Positivity bounds on dark energy: when matter matters

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