We study the connection between the early and late accelerated expansion of the universe and string theory. In Part I of this thesis, the observational degeneracy between single field models of inflation with canonical kinetic terms and non-canonical kinetic terms, in particular string theory inspired models, is discussed. The 2-point function observables of a given non-canonical theory and its canonical transform that is obtained by matching the inflationary trajectories in phase space are

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... d to match in the case of Dirac-Born-Infeld (DBI) inflation. At the level of the 3-point function observables (non-Gaussianities), we find degeneracy between non-canonical inflation and canonical inflation with a potential that includes a sum of modulated terms. In Part II, we present explicit examples for de Sitter vacua in type IIB string theory. After deriving a sufficient condition for de Sitter vacua in the K\"ahler uplifting scenario, we show that a globally consistent de Sitter model can be realized on a certain Calabi-Yau manifold. All geometric moduli are stabilized and all known consistency constraints are fulfilled. The complex structure moduli stabilization by fluxes is studied explicitly for a small number of cycles. Extrapolating to a larger number of flux carrying cycles, we verify statistical studies in the literature which show that, in principle, the string landscape can account for a universe with an extremely small cosmological constant.