The Universe as a Cosmic String
  The cosmology of brane induced gravity in six infinite dimensions is
investigated. It is shown that a brane with Friedmann-Robertson-Walker
symmetries necessarily acts as a source of cylindrically symmetric
gravitational waves, so called Einstein-Rosen waves. Their existence
essentially distinguishes this model from its codimension-one counterpart and
necessitates solving the nonlinear system of bulk and brane-matching equations.
A numerical analysis is performed and two qualitatively different and
dynamically separated classes of cosmologies are derived: degravitating
solutions for which the Hubble parameter settles to zero despite the presence
of a non-vanishing energy density on the brane and super-accelerating solutions
for which Hubble grows unbounded. The parameter space of both the stable and
unstable regime is derived and observational consequences are discussed: It is
argued that the degravitating regime does not allow for a phenomenologically
viable cosmology. On the other hand, the super-accelerating solutions are
potentially viable, however, their unstable behavior questions their physical
relevance.
