Constraints on modified gravity from Planck 2015: when the health of your theory makes the difference

TL;DR

This study uses Planck 2015 data within the effective field theory framework to constrain Horndeski class dark energy models, finding that standard ΛCDM remains the best fit and that many modified theories are ruled out by stability and speed constraints.

Contribution

It applies the EFT of dark energy formalism to impose stability and speed constraints on Horndeski models using Planck data, narrowing the viable parameter space.

Findings

No alternative theory outperforms ΛCDM with CMB data.

Stability and speed constraints significantly restrict model parameters.

Healthy dark energy models cannot replicate certain phenomenological features of gravity.

Abstract

We use the effective field theory of dark energy (EFT of DE) formalism to constrain dark energy models belonging to the Horndeski class with the recent Planck 2015 CMB data. The space of theories is spanned by a certain number of parameters determining the linear cosmological perturbations, while the expansion history is set to that of a standard $\Lambda$CDM model. We always demand that the theories be free of fatal instabilities. Additionally, we consider two optional conditions, namely that scalar and tensor perturbations propagate with subliminal speed. Such criteria severely restrict the allowed parameter space and are thus very effective in shaping the posteriors. As a result, we confirm that no theory performs better than $\Lambda$CDM when CMB data alone are analysed. Indeed, the healthy dark energy models considered here are not able to reproduce those phenomenological behaviours of the effective Newton constant and gravitational slip parameters that, according to previous studies, best fit the data.