This series consists of talks in the areas of Cosmology, Gravitation and Particle Physics.
From the Quantum Field Theory point of view, matter and gauge fields are generally expected to be localised around branes (topological defects) occurring in extra dimensions. I will discuss a simple scenario where, by starting with a five dimensional SU(3) gauge theory, we end up with several 4-D parallel braneworlds with localised 'chiral' fermions and gauge fields to them. I will show that it is possible to reproduce the electroweak model confined to a single brane, allowing a simple and geometrical approach to the hierarchy problem.
Cosmic strings are a generic by-product of string theory models of the inflationary epoch. These new cosmic "superstrings," as they are called, are distinct from the grand unified strings once thought to generate large scale structure. I will discuss what limits the WMAP and SDSS data have already placed on the properties of networks of cosmic strings, as well as avenues for their direct detection.
We study the generation of cosmological perturbations during the Hagedorn phase of string gas cosmology. Using tools of string thermodynamics we provide indications that it may be possible to obtain a nearly scale-invariant spectrum of cosmological fluctuations on scales which are of cosmological interest today. In our cosmological scenario, the early Hagedorn phase of string gas cosmology goes over smoothly into the radiation-dominated phase of standard cosmology, without having a period of cosmological inflation.
A definite prediction of string theory is the existence of a scalar field, the dilaton. The presence of the dilaton generally leads to strong violations of the equivalence principle and thus describe a kind of gravitational force radically different from what we experience. String loop corrections, however, may render phenomenologically acceptable the region of the theory characterized by large values of the dilaton field i.e. the region with a strong tree level-coupling.
The theory of cosmological perturbations provides a bridge between theoretical models of the early universe (often motivated by string theory) and astrophysical observation, e.g of the CMBR. Since extra dimensions are pivotal to string theory, the known lore of perturbation theory needs to be adjusted accordingly. After introducing the needed formalism, I will illustrate its use on an example within the framework of String Gas Cosmology