Quintessence, fifth forces and string theory

This thesis focuses on the study of the dynamical systems of late time cosmology including scalar field driven dark energy models. Light gravitationally coupled scalars are known to mediate fifth forces that lead to deviations of Newton's gravitational inverse square law. These bounds severely constrain dynamical dark energy models. In this thesis we explore a recently proposed mechanism, easily embedded in the moduli sector of string compactifications, that exploits field space curvature to evade fifth force bounds. This mechanism can be realised with an axion and its dilaton in the presence of a small axion-matter coupling. We perform a systematic study of the dynamics of systems and of their ultraviolet embedding into string theory.