Sometimes, the holographic principle is stated as follows: "Every theory with gravity in D-dimensions (in certain preexistent space-time background) can be described by a (D-1)-field theory WITHOUT gravity on the boundary (of the space-time background, I presume)". My question is: What is the holographic dual of the most general gravity described by general relativity in 4 space-time dimensions? That is, what is the dual of general relativity metric in general (if it exists)? AdS metrics are dual to CFT, but what about dS space-times in Cosmology/cosmological settings and more general Petrov D-type space-times? And furthermore...Does the holographic principle in this form remain valid for ANY metric?