Challenges to the DGP Model from Horizon-Scale Growth and Geometry

TL;DR

This study tests the DGP braneworld gravity model against cosmological data, finding significant tensions with observations that challenge its viability as an alternative to dark energy.

Contribution

The paper provides a comprehensive MCMC analysis of the DGP model using multiple cosmological datasets, revealing its inability to reconcile growth and distance measures.

Findings

DGP model fits are significantly worse than Lambda CDM.

Growth and distance measure tensions cannot be alleviated simultaneously.

Modifications to initial conditions conflict with polarization data.

Abstract

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.