Aspects of dark bubble cosmology from string theory

This work investigates thoroughly a brane world scenario called "Dark Bubble Cosmology", which differs in crucial aspects from other proposals in string phenomenology: our four-dimensional universe rides an expanding bubble whose embedding naturally induces a positive cosmological constant. This model avoids the Swampland constraints of constructing fundamental de Sitter vacua based on pure compactifications of String Theory and, at the same time, provides a concrete realization of a brane world with an asymmetric inside-outside construction. The bubble represents a Coleman-de Luccia or Brown-Teitelboim instanton which nucleates in an unstable five-dimensional Anti de Sitter spacetime. This proposal therefore involves inducing four-dimensional gravity on a brane which mediates the decay from a non-supersymmetric false AdS-5 vacuum to a lower energy vacuum. In this thesis, brane world constructions based both on Type IIB and 0’B string theories are examined and a general mechanism to embed electromagnetic gauge fields living on the brane is found. The latter requires a non-trivial interplay between gauge fields on the brane and stringy fields in the bulk, and can be hopefully generalized to include non-abelian gauge fields, with the perspective of embedding the full Standard Model of Particle Physics in the Dark Bubble scenario. In the context of the (non-supersymmetric) Type 0'B string theory, the Dark Bubble model yields precise predictions for the evolution of the cosmological constant Λ and of the scale factor a(t). However, it turns out this setup is incompatible with experimental observations and has to be discarded. A Swampland approach is also justified, especially in connecting the emerging scales with hypotheses on scale separation and the dark dimension.