Real-time gauge/gravity duality and ingoing boundary conditions

In Lorentzian gauge/gravity duality, a proper understanding of initial conditions is essential. I discuss the precise relation between purely ingoing conditions at the horizon for bulk fields and retarded boundary correlation functions, as well as the generalization to higher-point functions. Some open questions can be answered only within the recently developed framework of [K. Skenderis and B. C. van Rees Phys. Rev. Lett. 101 (2008) 081601, arXiv:0805.0150 [hep-th], K. Skenderis and B. C. van Rees arXiv:0812.2909 [hep-th]].