Uniqueness of the Fock quantization of scalar fields and processes with signature change in cosmology

We study scalar fields subject to an equation of the Klein-Gordon type in nonstationary spacetimes, such as those found in cosmology, assuming that all the relevant spatial dependence is contained in the Laplacian. We show that the field description ---with a specific canonical pair--- and the Fock representation for the quantization of the field are fixed indeed in a unique way (except for unitary transformations that do not affect the physical predictions) if we adopt the combined criterion of (a) imposing the invariance of the vacuum under the group of spatial symmetries of the field equations and (b) requiring a unitary implementation of the dynamics in the quantum theory. Besides, we provide a spacetime interpretation of the field equations as those corresponding to a scalar field in a cosmological spacetime that is conformally ultrastatic. In addition, in the privileged Fock quantization, we investigate the generalization of the evolution of physical states from the hyperbolic dynamical regime to an elliptic regime. In order to do this, we contemplate the possibility of processes with signature change in the spacetime where the field propagates and discuss the behavior of the background geometry when the change happens, proving that the spacetime metric degenerates. Finally, we argue that this kind of signature change leads naturally to a phenomenon of particle creation, with exponential production.Comment: 11 pages, version accepted for publication in Physical Review

Comparison of different repetitive control architectures: synthesis and comparison. Application to VSI Converters

Repetitive control is one of the most used control approaches to deal with periodic references/disturbances. It owes its properties to the inclusion of an internal model in the controller that corresponds to a periodic signal generator. However, there exist many different ways to include this internal model. This work presents a description of the different schemes by means of which repetitive control can be implemented. A complete analytic analysis and comparison is performed together with controller synthesis guidance. The voltage source inverter controller experimental results are included to illustrative conceptual developmentsPeer ReviewedPostprint (published version

Los museos de ciencias como instrumentos de alfabetización científica

Esta comunicación ha sido concebida como contribución a la Década de la Educación para un futuro sostenible, instituida por Naciones Unidas para el periodo 2005-2014. (Ver http://www.oei.es/decada/).Numerosas investigaciones han mostrado que las visiones transmitidas de la naturaleza de la ciencia y la tecnología, tanto por la educación formal como por la no reglada, se caracterizan por un conjunto de deformaciones estrechamente relacionadas, que contribuyen al desinterés hacia su estudio. Atendiendo a esa problemática, nuestra investigación se centra en el papel educativo de las visitas de los estudiantes a los museos de ciencia y tecnología, analizando las visiones que suelen transmitir de las mismas. En la comunicación se mostrarán los primeros resultados del análisis de diez museos visitados, con el fin de diseñar propuestas que favorezcan el uso de los museos como instrumentos de alfabetización científica, para proporcionar una imagen más real de las actividades tecnocientíficas y, por tanto, más rica y contextualizada, susceptible de generar un mayor interés