Galileon Cosmology

TL;DR

This paper explores a covariant galileon scalar-tensor theory's cosmological implications, showing its evolution closely mirrors the DGP model, with a screening mechanism that suppresses early-time effects and contributes to late-time acceleration.

Contribution

It introduces a covariant galileon model that reproduces DGP-like cosmology and uncovers a cosmological Vainshtein screening mechanism affecting dark energy behavior.

Findings

The model's expansion history aligns with observations if r_c > 15 Gpc.

Perturbations are stable on one solution branch, ghost-like on the other.

A screening mechanism suppresses galileon effects at early times.

Abstract

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.