  In this thesis we study the dynamics of higher-dimensional gravity in a
universe emerging from a brane collision. We develop a set of powerful analytic
methods which, we believe, render braneworld cosmological perturbation theory
solvable. Our particular concern is to determine the extent to which the
four-dimensional effective theory accurately captures the higher-dimensional
dynamics about the cosmic singularity. We begin with an overview of the
generation of primordial density perturbations, both in inflation and in the
ekpyrotic mechanism, followed by an introduction to braneworld cosmology. A
simple derivation of the low-energy effective action for braneworlds is
presented, highlighting the role of conformal invariance. We then solve for
cosmological perturbations in a five-dimensional background consisting of two
separating or colliding boundary branes, as an expansion in the collision
rapidity. We show that the four-dimensional effective description fails at
first non-trivial order in this expansion, and highlight the implications for
cosmology. Finally, we extend our methods to find a colliding brane solution of
heterotic M-theory.
