A Simplified Approach to General Scalar-Tensor Theories

TL;DR

This paper simplifies the complex Horndeski scalar-tensor theories by reducing the number of functions needed to describe linear perturbations around a cosmological background, aiding in constraining dark energy models.

Contribution

It demonstrates that Horndeski's theory can be characterized by only six functions of time for linear perturbations, reducing the parameter space for easier analysis and constraints.

Findings

Six functions of time fully specify linear perturbations.

Four functions suffice in the quasistatic approximation.

Parameter space reduction facilitates observational constraints.

Abstract

The most general covariant action describing gravity coupled to a scalar field with only second order equations of motion, Horndeski's theory (also known as "Generalized Galileons"), provides an all-encompassing model in which single scalar dark energy models may be constrained. However, the generality of the model makes it cumbersome to manipulate. In this paper, we demonstrate that when considering linear perturbations about a Friedmann-Robertson-Walker background, the theory is completely specified by only six functions of time, two of which are constrained by the background evolution. We utilise the ideas of the Effective Field Theory of Inflation/Dark Energy to explicitly construct these six functions of time in terms of the free functions appearing in Horndeski's theory. These results are used to investigate the behavior of the theory in the quasistatic approximation. We find that only four functions of time are required to completely specify the linear behavior of the theory in this limit, which can further be reduced if the background evolution is fixed. This presents a significantly reduced parameter space from the original presentation of Horndeski's theory, giving hope to the possibility of constraining the parameter space. This work provides a cross-check for previous work on linear perturbations in this theory, and also generalizes it to include spatial curvature.