The observational status of Galileon gravity after Planck

TL;DR

This study uses Planck and BAO data to constrain Galileon gravity models, finding all fit well and can be distinguished from Lambda-CDM through specific CMB lensing features and local gravity tests.

Contribution

It provides the first comprehensive constraints on all main Galileon models using the latest cosmological data, highlighting their compatibility and distinguishing features.

Findings

All Galileon models fit the data well.

Galileon models are compatible with local Hubble measurements.

Quartic and Quintic models slightly preferred over Lambda-CDM.

Abstract

We use the latest CMB data from Planck, together with BAO measurements, to constrain the full parameter space of Galileon gravity. We constrain separately the three main branches of the theory known as the Cubic, Quartic and Quintic models, and find that all yield a very good fit to these data. Unlike in $\Lambda{\rm CDM}$, the Galileon model constraints are compatible with local determinations of the Hubble parameter and predict nonzero neutrino masses at over $5\sigma$ significance. We also identify that the low-$l$ part of the CMB lensing spectrum may be able to distinguish between $\Lambda{\rm CDM}$ and Galileon models. In the Cubic model, the lensing potential deepens at late times on sub-horizon scales, which is at odds with the current observational suggestion of a positive ISW effect. Compared to $\Lambda$CDM, the Quartic and Quintic models predict less ISW power in the low-$l$ region of the CMB temperature spectrum, and as such are slightly preferred by the Planck data. We illustrate that residual local modifications to gravity in the Quartic and Quintic models may render the Cubic model as the only branch of Galileon gravity that passes Solar System tests.