Parametric Resonance in the Einstein frame: the Jordan-frame Doppelg\"anger

Modified $f(R)$ theories of gravity have been investigated for quite a long time in the literature as a possible explanation for the inflationary period of the universe. The correspondence to General Relativity with an extra scalar field $\tilde\phi$ in the so-called Einstein Frame via a conformal transformation is a major tool in this class of theories. Here, we assume three different potentials $V(\tilde\phi)$ and a parametric-resonance coupling between $\tilde\phi$ and a secondary scalar field $\tilde\psi$ such that one can have both inflation and preheating in the Einstein frame. We study the instability resonance band structure for our models. Further, we determine the correspondent mechanism -- and the function $f(R)$ itself -- in the Jordan frame, that is possibly related to the so-called vacuum awakening mechanism.