Equivalence principle, de-Sitter space, and cosmological twistors

I discuss the impact of the positive cosmological constant on the interplay between the equivalence principle in general relativity, and the rules of quantum mechanics. At the non--relativistic level there is an ambiguity in the definition of a phase of a wave function measured by inertial and accelerating observes. This is the cosmological analogue of the Penrose effect, which can also be seen as a non--relativistic limit of the Unruh effect. The symmetries of the associated Schr\"odinger equation are generated by the Newton--Hooke algebra, which arises from a non--relativistic limit of a cosmological twistor space.