  The subject of this thesis is cosmological implications of string
compactifications understood in a broad sense. In the first half of the thesis,
we will begin by reviewing the four-dimensional description of the tree-level
perturbative type IIB action. We will then review a number of open questions in
cosmology and their relevance with regards to the remainder of the thesis. We
will first explore some of these cosmological questions from the perspective of
effective field theories motivated by supergravity. From the naturalness of
dark energy and how to obtain a naturally light dark energy field in terms of
the clockwork mechanism and the Dvali-Kaloper-Sorbo four-form mixing. We also
discuss the coincidence problem for dynamical models of dark energy consistent
with a quintessence field slowly rolling down a potential slope, of the type
one would expect from the asymptotics of moduli space.
  In the second half of the thesis, we introduce the effects of perturbative
and non-perturbative corrections to the tree-level type IIB action. We then
focus on obtaining a viable model of quintessence from the type IIB effective
field theory. However, we are able to show that such a model must have a
non-supersymmetric Minkowski vacuum at leading order. When we consider the
effects of quantum fluctuations during the early Universe, we see that such
models must have extremely fine-tuned initial conditions to describe a
slow-rolling scalar field at present times. We conclude that quintessence faces
more challenges than a true cosmological constant, to the point that
quintessence is very unattractive for model building modulo a ruling out of the
cosmological constant by observations. Following this line of reasoning, we
consider whether other perturbative corrections can generate de Sitter
solutions in an appropriate setting.
