A model for time-dependent cosmological constant and its consistency with the present Friedmann universe

We use a model where the cosmological term can be related to the chiral gauge anomaly of a possible quantum scenario of the initial evolution of the universe. We show that this term is compatible with the Friedmann behavior of the present universe.Comment: 5 pages, Revtex 4, twocolumn (minor corrections and improved reference list. To appear in Classical and Quantum Gravity

Cyclic Magnetic Universe

Recent works have shown the important role Nonlinear Electrodynamics (NLED) can have in two crucial questions of Cosmology, concerning particular moments of its evolution for very large and for low-curvature regimes, that is for very condensed phase and at the present period of acceleration. We present here a toy model of a complete cosmological scenario in which the main factor responsible for the geometry is a nonlinear magnetic field which produces a FRW homogeneous and isotropic geometry. In this scenario we distinguish four distinct phases: a bouncing period, a radiation era, an acceleration era and a re-bouncing. It has already been shown that in NLED a strong magnetic field can overcome the inevitability of a singular region typical of linear Maxwell theory; on the other extreme situation, that is for very weak magnetic field it can accelerate the expansion. The present model goes one step further: after the acceleration phase the universe re-bounces and enter in a collapse era. This behavior is a manifestation of the invariance under the dual map of the scale factor $a(t) \to 1/ a(t),$ a consequence of the corresponding inverse symmetry of the electromagnetic field ($F \to 1/ F,$ where $F \equiv F^{\mu\nu}F_{\mu\nu}$) of the NLED theory presented here. Such sequence collapse-bouncing-expansion-acceleration-re-bouncing-collapse constitutes a basic unitary element for the structure of the universe that can be repeated indefinitely yielding what we call a Cyclic Magnetic Universe

Back-reaction effects in acoustic black holes

Acoustic black holes are very interesting non-gravitational objects which can be described by the geometrical formalism of General Relativity. These models can be useful to experimentally test effects otherwise undetectable, as for example the Hawking radiation. The back-reaction effects on the background quantities induced by the analogue Hawking radiation could be the key to indirectly observe it. We briefly show how this analogy works and derive the backreaction equations for the linearized quantum fluctuations in the background of an acoustic black hole. A first order in hbar solution is given in the near horizon region. It indicates that acoustic black holes, unlike Schwarzschild ones, get cooler as they radiate phonons. They show remarkable analogies with near-extremal Reissner-Nordstrom black holes.Comment: 10 pages, 1 figure; Talk given at the conference ``Constrained Dynamics and Quantum Gravity (QG05)", Cala Gonone (Italy), September 200

Dyadosphere bending of light

In the context of the static and spherically symmetric solution of a charged compact object, we present the expression for the bending of light in the region just outside the event horizon (the dyadosphere) where vacuum polarization effects are taken into account.Comment: 4 pages, LaTeX, to appear in A&A (2001

Regular Magnetic Black Holes and Monopoles from Nonlinear Electrodynamics

It is shown that general relativity coupled to nonlinear electrodynamics (NED) with the Lagrangian $L(F)$, $F = F_mn F^mn$ having a correct weak field limit, leads to nontrivial static, spherically symmetric solutions with a globally regular metric if and only if the electric charge is zero and $L(F)$ tends to a finite limit as $F \to \infty$. Properties and examples of such solutions, which include magnetic black holes and soliton-like objects (monopoles), are discussed. Magnetic solutions are compared with their electric counterparts. A duality between solutions of different theories specified in two alternative formulations of NED (called $FP$ duality) is used as a tool for this comparison.Comment: 6 pages, Latex2e. One more theorem, some comments and two references have been added. Final journal versio

Simulation of Acoustic Black Hole in a Laval Nozzle

A numerical simulation of fluid flows in a Laval nozzle is performed to observe formations of acoustic black holes and the classical counterpart to Hawking radiation under a realistic setting of the laboratory experiment. We determined the Hawking temperature of the acoustic black hole from obtained numerical data. Some noteworthy points in analyzing the experimental data are clarified through our numerical simulation.Comment: 26 pages, published versio

Beyond Analog Gravity: The Case of Exceptional Dynamics

We show that it is possible to go beyond the simple kinematical aspects of the analog models of gravity. We exhibit the form of the Lagrangian that describes the dynamics of a self-interacting field $\phi$ as an interaction between $\phi$ and its associated effective metric $\hat{g}^{\mu\nu}.$ In other words the non-linear equation of motion of the field may be interpreted as the gravitational influence on $\phi$ by its own effective metric which, in our scheme becomes an active partner of the dynamics of $\phi.$Comment: This new version is an extended version of the previous one. All previous results are mantaine

Classical and Quantum Bianchi Type III vacuum Horava - Lifshitz Cosmology

A diagonal Bianchi Type III space-time is treated, both at the classical and quantum level, in the context of Horava - Lifshitz gravity. The system of the classical equations of motion is reduced to one independent Abel's equation of the first kind. Closed form solution are presented for various values of the coupling constants appearing in the action. Due to the method used, solutions of Euclidean, Lorentzian and neutral signature are attained. The solutions corresponding to \lamda 1 are seen to develop curvature singularities as the other constants approach their Einsteinian values, in contrast to those with \lamda = 1 which tend to the known Einstein gravity solutions. At the quantum level, the resulting Wheeler-DeWitt equation is explicitly solved for \lamda = 1, \sigma = 0 and \lamda = 1/3 . The ensuing wave-functions diverge in the Einsteinian limit.Comment: LaTeX 2e source file, 17 pages, no figure

Teleparallel origin of the Fierz picture for spin-2 particle

A new approach to the description of spin-2 particle in flat and curved spacetime is developed on the basis of the teleparallel gravity theory. We show that such an approach is in fact a true and natural framework for the Fierz representation proposed recently by Novello and Neves. More specifically, we demonstrate how the teleparallel theory fixes uniquely the structure of the Fierz tensor, discover the transparent origin of the gauge symmetry of the spin 2 model, and derive the linearized Einstein operator from the fundamental identity of the teleparallel gravity. In order to cope with the consistency problem on the curved spacetime, similarly to the usual Riemannian approach, one needs to include the non-minimal (torsion dependent) coupling terms.Comment: 5 pages, Revtex4, no figures. Accepted for publication in Phys. Rev.