Cosmological perturbations in generalised dark Lagrangians

TL;DR

This paper introduces a new parameterisation method for a broad class of dark energy theories, including massive gravity and elastic dark energy, enabling easier evaluation of their properties and constraints.

Contribution

The authors develop a general framework to describe second order Lagrangians depending on metric variations, extending to Lorentz violating theories and tensor-metric theories.

Findings

New constraints on Lagrangian parameters

Framework describes massive gravity and elastic dark energy

Rules out theories depending only on time metric variation

Abstract

We describe a new method to parameterise dark energy theories including massive gravity, elastic dark energy and tensor-metric theories. We first examine a framework to describe any second order Lagrangian which depends on the variation of the metric and find new constraints on the parameters. We extend the method to Lorentz violating theories which depend on the variation of the time and spatial parts of the metric separately. We show how this can describe massive gravity and elastic dark energy, while ruling out the whole class of theories where the Lagrangian depends only on the variation of the time part of the metric. We further generalise our method to tensor-metric theories, both with and without splitting the metric into time and spatial parts. Our method extends existing physics by providing a mechanism to easily evaluate large classes of dark energy theories.