Quantum Linear Gravity in de Sitter Universe On Gupta-Bleuler vacuum   state
  Application of Krein space quantization to the linear gravity in de Sitter space-time have constructed on Gupta-Bleuler vacuum state, resulting in removal of infrared divergence and preserving de Sitter covariant. By pursuing this path, the non uniqueness of vacuum expectation value of the product of field operators in curved space-time disappears as well. Then the vacuum expectation value of the product of field operators can be defined properly and uniquely. 
M. Enayati, S. Rouhani, M.V. Takook
gr-qc