Duality invariance of cosmological perturbation spectra

I show that cosmological perturbation spectra produced from quantum fluctuations in massless or self-interacting scalar fields during an inflationary era remain invariant under a two parameter family of transformations of the homogeneous background fields. This relates slow-roll inflation models to solutions which may be far from the usual slow-roll limit. For example, a scale-invariant spectrum of perturbations in a minimally coupled, massless field can be produced by an exponential expansion with a proportional to e(Ht), or by a collapsing universe with a proportional to(- t)(2/3). [S0556-2821(99)02814-3].