Spectrum of Gravitational Waves in Krein Space Quantization
  The main goal of this paper is to derive the primordial power spectrum for the scalar perturbations generated as a result of quantum fluctuations during an inflationary pe- riod by an alternative approach of field quantization[1-3]. Formulas are derived for the gravitational waves, special cases of which include power law inflation and inflation in the slow roll approximation, in Krein space quantization. 
M. Mohsenzadeh, A. Sojasi and E. Yusofi
gr-qc