Stringy Models of Modified Gravity: Space-time defects and Structure
  Formation
  Starting from microscopic models of space-time foam, based on brane universes
propagating in bulk space-times populated by D0-brane defects ("D-particles"),
we arrive at effective actions used by a low-energy observer on the brane world
to describe his/her observations of the Universe. These actions include, apart
from the metric tensor field, also scalar (dilaton) and vector fields, the
latter describing the interactions of low-energy matter on the brane world with
the recoiling point-like space-time defect (D-particle). The vector field is
proportional to the recoil velocity of the D-particle and as such it satisfies
a certain constraint. The vector breaks locally Lorentz invariance, which
however is assumed to be conserved on average in a space-time foam situation,
involving the interaction of matter with populations of D-particle defects. In
this paper we demonstrate that, already at the end of the radiation era, the
(constrained) vector field associated with the recoil of the defects provides
the seeds for a growing mode in the evolution of the Universe. Such a growing
mode survives during the matter dominated era, provided the variance of the
D-particle recoil velocities on the brane is larger than a critical value.
