Positivity Bounds on Dark Energy: When Matter Matters

TL;DR

This paper explores how incorporating matter interactions into positivity bounds refines constraints on dark energy theories and reveals potential superluminal gravitational wave propagation in certain models.

Contribution

It introduces the role of matter couplings in positivity bounds and demonstrates their impact on dark energy parameter constraints and UV completion requirements.

Findings

Matter couplings tighten dark energy constraints.

Positivity bounds imply superluminal gravitational waves in some models.

Implementing bounds as priors influences cosmological parameter estimation.

Abstract

Positivity bounds - constraints on any low-energy effective field theory imposed by the fundamental axioms of unitarity, causality and locality in the UV - have recently been used to constrain scalar-tensor theories of dark energy. However, the coupling to matter fields has so far played a limited role. We show that demanding positivity when including interactions with standard matter fields leads to further constraints on the dark energy parameter space. We demonstrate how implementing these bounds as theoretical priors affects cosmological parameter constraints and explicitly illustrate the impact on a specific Effective Field Theory for dark energy. We also show in this model that the existence of a standard UV completion requires that gravitational waves must travel superluminally on cosmological backgrounds.