Vacuum and semiclassical gravity: a difficulty and its bewildering significance

We review a long-standing difficulty in some semiclassical models of vacuum and vacuum decay. Surprisingly enough these models, careless of their transparent formulation, are affected by, both, technical and conceptual issues. After proving some general results that are relevant for, both, the Euclidean and Lorentzian sectors of their dynamics, we briefly highlight their importance in connection with the issues discussed before, arguing that their solution might be interesting in our quest for quantum gravity.Comment: 11 pages, LaTeX, 4 figure; talk given at the workshop "From Quantum to Emergent Gravity: Theory and Phenomenology", June, 11-15 2007, Trieste, Italy (http://www.sissa.it/app/QGconference); submitted to the "Proceedings of Science" (http://pos.sissa.it

Minisuperspace, WKB, Quantum States of General Relativistic Extended Objects

The dynamics of relativistic thin shells is a recurrent topic in the literature about the classical theory of gravitating systems and the still ongoing attempts to obtain a coherent description of their quantum behavior. Certainly, a good reason to make this system a preferred one for many models is the clear, synthetic description given by Israel junction conditions. Using some results from an effective Lagrangian approach to the dynamics of spherically symmetric shells, we show a general way to obtain WKB states for the system; a simple example is also analyzed.Comment: 3 pages, to appear in the proceedings of the 16th SIGRAV conference on general relativity and gravitational physic

WKB metastable quantum states of a de Sitter--Reissner-Nordstroem dust shell

We study the dynamics of a spherically symmetric dust shell separating two spacetime domains, the 'interior' one being a part of the de Sitter spacetime and the exterior one having the 'extremal' Reissner-Nordstroem geometry. Extending the ideas of previous works on the subject, we show that the it is possible to determine the (metastable) WKB quantum states of this gravitational system.Comment: IOPart class, 16 figure

Braneworlds and Quantum States of Relativistic Shells

We review some applications of relativistic shells that are relevant in the context of quantum gravity/quantum cosmology. Using a recently developed approach, the stationary states of this general relativistic system can be determined in the semiclassical approximation. We suggest that this technique might be of phenomenological relevance in the context of the brane-world scenario and we draw a picture of the general set-up and of the possible developments.Comment: 4 pages, LaTeX. To appear in the proceedings of the Eleventh Marcel Grossmann Meeting on General Relativity, July 23-29, 2006, Freie Universitaet Berlin, Berlin, Germany (http://www.icra.it/mg/mg11

Unified Dark Energy-Dark Matter model with Inverse Quintessence

We consider a model where both dark energy and dark matter originate from the coupling of a scalar field with a non-conventional kinetic term to, both, a metric measure and a non-metric measure. An interacting dark energy/dark matter scenario can be obtained by introducing an additional scalar that can produce non constant vacuum energy and associated variations in dark matter. The phenomenology is most interesting when the kinetic term of the additional scalar field is ghost-type, since in this case the dark energy vanishes in the early universe and then grows with time. This constitutes an "inverse quintessence scenario", where the universe starts from a zero vacuum energy density state, instead of approaching it in the future.Comment: 13 pages; minor changes with references adde