On the stability of field-theoretical regularizations of negative tension branes

Any attempt to regularize a negative tension brane through a bulk scalar requires that this field is a ghost. One can try to improve in this aspect in a number of ways. For instance, it has been suggested to employ a field whose kinetic term is not sign definite, in the hope that the background may be overall stable. We show that this is not the case; the physical perturbations (gravity included) of the system do not extend across the zeros of the kinetic term; hence, all the modes are entirely localized either where the kinetic term is positive, or where it is negative; this second type of modes are ghosts. We show that this conclusion does not depend on the specific choice for the kinetic and potential functions for the bulk scalar.Comment: 7 pages, 3 figure

Moduli Evolution in Heterotic Scenarios

We discuss several aspects of the cosmological evolution of moduli fields in heterotic string/M-theory scenarios. In particular we study the equations of motion of both the dilaton and overall modulus of these theories in the presence of an expanding Universe and under different assumptions. First we analyse the impact of their couplings to matter fields, which turns out to be negligible in the string and M-theory scenarios. Then we examine in detail the possibility of scaling in M-theory, i.e. how the moduli would evolve naturally to their minima instead of rolling past them in the presence of a dominating background. In this case we find interesting and positive results, and we compare them to the analogous situation in the heterotic string.Comment: 12 pages, 4 postscript figure

A Review on the Cosmology of the de Sitter Horndeski Models

We review the most general scalar-tensor cosmological models with up to second-order derivatives in the field equations that have a fixed spatially flat de Sitter critical point independent of the material content or vacuum energy. This subclass of the Horndeski Lagrangian is capable of dynamically adjusting any value of the vacuum energy of the matter fields at the critical point. We present the cosmological evolution of the linear models and the non-linear models with shift symmetry. We come to the conclusion that the shift symmetric non-linear models can deliver a viable background compatible with current observations.Comment: 7 pages, 2 figures. Proceedings accepted for publication in "Universe", Special Issue "Varying Constants and Fundamental Cosmology" for the VARCOSMOFUN16 in Szczecin, Poland, 12-17 September, 201

Status of reaction theory for studying rare isotopes

Reactions are an important tool to study nuclear structure and for extracting reactions relevant for astrophysics. In this paper we focus on deuteron induced reactions which can provide information on neutron shell evolution as well as neutron capture cross sections. We review recent work on the systematic comparison of the continuum discretized coupled channel method, the adiabatic wave approximation and the Faddeev momentum-space approach. We also explore other aspects of the reaction mechanism and discuss in detail difficulties encountered in the calculations.Comment: 7 pages, 5 figures, proceeding for HITES 201