  We conduct a Markov Chain Monte Carlo study of the Dvali-Gabadadze-Porrati
(DGP) self-accelerating braneworld scenario given the cosmic microwave
background (CMB) anisotropy, supernovae and Hubble constant data by
implementing an effective dark energy prescription for modified gravity into a
standard Einstein-Boltzmann code. We find no way to alleviate the tension
between distance measures and horizon scale growth in this model. Growth
alterations due to perturbations propagating into the bulk appear as excess CMB
anisotropy at the lowest multipoles. In a flat cosmology, the maximum
likelihood DGP model is nominally a 5.3 sigma poorer fit than Lambda CDM.
Curvature can reduce the tension between distance measures but only at the
expense of exacerbating the problem with growth leading to a 4.8 sigma result
that is dominated by the low multipole CMB temperature spectrum. While changing
the initial conditions to reduce large scale power can flatten the temperature
spectrum, this also suppresses the large angle polarization spectrum in
violation of recent results from WMAP5. The failure of this model highlights
the power of combining growth and distance measures in cosmology as a test of
gravity on the largest scales.
