The density perturbations generated when the inflaton decay rate is perturbed by a light scalar field χ are studied. By explicitly solving the perturbation equations for the system of two scalar fields and radiation, we show that even in low energy-scale inflation nearly scale-invariant spectra of scalar perturbations with an amplitude set by observations are obtained through the conversion of χ fluctuations into adiabatic density perturbations. We demonstrate that the spectra depend on the average decay rate of the inflaton and on the inflaton fluctuations. We then apply this new mechanism to string cosmologies and generalized Einstein theories and discuss the conditions under which scale-invariant spectra are possible.
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