Alexander Feinstein

Exponential-potential scalar field universes. I. Bianchi type I models.

We obtain a general exact solution of the Einstein field equations for the anisotropic Bianchi type I universes filled with an exponential-potential scalar field and study their dynamics. It is shown, in agreement with previous studies, that for a wide range of initial conditions the late-time behavior of the models is that of a power-law inflating Friedmann-Robertson-Walker (FRW) universe. This property does not hold, in contrast, when some degree of inhomogeneity is introduced, as discussed in our following paper.

Initial conditions and the structure of the singularity in pre - big bang cosmology

We propose a picture, within the pre-big-bang approach, in which the universe emerges from a bath of plane gravitational and dilatonic waves. The waves interact gravitationally breaking the exact plane symmetry and lead generically to gravitational collapse resulting in a singularity with Kasner-like structure. The analytic relations between the Kasner exponents and the initial data are evaluated explicitly and it is shown that pre-big-bang inflation may occur within a dense set of initial data. Finally, we argue that plane waves carry zero gravitational entropy and thus are, from a thermodynamical point of view, good candidates for the universe to emerge from.

Homogeneous scalar field and the wet dark sides of the universe

We study the possibility that a generalized real scalar field minimally coupled to gravity could explain both the galactic and the cosmological dark components of the universe. Within the framework of Einsteins Relativity we model static galactic halos by considering the most general action built from the scalar field and its first derivatives. Although the gravitational configuration is static, the scalar field may be either static, or homogeneous and linear in time. In the case of the static scalar field, the models we look at inevitably posses unphysical negative energies, and we are led to a sort of no-go result. In the case of the homogeneous scalar field, on the contrary, we find that compact objects with flat rotational curves and with the mass and the size of a typical galaxy can be successfully modeled and the Tully-Fisher relation recovered. We further show that the homogeneous scalar field deduced from the galactic halo spacetimes has an action compatible with the kinetic Unified Dark Matter models recently proposed by Scherrer. Therefore, such a homogeneous kinetic Unified Dark Matter not only may correctly mimic the galactic dynamics, but could also be used to model the present day accelerated expansion in the universe.

Curved dilatonic brane worlds

We construct a broad family of exact solutions to the five-dimensional Einstein equations coupled to a scalar field with an exponential potential. Embedding a three-brane in these bulk space-times in a particular way we obtain a class of self-tuned curved brane worlds in which the vacuum energy on the brane is gravitationally idle, the four-dimensional geometry being insensitive to the value of the brane tension. This self-tuning arises from cancellations, enforced by the junction conditions, between the scalar field potential, the brane vacuum energy and the matter on the brane. Finally, we study some physically relevant examples and their dynamics.

Exponential-potential scalar field universes. II. Inhomogeneous models.

We obtain exact solutions for the Einstein equations with an exponential-potential scalar field (

Can the type-IIB axion prevent Pre-Big Bang inflation?

V=\ensuremath{\Lambda}{e}^{k\ensuremath{\varphi}}

Tilted string cosmologies

) which represent simple inhomogeneous generalizations of Bianchi type I cosmologies. Studying these equations numerically we find that in most of the cases there is a certain period of inflationary behavior for

Particle creation in the Bell - Szekeres spacetime

{k}^{2}l2

5D gravitational waves from complexified black rings

. We as well find that for

Entropy generation and inflation in collision induced pre-big-bang cosmology

{k}^{2}g2