Cosmology on a cosmic ring
  We derive the modified Friedmann equations for a generalization of the
Dvali-Gabadadze-Porrati (DGP) model in which the brane has one additional
compact dimension. The main new feature is the emission of gravitational waves
into the bulk. We study two classes of solutions: First, if the compact
dimension is stabilized, the waves vanish and one exactly recovers DGP
cosmology. However, a stabilization by means of physical matter is not possible
for a tension-dominated brane, thus implying a late time modification of 4D
cosmology different from DGP. Second, for a freely expanding compact direction,
we find exact attractor solutions with zero 4D Hubble parameter despite the
presence of a 4D cosmological constant. The model hence constitutes an explicit
example of dynamical degravitation at the full non-linear level. Without
stabilization, however, there is no 4D regime and the model is ruled out
observationally, as we demonstrate explicitly by comparing to supernova data.
