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Abstract: We study the cosmology of a galileon scalar-tensor theory, obtained bycovariantizing the decoupling lagrangian of the Dvali-Gabadadze-Poratti DGPmodel. Despite being local in 3+1 dimensions, the resulting cosmologicalevolution is remarkably similar to that of the full 4+1-dimensional DGPframework, both for the expansion history and the evolution of densityperturbations. As in the DGP model, the covariant galileon theory yields twobranches of solutions, depending on the sign of the galileon velocity.Perturbations are stable on one branch and ghost-like on the other. Aninteresting effect uncovered in our analysis is a cosmological version of theVainshtein screening mechanism: at early times, the galileon dynamics aredominated by self-interaction terms, resulting in its energy density beingsuppressed compared to matter or radiation; once the matter density hasredshifted sufficiently, the galileon becomes an important component of theenergy density and contributes to dark energy. We estimate conservatively thatthe resulting expansion history is consistent with the observed late-timecosmology, provided that the scale of modification satisfies r c > 15 Gpc.

Autor: Nathan Chow, Justin Khoury


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