  The aim of this paper is to provide a more precise description of the
paradigm of corpuscular slow-roll inflation, which was previously introduced by
Casadio et al. in [1]. Specifically, we start by expanding the Starobinsky
theory on a curved background and then infer the number and nature of the
propagating degrees of freedom, both in the true inflationary phase and in a
quasi-de Sitter approximation. We correctly find that the particle spectrum
contains a transverse trace-free mode and a scalar one. The scalar mode
displays a tachyonic nature during the slow-roll phase, due to the instability
of the system, whereas it acquires the appropriate oscillatory behavior as the
background approaches a critical value of the curvature. These results confirm
the fact that the Einstein-Hilbert term acts as a perturbation to the quadratic
one, and is responsible for driving the early universe out of the inflationary
phase, thus realising the inflaton field in terms of pure (corpuscular)
gravity.
