Ekpyrotic loop quantum cosmology
  We consider the ekpyrotic paradigm in the context of loop quantum cosmology.
In loop quantum cosmology the classical big-bang singularity is resolved due to
quantum gravity effects, and so the contracting ekpyrotic branch of the
universe and its later expanding phase are connected by a smooth bounce. Thus,
it is possible to explicitly determine the evolution of scalar perturbations,
from the contracting ekpyrotic phase through the bounce and to the post-bounce
expanding epoch. The possibilities of having either one or two scalar fields
have been suggested for the ekpyrotic universe, and both cases will be
considered here. In the case of a single scalar field, the constant mode of the
curvature perturbations after the bounce is found to have a blue spectrum. On
the other hand, for the two scalar field ekpyrotic model where scale-invariant
entropy perturbations source additional terms in the curvature perturbations,
the power spectrum in the post-bounce expanding cosmology is shown to be nearly
scale-invariant and so agrees with observations.
