  The DGP brane-world model provides an alternative to the standard LCDM
cosmology, in which the late universe accelerates due to a modification of
gravity rather than vacuum energy. The cosmological constant $\Lambda$ in LCDM
is replaced by a single parameter, the crossover scale $r_c$, in DGP. The
Supernova redshift observations can be fitted by both models, with $\Lambda\sim
H_0^2$ and $r_c \sim H_0^{-1}$. This degeneracy is broken by structure
formation, which is suppressed in different ways in the two models. There is
some confusion in the literature about how the standard linear growth factor is
modified in DGP. While the luminosity distance can be computed purely from the
modified 4-dimensional Friedman equation, the evolution of density
perturbations requires an analysis of the 5-dimensional gravitational field. We
show that if the 5-dimensional effects are inappropriately neglected, then the
4-dimensional Bianchi identities are violated and the computed growth factor is
incorrect. By using the 5-dimensional equations, we derive the correct growth
factor.
