The nature of the fuel that drives today ’ s cosmic acceleration is an open and tantalizing mystery .
The braneworld theory of Dvali , Gabadadze , and Porrati ( DGP ) provides a context where late-time acceleration is driven not by some energy-momentum component ( dark energy ) , but rather is the manifestation of the excruciatingly slow leakage of gravity off our four-dimensional world into an extra dimension .
At the same time , DGP gravity alters the gravitational force law in a specific and dramatic way at cosmologically accessible scales .
We derive the DGP gravitational force law in a cosmological setting for spherical perturbations at subhorizon scales and compute the growth of large-scale structure .
We find that a residual repulsive force at large distances gives rise to a suppression of the growth of density and velocity perturbations .
Explaining the cosmic acceleration in this framework leads to a present day fluctuation power spectrum normalization \sigma _ { 8 } \leq 0.8 at about the two-sigma level , in contrast with observations .
We discuss further theoretical work necessary to go beyond our approximations to confirm these results .