Speed of Gravity

Low energy effective field theories are usefully constrained by positivity bounds, i.e. positive signs for EFT interactions, derived from S-matrix analyticity. For non-gravitational theories S-matrix analyticity is consistent with the requirement that fluctuations are (sub)luminal i.e. analyticity= causality. For gravitational effective field theories such as those relevant for cosmology, this connection is more subtle because of the ambiguity of which metric to use. In particular typical signs expected from analyticity imply that gravitational waves are mildly superluminal (relative to photons) in FRW spacetimes. Fortunately this effect is not in conflict with causality as I will explain. Our results have important implications for constraints on cosmological EFT model building. Based on https://arxiv.org/abs/1909.00881 and follow up work.