Linear perturbations in Galileon gravity models

TL;DR

This paper investigates the effects of Galileon gravity models on cosmological perturbations, revealing distinctive signatures in the CMB, matter growth, and lensing, and assessing their viability against current and future data.

Contribution

It derives gauge-invariant perturbation equations for Galileon models and explores their observational signatures, highlighting the models' rich phenomenology and current constraints.

Findings

Galileon models modify the CMB acoustic peaks.

Galileon fields cause the gravitational potential to grow at late times.

Some Galileon models are already ruled out by existing data.

Abstract

We study the cosmology of Galileon modified gravity models in the linear perturbation regime. We derive the fully covariant and gauge invariant perturbed field equations using two different methods, which give consistent results, and solve them using a modified version of the {\tt CAMB} code. We find that, in addition to modifying the background expansion history and therefore shifting the positions of the acoustic peaks in the cosmic microwave background (CMB) power spectrum, the Galileon field can cluster strongly from early times, and causes the Weyl gravitational potential to grow, rather than decay, at late times. This leaves clear signatures in the low-$l$ CMB power spectrum through the modified integrated Sachs-Wolfe effect, strongly enhances the linear growth of matter density perturbations and makes distinctive predictions for other cosmological signals such as weak lensing and the power spectrum of density fluctuations. The quasi-static approximation is shown to work quite well from small to the near-horizon scales. We demonstrate that Galileon models display a rich phenomenology due to the large parameter space and the sensitive dependence of the model predictions on the Galileon parameters. Our results show that some Galileon models are already ruled out by present data and that future higher significance galaxy clustering, ISW and lensing measurements will place strong constraints on Galileon gravity.