  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.

General Relativity and Quantum Cosmology