Galileon Cosmology
  We study the cosmology of a galileon scalar-tensor theory, obtained by
covariantizing the decoupling lagrangian of the Dvali-Gabadadze-Poratti (DGP)
model. Despite being local in 3+1 dimensions, the resulting cosmological
evolution is remarkably similar to that of the full 4+1-dimensional DGP
framework, both for the expansion history and the evolution of density
perturbations. As in the DGP model, the covariant galileon theory yields two
branches of solutions, depending on the sign of the galileon velocity.
Perturbations are stable on one branch and ghost-like on the other. An
interesting effect uncovered in our analysis is a cosmological version of the
Vainshtein screening mechanism: at early times, the galileon dynamics are
dominated by self-interaction terms, resulting in its energy density being
suppressed compared to matter or radiation; once the matter density has
redshifted sufficiently, the galileon becomes an important component of the
energy density and contributes to dark energy. We estimate conservatively that
the resulting expansion history is consistent with the observed late-time
cosmology, provided that the scale of modification satisfies r_c > 15 Gpc.
