  The braneworld model of Dvali, Gabadadze and Porrati (DGP) provides an
intriguing modification of gravity at large distances and late times. By
embedding a three-brane in an uncompactified extra dimension with separate
Einstein-Hilbert terms for both brane and bulk, the DGP model allows for an
accelerating universe at late times even in the absence of an explicit vacuum
energy. We examine the evolution of cosmological perturbations on large scales
in this theory. At late times, perturbations enter a DGP regime in which the
effective value of Newton's constant increases as the background density
diminishes. This leads to a suppression of the integrated Sachs-Wolfe effect,
bringing DGP gravity into slightly better agreement with WMAP data than
conventional LCDM. However, we find that this is not enough to compensate for
the significantly worse fit to supernova data and the distance to the
last-scattering surface in the pure DGP model. LCDM is, therefore, a better
fit.
