STRING COSMOLOGY · Strings theory in time-dependent backgrounds and cosmology

Final Activity Report Summary - STRING COSMOLOGY (Strings theory in time-dependent backgrounds and cosmology)

The main scientific objective of this research project was to investigate the physics of the very early universe using modern techniques of string theory. We have studied the propagation of strings in time-dependent geometries to understand the behaviour of quantum gravity in the very early universe. In particular, we made progress in the understanding of how string theory may resolve several inconsistencies that are present at the cosmological singularity. We considered very simple geometries that are time-dependent but that can be constructed starting from well know and time-independent geometries. Using a technique called 'orbifolding', we constructed toy models of the cosmological singularity. Then, we studied perturbative quantum field theory and string theory in such backgrounds, concluding that instabilities associated with the singularity may be resolved in string theory. In a related line of work, we studied the decay of the open string false vacuum in the presence of an external electric field. When such external field is present, we have the well know phenomena of pair creation, which in string theory is generalised to the phenomena of brane creation. We studied this phenomena from the open string theory description of the nucleated branes. In particular, we described the brane nucleation as the decay of the open string tachyon false vacuum. When the back reaction of the electric field is taken into account, the geometry will evolve and becomes time-dependent. In fact, this phenomenon describes the decay of a quintessence field in cosmology, using the effective supergravity description of string theory. Also, it may have implication for the late time evolution of the Universe. This grant was fundamental in helping to start a new research group in string theory at the Physics Department of the Oporto University Faculty of Sciences. We have been able to invite several scientists to lecture and to give seminars in our group. We have attracted last year undergraduate students to our field by giving them junior research grants. These students are now starting post-graduate studies in our field. Finally, we could update some of our group's computational resources and we could also buy several technical books that were missing in the department's library.

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