String Theory and its Applications in Cosmology and Particle Physics

Abstract

Finding experimental signatures of string theory is a central problem in modern physics. Recently, this problem has been revived in the context of the Swampland program. The purpose of this program is to identify the set of low energy effective theories that do not arise in string theory, called the Swampland. With this knowledge, string theory can be used to make predictions about low energy phenomena that are potentially accessible to near-future experiments. In this thesis, we explore the string Swampland and its observational implications for astroparticle physics. First, we provide evidence for the absence of de Sitter vacua in string theory and formulate the de Sitter Conjecture (dSC). Applying this to our universe, we find generic tensions with the simplest inflationary models and predictions for deviations of the dark energy equation of state from its cosmological constant value. Next, motivated by the dSC and dualities of string theory, we propose an early phase in our universe where our degrees of freedom are described by a topological theory. Finally, we use the Weak Gravity Conjecture (WGC) and Festina Lente (FL) to explore possibilities for physics of the dark sector and certain inflationary models. In all these cases, we show there is an interesting interplay between data and ideas from string theory demonstrating that the theoretical and observational efforts are highly complementary.