Structure formation in the Dvali–Gabadadze–Porrati cosmological model

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 in LCDM is replaced by a single parameter, the crossover scale rc, in DGP.The Supernova redshift observations can be fitted by both models, with H 2 0 and rc H -1 0 .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.