1,020 research outputs found
Evolution Operators for Linearly Polarized Two-Killing Cosmological Models
We give a general procedure to obtain non perturbative evolution operators in
closed form for quantized linearly polarized two Killing vector reductions of
general relativity with a cosmological interpretation. We study the
representation of these operators in Fock spaces and discuss in detail the
conditions leading to unitary evolutions.Comment: Accepted for publication in Physical Review
Hamiltonian Dynamics of Linearly Polarized Gowdy Models Coupled to Massless Scalar Fields
The purpose of this paper is to analyze in detail the Hamiltonian formulation
for the compact Gowdy models coupled to massless scalar fields as a necessary
first step towards their quantization. We will pay special attention to the
coupling of matter and those features that arise for the three-handle and
three-sphere topologies that are not present in the well studied three torus
case -in particular the polar constraints that come from the regularity
conditions on the metric. As a byproduct of our analysis we will get an
alternative understanding, within the Hamiltonian framework, of the appearance
of initial and final singularities for these models.Comment: Final version to appear in Classical and Quantum Gravit
ParticipaciĂłn en la Primera Competencia Nacional de Vivienda Sustentable Resistente a FenĂłmenos Naturales para Estudiantes de Arquitectura
En este reporte se presentará el proceso que llevamos a cabo para la realizaciĂłn de nuestro proyecto como participantes en la primera competencia nacional de vivienda sustentable resistente a fenĂłmenos naturales para estudiantes de arquitectura, desde el análisis que llevamos a cabo para escoger una locaciĂłn y sus fenĂłmenos naturales, asĂ como los materiales y el diseño necesarios para cumplir con los requisitos que el concurso nos pedĂa
Protecting Classical-Quantum Signals in Free Space Optical Channels
Due to turbulence and tracking errors, free-space optical channels involving
mobile transceivers are characterized by a signal's partial loss or complete
erasure. This work presents an error correction protocol capable of protecting
a signal passing through such channels by encoding it with an ancillary
entangled bipartite state. Beyond its ability to offer protection under
realistic channel conditions, novel to our protocol is its ability to encompass
both classical and quantum information on the encoded signal. We show how,
relative to non-encoded direct transmission, the protocol can improve the
fidelity of transmitted coherent states over a wide range of losses and erasure
probabilities. In addition, the use of ancillary non-Gaussian entangled
bipartite states in the signal encoding is considered, showing how this can
increase performance. Finally, we briefly discuss the application of our
protocol to the transmission of more complex input states, such as multi-mode
entangled states
CONTRIBUTION OF INFORMAL GREENSPACE TO BIRD CONSERVATION IN CITIES: A COMPARATIVE STUDY ON THE DIVERSITY OF BIRD COMMUNITIES IN VACANT LANDS, URBAN PARKS AND RESIDENTIAL AREAS
Vacant lands are part of the informal greenspace and could maintain native fauna in urban ecosystems. To provide scientific evidence that promotes bird conservation in cities, we compared different bird community attributes among vacant lands, urban parks, and residential areas in the city of Santiago de Chile. For this, we estimated taxonomic diversity in the three land uses, investigated the species richness and abundance (total and native, including three trophic guilds: granivores, insectivores and omnivores) among land-use types, and evaluated the influence of habitat variables on bird species richness and abundance recorded at sites. We found that vacant lands supported a diverse, with low species dominance, species rich and abundant bird community, comprised mainly by native granivorous and insectivorous birds. In contrast, birds with generalist diet (omnivores) reached high abundances in urban parks and residential areas. While parks and residential areas were dominated by a single omnivore species (Turdus falcklandii and the exotic Passer domesticus, respectively), in vacant lands a set of native species reached high abundances. They included granivorous birds (Sicalis luteola, Zenaida auriculata y Zonotrichia capensis) and insectivorous birds (Tachycineta meyeni y Anthus correndera). Sites with larger cover of herbaceous plants exhibited greater richness and abundance of native birds. In addition, sites with larger proportion of their area covered by bare ground exhibited greater abundance of native birds. Our results demonstrate that vacant lands maintain high bird diversity, as well as a high species richness and abundance of native birds with specialized diets (granivores and insectivores), offering new opportunities to conserve biodiversity in cities
Unitary evolution of free massless fields in de Sitter space-time
We consider the quantum dynamics of a massless scalar field in de Sitter
space-time. The classical evolution is represented by a canonical
transformation on the phase space for the field theory. By studying the
corresponding Bogoliubov transformations, we show that the symplectic map that
encodes the evolution between two instants of time cannot be unitarily
implemented on any Fock space built from a SO(4)-symmetric complex structure.
We will show also that, in contrast with some effectively lower dimensional
examples arising from Quantum General Relativity such as Gowdy models, it is
impossible to find a time dependent conformal redefinition of the massless
scalar field leading to a quantum unitary dynamics.Comment: 20 pages. Comments and references adde
Flux-area operator and black hole entropy
We show that, for space-times with inner boundaries, there exists a natural
area operator different from the standard one used in loop quantum gravity.
This new flux-area operator has equidistant eigenvalues. We discuss the
consequences of substituting the standard area operator in the
Ashtekar-Baez-Corichi-Krasnov definition of black hole entropy by the new one.
Our choice simplifies the definition of the entropy and allows us to consider
only those areas that coincide with the one defined by the value of the level
of the Chern-Simons theory describing the horizon degrees of freedom. We give a
prescription to count the number of relevant horizon states by using spin
components and obtain exact expressions for the black hole entropy. Finally we
derive its asymptotic behavior, discuss several issues related to the
compatibility of our results with the Bekenstein-Hawking area law and the
relation with Schwarzschild quasi-normal modes.Comment: 25 page
Wealth Inequality, Educational Environment and School Enrolment: Evidence from Mexico
Using data from the extended section of the 2010 Mexican census (2.9 million households), we study how school enrolment is associated with wealth inequality and with the educational environment the child is exposed to at the household and municipal levels. We provide robust evidence of wealth inequality as a negative predictor of school enrolment for children in primary, secondary and high school age ranges while a positive role is played by the educational environment. Through the introduction of interaction terms, we account for how economic and educational variables are intertwined at both the household and the municipal level, and we are able to illustrate the considerable heterogeneity in the role of adult education for households at different standards of living
uDDS: A Middleware for Real-time Wireless Embedded Systems
[EN] A Real-Time Wireless Distributed Embedded System (RTWDES) is formed by a large quantity of small devices with certain computing power, wireless communication and sensing/actuators capabilities. These types of networks have become popular as they have been developed for applications which can carry out a vast quantity of tasks, including home and building monitoring, object tracking, precision agriculture, military applications, disaster recovery, industry applications, among others. For this type of applications a middleware is used in software systems to bridge the gap between the application and the underlying operating system and networks. As a result, a middleware system can facilitate the development of applications and is designed to provide common services to the applications. The development of a middleware for sensor networks presents several challenges due to the limited computational resources and energy of the different nodes. This work is related with the design, implementation and test of a micro middleware for RTWDES; the proposal incorporates characteristics of a message oriented middleware thus allowing the applications to communicate by employing the publish/subscribe model. Experimental evaluation shows that the proposed middleware provides a stable and timely service to support different Quality of Service (QoS) levels. © 2011 Springer Science+Business Media B.V.This work was developed as a part of the D2ARS Project supported by CYTED. UNESCO code 120325;330417;120314;120305.González, A.; Mata, W.; Villaseñor, L.; Aquino, R.; SimĂł Ten, JE.; Chávez, M.; Crespo Lorente, A. (2011). uDDS: A Middleware for Real-time Wireless Embedded Systems. Journal of Intelligent and Robotic Systems. 64(3-4):489-503. https://doi.org/10.1007/s10846-011-9550-zS489503643-4Akyildiz, I.F., Su, W., Sankarasubramaniam, Y., Cayirci, E.: A survey on sensor networks. IEEE Commun. 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