Dynamics of the inflaton scalar field for a certain class of E-model potentials

We investigate the dynamics of the inflaton scalar field in a certain class of inflationary E-models that combine the properties of the Starobinsky model and the alpha-attractor model. The inflaton potential we are dealing with has an exponentially flat plateau at high field values and a sharply defined minimum at zero. Using the slow-roll approximation, we obtain the analytic expressions describing evolution of the background inflaton field at the inflationary stage. To describe the nonlinear field oscillations at the preheating stage, we use the technique of separation of fast (oscillation phase) and slow (field energy density) variables. The obtained expressions are in good agreement with the results of direct numerical integration of the field equations. These expressions are then used to study the evolution of perturbations at the preheating stage. Based on the Mukhanov-Sasaki equation, we derive the Hill equation with slowly varying parameters, which describes the scalar perturbation modes taking into account the anharmonicity of the background oscillations. Using Floquet theory, we analyze the structure of the resonance zones of this equation and then integrate it numerically. We show that the cosmological expansion limits the resonant growth of scalar modes, making their amplitudes nearly constant at late times. We also derive the corresponding Hill equation for tensor modes. We show that resonant amplification of tensor fluctuations of the metric does not occur.