A physical explanation for dark energy Λ is proposed by revisiting the subject of causality in cosmology, especially with regard to duality. Duality is the observation that local regions are contracting, even as the cosmos is expanding. “Local” means distances less than the center-to-center spacing of galaxy clusters, Ri; “non-local” means distances greater than Ri. It is argued that the Friedmann solution is causally incomplete by two parameters: Ri, denoting “inter-cluster” distance (twice the zero gravity radius RZG); and energy coupling η (0 < η < 1). Both factors--energy coupling and the distance across which it is coupled--are required to describe the duality of a gravitational system. Because Ri and η do not appear in the standard model, it is suggested that Λ may represent them in the relationship Λ = η/Ri^2. This requires η of the order of 0.0001%. It is concluded that modeling the universe gravitationally requires at least four causal parameters, two of which are missing in the Friedmann model. To clarify the situation, a solution is needed to Einstein’s field equations for general relativity that explicitly includes duality, i.e. a solution that includes Ri and η.