A No-Go Theorem in String Cosmology

A no-go theorem pertaining to the graceful exit problem in Pre-Big-Bang inflation is reviewed. It is shown that dilaton self-interactions and string fluid sources fail to facilitate branch changing necessary to avoid singularities. A comment on the failure of the higher genus corrections to induce graceful exit is also included.Comment: 7 pages, latex, no figures, contributed talk at the 6th Canadian Conference on General Relativity and Relativistic Astrophysics, University of New Brunswick, Fredericton, NB, May 24-2

A Closed Bianchi I Universe in String Theory

A special Bianchi I universe is constructed in $~4D~$ string theory. Geometrically it represents a $~3D~$ anti-de-Sitter space crossed with a flat direction whereas in terms of an associated conformal field theory it is an extremal case of a gauged WZWN theory with target the coset $~SU(1,1) \times R^2 / R~$. Some of its properties are discussed.Comment: 9 pages plain TeX (More typos corrected, a reference added in Ref. [3], and a new equation (Eq. 7) and some clarifications included

Topological Mass Generation in Three-Dimensional String Theory

The effective action of string theory in three dimensions is investigated, incorporating the Lorentz and gauge Chern-Simons terms in the definition of the Kalb-Ramond axion field strength. Since in three dimensions any three-form is trivial, the action can be reformulated by properly integrating the axion out. The circumstances under which it can be recast in form of topologically massive gravity coupled to a topologically massive gauge theory are pointed out. Finally, the strong coupling limit of the resulting action is inspected, with the focus on the roles played by the axion and dilaton fields.Comment: 11 pages LaTeX, Alberta preprint Alberta-Thy-46-9

Entropy Count for Extremal Three-Dimensional Black Strings

We compute the entropy of extremal black strings in three dimensions, using Strominger's approach to relate the Anti-de-Sitter near-horizon geometry and the conformal field theory at the asymptotic infinity of this geometry. The result is identical to the geometric Bekenstein-Hawking entropy. We further discuss an embedding of three-dimensional black strings in $N=1 D=10$ supergravity and demonstrate that the extremal strings preserve 1/4 of supersymmetries.Comment: 14 pages, latex, 1 .ps figure, a comment and some references added, as accepted for publication in Phys. Lett.