Superluminality sign constraints

Our friends have finished their paper about the constraints on the signs of various terms in the effective action that are required by causality, i.e. by the fact that the speed of light can't be exceeded, not even at non-trivial backgrounds.

These superluminality constraints are classically contained in the energy conditions, especially the dominant energy condition, but these Gentlemen start from a very satisfactory and fundamental principle, namely the condition of unitarity.