Electromagnetic Field in Some Anisotropic Stiff Fluid Universes

The electromagnetic field is studied in a family of exact solutions of the Einstein equations whose material content is a perfect fluid with stiff equation of state (p = $\epsilon$ ). The field equations are solved exactly for several members of the family.Comment: 6 pages, REVTEX,no figure

Cosmic Strings from N=2, D=5 Supergravity

Exact solutions of N=2 supergravity in five dimensions are found in the metric with cylindrical symmetry, a particular case corresponds to the exterior of a cosmic string.Comment: 5 pages, plain LATEX,no figure

Effects of Bulk Viscosity on Cosmological Evolution

The effect of bulk viscisity on the evolution of the homogeneous and isotropic cosmological models is considered. Solutions are found, with a barotropic equation of state, and a viscosity coefficient that is proportional to a power of the energy density of the universe. For flat space, power law expansions, related to extended inflation are found as well as exponential solutions, related to old inflation; also a solution with expansion that is an exponential of an exponential of the time is found.Comment: 8 pages, latex, no figure

Bianchi Type I Cosmology in N=2, D=5 Supergravity

The dynamics and evolution of Bianchi type I space-times is considered in the framework of the four-dimensional truncation of a reduced theory obtained from the N=2,D=5 supergravity. The general solution of the gravitational field equations can be represented in an exact parametric form. All solutions have a singular behavior at the initial/final moment, except when the space-time geometry reduces to the isotropic flat case. Generically the obtained cosmological models describe an anisotropic, expanding or collapsing, singular Universe with a non-inflationary evolution for all times.Comment: revised version to appear in PR

Cylindrically symmetric, static strings with a cosmological constant in Brans-Dicke theory

The static, cylindrically symmetric vacuum solutions with a cosmological constant in the framework of the Brans-Dicke theory are investigated. Some of these solutions admitting Lorentz boost invariance along the symmetry axis correspond to local, straight cosmic strings with a cosmological constant. Some physical properties of such solutions are studied. These strings apply attractive or repulsive forces on the test particles. A smooth matching is also performed with a recently introduced interior thick string solution with a cosmological constant.Comment: 8 pages, Revtex; Published versio