On String Vacua without Supersymmetry: brane dynamics, bubbles and holography

In this thesis we investigate some aspects of the dramatic consequences of supersymmetry breaking on string vacua. In particular, we focus on the issue of vacuum stability in ten-dimensional string models with broken, or without, supersymmetry, whose perturbative spectra are free of tachyons. After formulating the models at stake in Chapter 2, we introduce their low-energy effective description in Chapter 3, presenting a number of vacuum solutions to the classical equations of motion. In Chapter 4 we analyze their classical stability, studying linearized field fluctuations, and in Chapter 5 we turn to the issue of quantum stability. In Chapter 6 we frame the resulting instabilities in terms of brane dynamics, examining brane interactions and back-reacted geometries. In Chapter 7 we propose a holographic correspondence connecting bulk instabilities with dual renormalization group flows, and we explore a potentially concrete scenario involving world-volume gauge theories. Finally, in Chapter 8 we turn to cosmology, deriving generalized no-go results for warped flux compactifications and concocting a brane-world scenario along the lines of a recent proposal, providing a string-theoretic embedding of constructions of this type. In Chapter 9 we provide a summary and collect some concluding remarks.