Smooth Solutions and Discrete Imaginary Mass of the Klein-Gordon   Equation in the de Sitter Background
  Using methods in the theory of semisimple Lie algebras, we can obtain all smooth solutions of the Klein-Gordon equation on the 4-dimensional de Sitter spacetime (dS^4). The mass of a Klein-Gordon scalar on dS^4 is related to an eigenvalue of the Casimir operator of so(1,4). Thus it is discrete, or quantized. Furthermore, the mass m of a Klein-Gordon scalar on dS^4 is imaginary: m^2 being proportional to -N(N+3), with N >= 0 an integer. 
Bin Zhou and Zhen-Hua Zhou
hep-th gr-qc math-ph math.MP