Challenges to the DGP model from horizon-scale growth and geometry

We conduct a Markov Chain Monte Carlo study of the Dvali-Gabadadze-Porrati 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 Dvali-Gabadadze-Porrati model is nominally a 5.3⁢𝜎 poorer fit than 𝛬⁢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⁢𝜎 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 the five-year Wilkinson Microwave Anisotropy Probe. 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.