Perturbation of FRW Spacetime in NP Formalism
  Perturbation of FRW spacetime is carried out in NP formalism. The equation governing the scalar, vector and tensor modes take on a very simple and transparent form. All of them can be combined in one master equation for all helicities. The solutions for the closed, flat and open FRW are analytic continuation of the same functions, so only the solutions in the closed model are described. The scalar equation is the same as that of the conformally coupled massless Klein-Gordon field, the vectorial ones are the same as Maxwell equations, and the tensorial ones are for spin-2 fields. The corresponding eigen-functions are all determined, and in particular, the Green's function and the Lienard-Wiechert type potential also solved. These solutions reduce to the familiar form in flat space. 
S. K. Sharma and U. Khanal
astro-ph.CO