Wormholes in spacetimes with cosmological horizons

A generalisation of the asymptotic wormhole boundary condition for the case of spacetimes with a cosmological horizon is proposed. In particular, we consider de Sitter spacetime with small cosmological constant. The wave functions selected by this proposal are exponentially damped in WKB approximation when the scale factor is large but still much smaller than the horizon size. In addition, they only include outgoing gravitational modes in the region beyond the horizon. We argue that these wave functions represent quantum wormholes and compute the local effective interactions induced by them in low-energy field theory. These effective interactions differ from those for flat spacetime in terms that explicitly depend on the cosmological constant.Comment: 10 pages, LaTeX 2.O9, no figure

Einstein Gravity as an emergent phenomenon?

In this essay we marshal evidence suggesting that Einstein gravity may be an emergent phenomenon, one that is not ``fundamental'' but rather is an almost automatic low-energy long-distance consequence of a wide class of theories. Specifically, the emergence of a curved spacetime ``effective Lorentzian geometry'' is a common generic result of linearizing a classical scalar field theory around some non-trivial background. This explains why so many different ``analog models'' of general relativity have recently been developed based on condensed matter physics; there is something more fundamental going on. Upon quantizing the linearized fluctuations around this background geometry, the one-loop effective action is guaranteed to contain a term proportional to the Einstein--Hilbert action of general relativity, suggesting that while classical physics is responsible for generating an ``effective geometry'', quantum physics can be argued to induce an ``effective dynamics''. This physical picture suggests that Einstein gravity is an emergent low-energy long-distance phenomenon that is insensitive to the details of the high-energy short-distance physics.Comment: 8 pages, Essay awarded an honorable mention in the year 2001 Gravity Research Foundation essay competitio

The envelope of the power spectra of over a thousand \delta Scuti stars. The Tˉeff\bar{T}_{eff}-νmax\nu_{max} scaling relation

CoRoT and Kepler high-precision photometric data allowed the detection and characterization of the oscillation parameters in stars other than the Sun. Moreover, thanks to the scaling relations, it is possible to estimate masses and radii for thousands of solar-type oscillating stars. Recently, a \Delta\nu - \rho relation has been found for \delta Scuti stars. Now, analyzing several hundreds of this kind of stars observed with CoRoT and Kepler, we present an empiric relation between their frequency at maximum power of their oscillation spectra and their effective temperature. Such a relation can be explained with the help of the \kappa-mechanism and the observed dispersion of the residuals is compatible with they being caused by the gravity-darkening effect

Stability analysis of sonic horizons in Bose-Einstein condensates

We examine the linear stability of various configurations in Bose-Einstein condensates with sonic horizons. These configurations are chosen in analogy with gravitational systems with a black hole horizon, a white hole horizon and a combination of both. We discuss the role of different boundary conditions in this stability analysis, paying special attention to their meaning in gravitational terms. We highlight that the stability of a given configuration, not only depends on its specific geometry, but especially on these boundary conditions. Under boundary conditions directly extrapolated from those in standard General Relativity, black hole configurations, white hole configurations and the combination of both into a black hole--white hole configuration are shown to be stable. However, we show that under other (less stringent) boundary conditions, configurations with a single black hole horizon remain stable, whereas white hole and black hole--white hole configurations develop instabilities associated to the presence of the sonic horizons.Comment: 14 pages, 7 figures (reduced resolution

El uso de Sistemas Expertos en la interpretación de clasificaciones estadísticas multidimensionales

La investigación que se presenta consiste, básicamente, en el estudio de viabilidad de una base de conocimientos que formaliza el dominio científico de la interacción e identidad sociales; más concretamente, el uso de los objetos o artefactos "sociotécnicos" como símbolos de clase. Lo que se pretende es programar un sistema experto que simule el comportamiento de tal base de conocimientos. Los datos de entrada son los resultados de una clasificación automática (método de las escalas multidimensionales), y los datos de sliada constituyen la "interpretación" conceptual de los datos matemáticos de entrada en términos de los conceptos implementados en la base de conocimientos, la cual puede se descrita como una base de datos activa, con reglas deductivas asociadas. El propósito general de la investigación es estudiar la forma en que usamos teorías parcialmente estructuradas (sin conexiones causales) para obtener una teoría de la descripción, y cómo esa teoría de la descripción modifica la teoría científica propiamente dicha

Quasi-particle creation by analogue black holes

We discuss the issue of quasi-particle production by ``analogue black holes'' with particular attention to the possibility of reproducing Hawking radiation in a laboratory. By constructing simple geometric acoustic models, we obtain a somewhat unexpected result: We show that in order to obtain a stationary and Planckian emission of quasi-particles, it is not necessary to create an ergoregion in the acoustic spacetime (corresponding to a supersonic regime in the flow). It is sufficient to set up a dynamically changing flow either eventually generating an arbitrarily small sonic region v=c, but without any ergoregion, or even just asymptotically, in laboratory time, approaching a sonic regime with sufficient rapidity.Comment: 30 pages, 16 figure

Una interpretación socioeconómica del bronce final en el sudoeste de la Península Ibérica

En este artículo pretendo estudiar las transformaciones socioeconómicas en el Sudoeste de la Península Ibérica durante el Bronce Final o Bronce Reciente -los dos términos son sinónimos en este trabajo-. Se trata del análisis de un caso histórico específico que proporciona interesante información para la ulterior verificación de algunas de las teorías usadas para interpretar la transición de las economías de subsistencia «tribales» a la estratificación social. He intentado demostrar que en el Sudoeste de la Península Ibérica durante el Bronce Final, la introducción de una tecnología foránea (el auténtico bronce) y el inicio de nuevas redes de intercambio causaron una profunda transformación del orden social, que tuvo una gran relevancia en la transición hacia la sociedad de clasesIn this paper, I have tried to study socioeconomic transformations in the Southwestern Iberian Peninsula during the Late Bronze age. This case study gives interesting information to verify sorne of the theoretical constructs used to give meaning to the passage from «tribal» or subsistence economies to social stratification. We can say that in Southwestern Iberian during the Late Bronze age, the introduction of a foreign technology and the beginnings of new exchange networks caused deep transformations in the social order, that had great significance in the passage to a class-societ