Evolution of linear cosmological perturbations and its observational implications in Galileon-type modified gravity

TL;DR

This paper analyzes linear cosmological perturbations in Galileon modified gravity, revealing early-time instabilities and discussing observational signatures like ISW-LSS anticorrelation, with implications for cosmic acceleration models.

Contribution

It provides a fully relativistic analysis of perturbations in Galileon cosmology, highlighting stability issues and observational predictions distinct from standard models.

Findings

Existence of super-horizon growing modes indicating potential instability.

Late-time perturbation evolution can produce observable ISW-LSS anticorrelations.

Fine-tuning of initial conditions is necessary for stable perturbation evolution.

Abstract

A scalar-tensor theory of gravity can be made not only to account for the current cosmic acceleration, but also to satisfy solar-system and laboratory constraints, by introducing a non-linear derivative interaction for the scalar field. Such an additional scalar degree of freedom is called "Galileon". The basic idea is inspired by the DGP braneworld, but one can construct a ghost-free model that admits a self-accelerating solution. We perform a fully relativistic analysis of linear perturbations in Galileon cosmology. Although the Galileon model can mimic the background evolution of standard $\Lambda$CDM cosmology, the behavior of perturbation is quite different. It is shown that there exists a super-horizon growing mode in the metric and Galileon perturbations at early times, suggesting that the background is unstable. A fine-tuning of the initial condition for the Galileon fluctuation is thus required in order to promote a desirable evolution of perturbations at early times. Assuming the safe initial condition, we then compute the late-time evolution of perturbations and discuss observational implications in Galileon cosmology. In particular, we find anticorrelations in the cross-correlation of the integrated Sachs-Wolfe effect and large scale structure, similar to the normal branch of the DGP model.