Analytic Description of DGP Perturbations on All Scales
  We develop analytic solutions for the linear evolution of metric
perturbations in the DGP braneworld modified gravity scenario including
near-horizon and superhorizon modes where solutions in the bulk are required.
These solutions apply to both the self-accelerating and normal branch and
elucidate the nature of coordinate singularities and initial data in the bulk
as well as their effect on perturbation evolution on the brane. Even on
superhorizon scales, the evolution of metric perturbations is no longer
necessarily scale free due to multiple resonances in the bulk. Based on these
analytic solutions, we devise convenient fitting functions for the evolution
that bridge the various spatial and temporal regimes. Compared with a direct
numerical integration of the bulk equations, the fits are accurate at the
percent level and are sufficient for current and upcoming observational tests.
