupling plays the role of a control parameter. The fragility of global
dynamics with respect to changes of the coupling constant is studied in
details. We find that the future big rip singularity appearing in the phantom
scalar field cosmological models can be avoided due to non-minimal coupling
constant effects. We have shown the existence of a finite scale factor singular
point (future or past) where the Hubble function as well as its first
cosmological time derivative diverges.Comment: revtex4, 20 pages, 12 figs; (v2) title changed, analysis of critical
points at infinity added, accepted to JCA
Antimatter cosmic rays from dark matter annihilation: First results from an N-body experiment
[Abridged]. We take advantage of the galaxy-like 3D dark matter map extracted
from the HORIZON Project results to calculate the positron and antiproton
fluxes from dark matter annihilation, in a model-independent approach as well
as for dark matter particle benchmarks relevant at the LHC scale (from
supersymmetric and extra-dimensional theories). Such a study is dedicated to a
better estimate of the theoretical uncertainties affecting predictions, while
the PAMELA and GLAST satellites are currently taking data which will soon
provide better observational constraints. We discuss the predictions of the
antiproton and positron fluxes, and of the positron fraction as well, as
compared to the current data. We finally discuss the limits of the Nbody
framework in describing the dark matter halo of our Galaxy.Comment: 19 pages, 9 figures. Backgrounds included and additional comments and
figures on the positron fraction. Accepted for publication in PR
Contextual Object Detection with a Few Relevant Neighbors
A natural way to improve the detection of objects is to consider the
contextual constraints imposed by the detection of additional objects in a
given scene. In this work, we exploit the spatial relations between objects in
order to improve detection capacity, as well as analyze various properties of
the contextual object detection problem. To precisely calculate context-based
probabilities of objects, we developed a model that examines the interactions
between objects in an exact probabilistic setting, in contrast to previous
methods that typically utilize approximations based on pairwise interactions.
Such a scheme is facilitated by the realistic assumption that the existence of
an object in any given location is influenced by only few informative locations
in space. Based on this assumption, we suggest a method for identifying these
relevant locations and integrating them into a mostly exact calculation of
probability based on their raw detector responses. This scheme is shown to
improve detection results and provides unique insights about the process of
contextual inference for object detection. We show that it is generally
difficult to learn that a particular object reduces the probability of another,
and that in cases when the context and detector strongly disagree this learning
becomes virtually impossible for the purposes of improving the results of an
object detector. Finally, we demonstrate improved detection results through use
of our approach as applied to the PASCAL VOC and COCO datasets
Using the theory of planned behavior to predict gambling behavior
Gambling is an important public health concern. To better understand gambling behavior, we conducted a classroom-based survey that assessed the role of the theory of planned behavior (TPB; i.e., intentions, subjective norms, perceived behavioral control, and attitudes) in past year gambling and gambling frequency among college students. Results from this research support the utility of the TPB to explain gambling behavior in this population. Specifically, in TPB models to predict gambling behavior, friend and family subjective norms and perceived behavioral control predicted past year gambling and friend and family subjective norms, attitudes and perceived behavioral control predicted gambling frequency. Intention to gamble mediated these relationships. These findings suggest that college responsible gambling efforts should consider targeting misperceptions of approval regarding gambling behavior (i.e., subjective norms), personal approval of gambling behavior (i.e., attitudes), and perceived behavioral control to better manage gambling behavior in various situations
Morse index and causal continuity. A criterion for topology change in quantum gravity
Studies in 1+1 dimensions suggest that causally discontinuous topology
changing spacetimes are suppressed in quantum gravity. Borde and Sorkin have
conjectured that causal discontinuities are associated precisely with index 1
or n-1 Morse points in topology changing spacetimes built from Morse functions.
We establish a weaker form of this conjecture. Namely, if a Morse function f on
a compact cobordism has critical points of index 1 or n-1, then all the Morse
geometries associated with f are causally discontinuous, while if f has no
critical points of index 1 or n-1, then there exist associated Morse geometries
which are causally continuous.Comment: Latex, 20 pages, 3 figure
A combined data-driven, experimental and modelling approach for assessing the optimal composition of impregnation products for cementitious materials
The effectiveness of sol-gel based treatments for the protection of concrete depends on their capacity to penetrate into the material pores. Optimization of sol formulation to achieve maximum penetration depth is not a straightforward process, as the influence of different physical properties of the sol varies with the pore size distribution of each concrete. Thus, a comprehensive experimental programme to evaluate this large number of materials would require a significant number of experiments. This manuscript describes an approach, using combined computational and experimental approach to design tailor-made impregnation products with optimized penetration depth on concrete or cementitious materials with different pore size distributions. First, a process-based numerical model, calibrated experimentally for one sol composition and several cementitious material samples with different pore structures is developed. The model calculates the penetration depth for a specific pore structure. The optimization process utilizes the probabilistic and non-parametric Gaussian Processes regression method Gaussian Processes at two steps; first to make the choice of the optimal experimental design, and second to make predictions of physical properties based on the obtained training points. In the final step, the penetration depth is calculated for each mix combination in defined parameter range. The effectiveness of this approach is demonstrated on three cases. In the first instance, we optimized the impregnation product for the maximum penetration depth without any restrictions. With another two cases, we impose the restrictions on the gelation time, i.e. the time in which the sol reacts to gel. The validation of the procedure has been made by the use of a blind validation and shows promising results. The impregnation product penetrated significantly deeper with a product selected by using the described procedure compared to the considered best product before this optimization. The proposed procedure can be applied to a wide range of cementitious materials based on their pore size distribution data. This offers significant advantage compared to purely experimental approaches, where a set of experiments is required for each considered material
A new proof of the Bianchi type IX attractor theorem
We consider the dynamics towards the initial singularity of Bianchi type IX
vacuum and orthogonal perfect fluid models with a linear equation of state. The
`Bianchi type IX attractor theorem' states that the past asymptotic behavior of
generic type IX solutions is governed by Bianchi type I and II vacuum states
(Mixmaster attractor). We give a comparatively short and self-contained new
proof of this theorem. The proof we give is interesting in itself, but more
importantly it illustrates and emphasizes that type IX is special, and to some
extent misleading when one considers the broader context of generic models
without symmetries.Comment: 26 pages, 5 figure
Cosmological zoo -- accelerating models with dark energy
ecent observations of type Ia supernovae indicate that the Universe is in an
accelerating phase of expansion. The fundamental quest in theoretical cosmology
is to identify the origin of this phenomenon. In principle there are two
possibilities: 1) the presence of matter which violates the strong energy
condition (a substantial form of dark energy), 2) modified Friedmann equations
(Cardassian models -- a non-substantial form of dark matter). We classify all
these models in terms of 2-dimensional dynamical systems of the Newtonian type.
We search for generic properties of the models. It is achieved with the help of
Peixoto's theorem for dynamical system on the Poincar{\'e} sphere. We find that
the notion of structural stability can be useful to distinguish the generic
cases of evolutional paths with acceleration. We find that, while the
CDM models and phantom models are typical accelerating models, the
cosmological models with bouncing phase are non-generic in the space of all
planar dynamical systems. We derive the universal shape of potential function
which gives rise to presently accelerating models. Our results show explicitly
the advantages of using a potential function (instead of the equation of state)
to probe the origin of the present acceleration. We argue that simplicity and
genericity are the best guide in understanding our Universe and its
acceleration.Comment: RevTeX4, 23 pages, 10 figure
Visualizing Spacetime Curvature via Gradient Flows I: Introduction
Traditional approaches to the study of the dynamics of spacetime curvature in
a very real sense hide the intricacies of the nonlinear regime. Whether it be
huge formulae, or mountains of numerical data, standard methods of presentation
make little use of our remarkable skill, as humans, at pattern recognition.
Here we introduce a new approach to the visualization of spacetime curvature.
We examine the flows associated with the gradient fields of invariants derived
from the spacetime. These flows reveal a remarkably rich structure, and offer
fresh insights even for well known analytical solutions to Einstein's
equations. This paper serves as an overview and as an introduction to this
approach.Comment: 10 pages twocolumn revtex 4-1 two figures. Final form to appear in
Phys Rev
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