  In a braneworld description of our universe, we must allow for the
possibility of having dynamical branes around the time of the big bang. Some
properties of such domain walls in motion are discussed here, for example the
ability of negative-tension domain walls to bounce off spacetime singularities
and the consequences for cosmological perturbations. In this context, we will
also review a colliding branes solution of heterotic M-theory that has been
proposed as a model for early universe cosmology.
