  In this thesis we mainly explore the evolution of both scalar and tensor
cosmological perturbations in a Randall-Sundrum braneworld having an arbitrary
expansion history. We adopt a four-dimensional perspective in which the
localized degrees of freedom on the brane are regarded as an open quantum
system coupled to the large environment composed of the Anti-de Sitter ($AdS$)
bulk gravitons. Due to the non-uniform expansion of the universe, the brane
degrees of freedom and the bulk degrees of freedom interact as they propagate
forward in time, leading to an effective dissipation as well as a nonlocality
from the four-dimensional point of view of an observer on the brane. Using both
the "bare" retarded propagator on the brane and the retarded propagator in the
bulk, we compute the effective propagator on the brane for the "dressed" brane
modes by resumming the bulk backreaction effects at all order in the brane-bulk
coupling. Dissipation and nonlocality are encoded into the effective brane
propagator. We find the dissipation rates of various matter perturbations on
the brane as well as of the graviton bound state. Next we present an explicit
calculation of the covariant retarded Green function for the graviton in $AdS$
space. In the last part, we investigate the reconstruction of CMB lensing in
standard cosmology. We construct a statistical estimator of the gravitational
lensing dilatation and shear fields directly in real space. The real space
method developed here is useful for analysing realistic CMB maps containing a
galactic cut and possibly numerous small excisions to exclude point sources
that cannot be reliably subtracted.

High Energy Physics - Theory