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.
