PhD defence: Non-Geometric Compactifications and String Cosmology in the Swampland

PLEASE NOTE: If a candidate gives a layman's talk, the livestream will start fifteen minutes earlier.

Our universe is described microscopically by particle physics and macroscopically by cosmology. A unified account is expected from a quantum theory of gravity, with string theory a leading candidate.

String theory is formulated in ten dimensions. To obtain a four-dimensional universe, six extra dimensions are compactified. The enormous variety of such compactifications generates a vast string “Landscape” of effective theories. By contrast, seemingly consistent theories that cannot arise from string theory lie in the “Swampland”, and several conjectural criteria aim to distinguish the two sets.

This thesis investigates dark energy and the structure of the extra dimensions and tests Swampland conjectures by analysing asymptotic regions of moduli space, the parameter space encoding the geometry of the extra dimensions.

First, assuming Swampland criteria, I show that accelerated expansion of the universe does not occur in asymptotic regions of moduli space within string theory. Consequently, our universe must reside in the interior of moduli space. Second, I develop a string-theoretic realisation of the recently proposed Dark Dimension scenario, motivated by Swampland ideas, in which one extra dimension is much larger than the others. This is achieved using non-geometric compactifications, whose extra-dimensional structure transcends conventional differential geometry. Third, I analyse the particle spectrum and quantum corrections of a distinct class of non-geometric compactifications based on asymmetric orbifolds and test Swampland criteria in this previously unexplored setting.