Gravity with Perturbative Constraints: Dark Energy Without New Degrees of Freedom

TL;DR

This paper introduces a framework to analyze dark energy and modifications to gravity without requiring new dynamical degrees of freedom, using perturbative constraints and current cosmological observations.

Contribution

It proposes a novel perturbative approach to test for non-dynamical gravitational degrees of freedom in cosmology, challenging the assumption that modifications imply new fields.

Findings

Current observations support the perturbative validity of the framework.

The universe can accommodate non-dynamical gravitational degrees of freedom at cosmological scales.

Abstract

Major observational efforts in the coming decade are designed to probe the equation of state of dark energy. Measuring a deviation of the equation-of-state parameter w from -1 would indicate a dark energy that cannot be represented solely by a cosmological constant. While it is commonly assumed that any implied modification to the LambdaCDM model amounts to the addition of new dynamical fields, we propose here a framework for investigating whether or not such new fields are required when cosmological observations are combined with a set of minimal assumptions about the nature of gravitational physics. In our approach, we treat the additional degrees of freedom as perturbatively constrained and calculate a number of observable quantities, such as the Hubble expansion rate and the cosmic acceleration, for a homogeneous Universe. We show that current observations place our Universe within the perturbative validity of our framework and allow for the presence of non-dynamical gravitational degrees of freedom at cosmological scales.