Starobinsky-like two-field inflation

Abstract: We consider an extension of the Starobinsky model, whose parameters are functions of an extra scalar field. Our motivation is to test the robustness (or sensitivity) of the Starobinsky inflation against mixing scalaron with another (matter) scalar field. We find that the extended Starobinsky model is (classically) equivalent to the two-field inflation, with the scalar potential having a flat direction. For the sake of fully explicit calculations, we perform a numerical scan of the parameter space. Our numerical calculations support the viability of the Starobinsky-like two-field inflation for the certain range of its parameters, which are characterized by the scalar index $n_s=0.96\pm 0.01$, the tensor-to-scalar ratio $r<0.06$, and small running of the scalar index at $|\alpha_s|<0.05$.