Parametric Resonance of Entropy Perturbations in Massless Preheating

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Here, we revisit the question of possible preheating of entropy modes in a two field model with a massless inflaton coupled to a matter scalar field.

• Using a perturbative approximation to the covariant method we demonstrate that there is indeed a parametric instability of the entropy mode which then at second order leads to exponential growth of the curvature fluctuation on super-Hubble scale.
• Back-reaction effects shut off the induced curvature fluctuations, but possibly not early enough to prevent phenomenological problems.
• This confirms previous results obtained using different methods and resolves a controversy in the literature.