We consider the origin of fluctuations in the early universe. Inflation is one of the possible mechanisms to produce the initial seeds which are needed for formation of structures in the universe. Most of the inflationary theories lead to a "scale-invariant" spectrum, but generally the amplitude of perturbation is too large. This amplitude can be brought down only if the non-supersymmetric inflationary potential is fine-tuned in a very unnatural way. Correct values for the amplitude may be obtained if the divergent expressions in field theory are regularized using a cutoff at Planck energy.
For that we use the hypothesis of path integral duality. Assuming the path integral amplitude to be invariant under the duality transformation, we evaluate the modified Feynman propagator in a de Sitter inflationary phase and from there the amplitude of fluctuations at the time when perturbations re-enter the Hubble radius. In this approach the power spectrum is almost scale-invariant. At the end we obtain a very reasonable value for the amplitude of cosmic gravitational waves.