The causal order on the ambient boundary
  We analyse the causal structure of the ambient boundary, the conformal infinity of the ambient (Poincar\'e) metric. Using topological tools we show that the only causal relation compatible with the global topology of the boundary spacetime is the horismos order. This has important consequences for the notion of time in the conformal geometry of the ambient boundary. 
Ignatios Antoniadis, Spiros Cotsakis, Kyriakos Papadopoulos
hep-th gr-qc