1,114 research outputs found
The MESS of cosmological perturbations
We introduce two new effective quantities for the study of comoving curvature
perturbations : the space dependent effective sound speed (SESS) and the
momentum dependent effective sound speed (MESS) . We use the SESS and the MESS
to derive a new set of equations which can be applied to any system described
by an effective stress-energy-momentum tensor (EST), including multi-fields
systems, supergravity and modified gravity theories. We show that this approach
is completely equivalent to the standard one and it has the advantage of
requiring to solve only one differential equation for instead of a
system, without the need of explicitly computing the evolution of entropy
perturbations. The equations are valid for perturbations respect to any
arbitrary flat spatially homogeneous background, including any inflationary and
bounce model.
As an application we derive the equation for for multi-fields
models and show that observed features of the primordial curvature perturbation
spectrum are compatible with the effects of an appropriate local variation of
the MESS in momentum space. The MESS is the natural quantity to parametrize in
a model independent way the effects produced on curvature perturbations by
multi-fields systems, particle production and modified gravity theories and
could be conveniently used in the analysis of LSS observations, such as the
ones from the upcoming EUCLID mission or CMB radiation measurements.Comment: We study the MESS of cosmological perturbations, version accepted in
Physics Letters
Effects of particle production during inflation
The impact of particle production during inflation on the primordial
curvature perturbation spectrum is investigated both analytically and
numerically. We obtain an oscillatory behavior on small scales, while on large
scales the spectrum is unaffected. The amplitude of the oscillations is
proportional to the number of coupled fields, their mass, and the square of the
coupling constant. The oscillations are due a discontinuity in the second time
derivative of the inflaton, arising from a temporary violation of the slow-roll
conditions. A similar effect on the power spectrum should be produced also in
other inflationary models where the slow-roll conditions are temporarily
violated.Comment: 7 pages, 5 figure
An Analytic Hierarchy Process for The Evaluation of Transport Policies to Reduce Climate Change Impacts
Transport is the sector with the fastest growth of greenhouse gases emissions, both in developed and in developing countries, leading to adverse climate change impacts. As the experts disagree on the occurrence of these impacts, by applying the analytic hierarchy process (AHP), we have faced the question on how to form transport policies when the experts have different opinions and beliefs. The opinions of experts have been investigated by a means of a survey questionnaire. The results show that tax schemes aiming at promoting environmental-friendly transport mode are the best policy. This incentives public and environmental-friendly transport modes, such as car sharing and car pooling.Analytic Hierarchy Process, Transport Policies, Climate Change
Large and strong scale dependent bispectrum in single field inflation from a sharp feature in the mass
We study an inflationary model driven by a single minimally coupled standard
kinetic term scalar field with a step in its mass modeled by an Heaviside step
function. We present an analytical approximation for the mode function of the
curvature perturbation, obtain the power spectrum analytically and compare it
with the numerical result. We show that, after the scale set by the step, the
spectrum contains damped oscillations that are well described by our analytical
approximation. We also compute the dominant contribution to the bispectrum in
the equilateral and the squeezed limits and find new shapes. In the equilateral
and squeezed limits the bispectrum oscillates and it has a linear growth
envelope towards smaller scales. The bispectrum size can be large depending on
the model parameters.Comment: 18 pages, 10 figures; v2: Plots and captions of figs. 7 and 10
correcte
Godel-type space-time metrics
A simple group theoretic derivation is given of the family of space-time
metrics with isometry group SO(2,1) X SO(2) X R first described by Godel, of
which the Godel stationary cosmological solution is the member with a
perfect-fluid stress-energy tensor. Other members of the family are shown to be
interpretable as cosmological solutions with a electrically charged perfect
fluid and a magnetic field.Comment: Heavly rewritten respect to the orginal version, corrected some typos
due to files transfer in the last submitted versio
Component-wise damage detection by neural networks and refined FEs training
Multilayer perceptrons are utilized in this work for vibration-based damage detection of multicomponent aerospace structures. A back-propagation algorithm is utilized along with Monte
Carlo simulations and advanced structural theories for training Artificial Neural Networks
(ANN’s), which are able to detect and classify local damages in structures given the natural
frequencies and the associated vibrations modes. The latter ones are feed into the network
in terms of Modal Assurance Criterion (MAC), which is a scalar representing the degree of
consistency between undamaged and damaged modal vectors. Dataset and ANN training process
is carried out by means of Carrera Unified Formulation (CUF), according to which refined finite
elements with component-wise capabilities can be implemented in a hierarchical and unified
manner. The proposed results demonstrate that CUF-trained ANNs can approximate complete
mapping of the damage distribution, even in case of low damage intensities and local defects
in localized components (stringers, spar caps, webs, etc.
Damage detection in composites by AI and high-order modelling surface-strain-displacement analysis
In the recent years, machine learning algorithms have been widely employed for structural health monitoring applications. As an example, Artificial Neu-ral Networks (ANN) could be useful in giving a precise and complete map-ping of damage distribution in a structure, including low-intensity or local-ized defects, which could be difficult to detected via traditional testing tech-niques. In this domain, Convolutional Neural Network (CNN) are employed in this work along with one-dimensional refined models based on the Carrera Unified formulation (CUF) for surface strain\displacement based damage detection in composite laminates. A layer-wise kinematic is adopted, while both an isotropic and orthotropic damage formulation is implemented. In de-tail, CUF-based finite element models have been exploited in combination with Monte Carlo simulations for the creation of a dataset of damage scenar-ios used for the training of the CNN. Therefore, the latter is fed with images of the strain or displacement field in a region of particular interest for each sample, which are subjected to the same boundary conditions. The trained ANN, given the strain\displacement mapping of an unknown structure, is therefore able to detect and classify all the damages within the structure, solving the so-called inverse problem
A Multi-Objective Approach to Optimize a Periodic Maintenance Policy
The present paper proposes a multi-objective approach to find out an optimal periodic maintenance policy for a repairable and stochastically deteriorating multi-component system over a finite time horizon. The tackled problem concerns the determination of the system elements to replace at each scheduled and periodical system inspection by ensuring the simultaneous minimization of both the expected total maintenance cost and the expected global system unavailability time. It is assumed that in the case of system elements failure they are instantaneously detected and repaired by means of minimal repair actions in order to rapidly restore the system. A non-linear integer mathematical programming model is developed to solve the treated multi-objective problem whereas the Pareto optimal frontier is described by the Lexicographic Goal Programming and the \u3b5-constraint methods. To explain the whole procedure a case study is solved and the related considerations are given
Decision Procedure for Entailment of Symbolic Heaps with Arrays
This paper gives a decision procedure for the validity of en- tailment of
symbolic heaps in separation logic with Presburger arithmetic and arrays. The
correctness of the decision procedure is proved under the condition that sizes
of arrays in the succedent are not existentially bound. This condition is
independent of the condition proposed by the CADE-2017 paper by Brotherston et
al, namely, one of them does not imply the other. For improving efficiency of
the decision procedure, some techniques are also presented. The main idea of
the decision procedure is a novel translation of an entailment of symbolic
heaps into a formula in Presburger arithmetic, and to combine it with an
external SMT solver. This paper also gives experimental results by an
implementation, which shows that the decision procedure works efficiently
enough to use
Function Allocation between Automation and Human Pilot for Airborne Separation Assurance
Maintaining safe separation between aircraft is a key determinant of the airspace capacity to handle air transportation. With the advent of satellite-based surveillance, aircraft equipped with the needed technologies are now capable of maintaining awareness of their location in the airspace and sharing it with their surrounding traffic. As a result, concepts and cockpit automation are emerging to enable delegating the responsibility of maintaining safe separation from traffic to the pilot; thus increasing the airspace capacity by alleviating the limitation of the current non-scalable centralized ground-based system. In this paper, an analysis of allocating separation assurance functions to the human pilot and cockpit automation is presented to support the design of these concepts and technologies. A task analysis was conducted with the help of Petri nets to identify the main separation assurance functions and their interactions. Each function was characterized by three behavior levels that may be needed to perform the task: skill, rule and knowledge based levels. Then recommendations are made for allocating each function to an automation scale based on their behavior level characterization and with the help of Subject matter experts
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