Null geodesics in black hole metrics with non-zero cosmological constant.

The authors study the radial motion along null geodesics in the Reissner-Nordström-de Sitter and Kerr-de Sitter space-times. They analyze the properties of the effective potential and discuss circular orbits. They find that the radii of circular geodesics in the Reissner-Nordström-de Sitter space-time do not depend on the cosmological constant, and explain this property using the optical reference geometry. In addition, the authors describe the unusual consequences of the interplay between rotation of the source and cosmological repulsion.