Do current cosmological observations rule out all Covariant Galileons?

TL;DR

This work tests Covariant Galileon gravity against an up-to-date cosmological data set that includes Planck CMB, BAO, local $H_0$, SN, and KiDS weak lensing, exploring three CG branches (G3, G4, G5) and three neutrino-mass hierarchies. Using EFTCAMB, CG is mapped to EFT functions with explicit background-perturbation relations, and priors are placed on $(c_3,\xi)$ and $\Sigma m_\nu$; two data sets (Planck+BAO and Planck+BAO+$H_0$+WL+SN) are employed with WL cut to linear scales. The results show that a nonzero neutrino mass improves fits but differences among hierarchies are negligible; more crucially, Bayesian evidence strongly disfavors all CG models relative to $\Lambda$CDM, with the Quartic and Quintic branches further constrained by GW170817. The findings imply that cosmological data, including weak lensing, can decisively rule out the full CG class as a viable alternative to $\Lambda$CDM for dark energy or modified gravity.

Abstract

We revisit the cosmology of Covariant Galileon gravity in view of the most recent cosmological data sets, including weak lensing. As a higher derivative theory, Covariant Galileon models do not have a $Λ$CDM limit and predict a very different structure formation pattern compared with the standard $Λ$CDM scenario. Previous cosmological analyses suggest that this model is marginally disfavoured, yet can not be completely ruled out. In this work we use a more recent and extended combination of data, and we allow for more freedom in the cosmology, by including a massive neutrino sector with three different mass hierarchies. We use the Planck measurements of Cosmic Microwave Background temperature and polarization; Baryonic Acoustic Oscillations measurements by BOSS DR12; local measurements of $H_0$; the joint light-curve analysis supernovae sample; and, for the first time, weak gravitational lensing from the KiDS collaboration. We find, that in order to provide a reasonable fit, a non-zero neutrino mass is indeed necessary, but we do not report any sizable difference among the three neutrino hierarchies. Finally, the comparison of the Bayesian Evidence to the $Λ$CDM one shows that in all the cases considered, Covariant Galileon models are statistically ruled out by cosmological data.

Figures (5)

• Figure 1: The joint marginalized posterior of $\Lambda$CDM runs with PBHWS data set. The lines correspond to the 68% C.L. and the 95% C.L. regions. Different colours correspond to different neutrino scenarios as stated in the legend.
• Figure 2: The joint marginalized posterior of $G_3$ runs with the PBHWS data set. The lines correspond to the 68% C.L. and the 95% C.L. regions. Different colours correspond to different neutrino scenarios as stated in the legend.
• Figure 3: The joint marginalized posterior of $G_4$ runs with the PBHWS data set. The lines correspond to the 68% C.L. and the 95% C.L. regions. Different colours correspond to different neutrino scenarios as stated in the legend.
• Figure 4: The joint marginalized posterior of $G_5$ runs with the PBHWS data set. The lines correspond to the 68% C.L. and the 95% C.L. regions. Different colours correspond to different neutrino scenarios as stated in the legend.
• Figure 5: Deviation in the CMB TT power spectra in units of TT variance, $\sigma_\ell = \sqrt{2/(2 \ell+1)} C_\ell^{\Lambda {\rm CDM}}$, for best fit parameters for PB (top) and PBHWS (bottom), computed with respect to $\Lambda$CDM.