Santiago E. Perez Bergliaffa

Centaurus A as a source of extragalactic cosmic rays with arrival energies well beyond the GZK cutoff

Abstract The ultra—high energy cosmic rays recently detected by several air shower experiments could have an extragalactic origin. In this case, the nearest active galaxy Centaurus A might be the source of the most energetic particles ever detected on Earth. We have used recent radio observations in order to estimate the arrival energy of the protons accelerated by strong shock fronts in the outer parts of this southern radio source. We expect detections corresponding to particles with energies up to ∼ 2.2 × 10 21 eV and an arrival direction of ( l ≈ 310°, b ≈ 20°) in galactic coordinates. The future Southern Hemisphere Pierre Auger Observatory might provide a decisive test for extragalactic models of the origin of the ultra—high energy cosmic rays.

Brans-Dicke wormholes in nonvacuum spacetime

Analytical wormhole solutions in Brans-Dicke theory in the presence of matter are presented. It is shown that the wormhole throat must not be necessarily threaded with exotic matter.

Evolving wormhole geometries

We present here analytical solutions of General Relativity that describe evolving wormholes with a non-constant redshift function. We show that the matter that threads these wormholes is not necessarily exotic. Finally, we investigate some issues concerning WEC violation and human traversability in these time-dependent geometries.

Static and spherically symmetric black holes in f(R) theories

We study some features of static and spherically symmetric solutions (SSS) with a horizon in f(R) theories of gravitation by means of a near-horizon analysis. A necessary condition for an f(R) theory to have this type of solution is obtained. General features of the effective potential are deduced, and it is shown that there exists a limit on the curvature at the horizon, in both cases for any f(R). Finally, we calculate the expression for the energy of the collision of two massive particles in the center of mass frame.

Brane worlds, string cosmology, and AdS / CFT

Using the thin-shell formalism we discuss the motion of domain walls in de Sitter and anti--de Sitter (AdS) time-dependent bulks. This motion results in a dynamics for the brane scale factor. We show that in the case of a clean brane the scale factor describes both singular and nonsingular universes, with phases of contraction and expansion. These phases can be understood as quotients of AdS spacetime by a discrete symmetry group. We discuss this effect in some detail, and suggest how the AdS/CFT correspondence could be applied to obtain a nonperturbative description of brane-world string cosmology.

Effective metric in nonlinear scalar field theories

We discuss several features of the propagation of perturbations in nonlinear scalar field theories using the effective metric. It is shown that the effective metric can be classified according to whether the gradient of the scalar field is timelike, null, or spacelike, and this classification is illustrated with two examples. We shall also show that different signatures for the effective metric are allowed.

INHOMOGENEOUS MULTIDIMENSIONAL COSMOLOGIES

Einsteins equations for a (4 + n)-dimensional inhomogeneous space–time are presented, and a special family of solutions is exhibited for an arbitrary n. The solutions depend on two arbitrary functions of time. The time development of a particular member of this family is studied. This solution exhibits a singularity at t = 0 and dynamical compactification of the n dimensions. It is shown that the behavior of the system in the four-dimensional(i.e. post-compactification) phase is constrained by the way in which the compactified dimensions are stabilized. The fluid that generates this solution is analyzed by means of the energy conditions.

A Born-Infeld-like f(R) gravity

Several features of an

AN ANALYSIS OF A REGULAR BLACK HOLE INTERIOR

f(R)

CYLINDRICALLY SYMMETRIC SPINNING BRANS–DICKE SPACE–TIMES WITH CLOSED TIMELIKE CURVES

theory in which there is a maximum value for the curvature are analyzed. The theory admits the vaccuum solutions of GR, and also the radiation evolution for the scale factor of the standard cosmological model. Working in the Jordan frame, a complete analysis of the phase space is performed, and its results supported with examples obtainted by numerical integration. In particular, we showed that theory has nonsingular cosmological solutions which after the bounce enter a phase of de Sitter expansion and subsequently relax to a GR-like radiation-dominated evolution.