On Causality Conditions in de Sitter Spacetime

We carefully consider the Shapiro time delay due to black holes and shockwaves in de Sitter spacetime and study the implications for causality. We discuss how causality conditions of AdS and ﬂat spacetime can be applied in de Sitter spacetime, using spatial shifts measured on the boundary to deﬁne “fastest null geodesics” and taking into account the “stretching” of the de Sitter Penrose diagram. We consider the propagation of a massless spin-1 ﬁeld with an RF F coupling in a de Sitter shockwave background as an illustrative example. We also brieﬂy discuss connections to the ANEC.