  It has been shown that the negative norm states necessarily appear in a
covariant quantization of the free minimally coupled scalar field in de Sitter
space [1,2]. In this process ultraviolet and infrared divergences have been
automatically eliminated [3]. A natural renormalization of the one-loop
interacting quantum field in Minkowski space-time ($\lambda\phi^4$ theory) has
been achieved through the consideration of the negative norm states [4].
One-loop effective action for scalar field in a general curved space-time has
been calculated by this method and a natural renormalization procedure in the
one-loop approximation has been established.
