Matter perturbations in Galileon cosmology

TL;DR

This paper analyzes how matter density perturbations evolve in Galileon cosmology, revealing distinctive growth patterns and gravitational potential behaviors that could differentiate it from the standard LCDM model in future observations.

Contribution

It derives full linear perturbation equations for Galileon models and demonstrates their implications for structure growth and CMB anisotropies, highlighting observable differences from LCDM.

Findings

Growth rate of matter perturbations is larger than in LCDM.

Growth index gamma is typically smaller than 0.4.

Effective gravitational potential grows during the matter to acceleration transition.

Abstract

We study the evolution of matter density perturbations in Galileon cosmology where the late-time cosmic acceleration can be realized by a field kinetic energy. We obtain full perturbation equations at linear order in the presence of five covariant Lagrangians ${cal L}_i$ ($i=1,...,5$) satisfying a Galilean symmetry in the flat space-time. The equations for a matter perturbation as well as an effective gravitational potential are derived under a quasi-static approximation on sub-horizon scales. This approximation can reproduce full numerical solutions with high accuracy for the wavelengths relevant to large-scale structures. For the model parameters constrained by the background expansion history of the Universe the growth rate of matter perturbations is larger than that in the LCDM model, with the growth index $gamma$ today typically smaller than 0.4. We also find that, even on very large scales associated with the Integrated-Sachs-Wolfe (ISW) effect in Cosmic Microwave Background (CMB) temperature anisotropies, the effective gravitational potential exhibits a temporal growth during the transition from the matter era to the epoch of cosmic acceleration. These properties are useful to distinguish the Galileon model from the LCDM in future high-precision observations.