31.12.05 - Topological defects and gravity in theories with extra dimensions. Thesis EPFL, n° 3088 (2004). Dir.: Prof. M. Chapochnikov.

For fundamental contributions to the theory of extra dimensions and brane models.

Topological defects and gravity in theories with extra dimensions

Recent proposals of large and infinite extra dimensions triggered a strong research activity in theories in which our universe is considered as a sub-manifold of some higher-dimensional space-time, a so-called $3$-brane. In this context, it is generally assumed that some mechanism is at work which binds Standard Model particles to the $3$-brane, an effect often referred to as the localization of matter on the brane. Gravity, however, is allowed to propagate in general also in the extra dimensions. As demonstrated by Randall and Sundrum in 1999,it is also possible to localize gravity itself on a $3$-brane. In the setup they proposed, the $3$-brane is realized as a singular domain wall separating two patches of 5-dimensional anti-de-Sitter ($AdS_5$) space-time.
The potential between two test masses on the brane at distances larger than the $AdS_5$-radius was shown to be the usual $4$-dimensional Newtonian $1/r$ potential with strongly suppressed corrections.

The model of Randall and Sundrum, usually referred to as the Randall-Sundrum II setup, constitutes the center of interest for this thesis. The main goal of this work is to find possible generalizations to higher dimensions of the simple setup considered by Randall and Sundrum.
One of the motivations for such a generalization is that a realistic theory should possibly be able to explain the chiral nature of $4$-dimensional fermions on the brane. One way to explain chiral fermions from higher dimensions is to consider $3$-branes identified with the cores of topological defects located in a higher-dimensional transverse space. Naturally a richer topological structure of the field configuration in transverse space provides the possibility of a more realistic spectrum of chiral fermions localized on the $3$-brane.

After two introductory chapters on extra dimensions and non-factorizable geometries which are relevant for the Randall-Sundrum II model, we briefly discuss basics of topological defects in the following third chapter. In the rest of the third chapter we consider various solutions to higher-dimensional
Einstein equations coupled to a series of physically different sources and discuss their properties of localization of gravity. Due to their asymptotic nature, these solutions are only valid far from the cores of the defects in transverse space. Therefore, it seems reasonable to complement the considerations by presenting a particular numerical example of a solution to the Einstein equations coupled to a set of scalar and gauge fields: this solution describes a $3$-brane realized as a 't~Hooft-Polyakov monopole residing in the $3$-dimensional transverse space of a $7$-dimensional space-time.

The last chapter of this work is dedicated to the study of a modification of the original Randall-Sundrum II model of another kind. The motivation is given by the geodesic incompleteness of the latter scenario with respect to time-like and light-like geodesics. We will describe a model which resembles the Randall-Sundrum II model with respect to its properties of gravity localization but with the advantage that the underlying space-time manifold is geodesically complete. Parts of the calculations related to the properties of gravity at low energies in this model are rather technical in nature and we therefore preferred to assemble them in several appendices. We finally add some concluding remarks and discuss possible further developments.