Global Portraits of Nonminimal Inflation: Metric and Palatini

In this paper, we study the global phase space dynamics of single nonminimally coupled scalar field inflation models in the metric and Palatini formalisms. Working in the Jordan frame, we derive the scalar-tensor general field equations and flat FLRW cosmological equations, and present the Palatini and metric equations in a common framework. We show that inflation is characterized by a "master" trajectory from a saddle-type de Sitter fixed point to a stable node fixed point, approximated by slow roll conditions (presented for the first time in the Palatini formalism). We show that, despite different underlying equations, the fixed point structure and properties of many models are congruent in metric and Palatini, which explains their qualitative similarities and their suitability for driving inflation. On the other hand, the global phase portraits reveal how even models which predict the same values for observable perturbations differ, both to the extent of the phase space physically available to their trajectories, as well as their past asymptotic states. We also note how the slow roll conditions tend to underestimate the end of inflationary accelerated expansion experienced by the true nonlinear "master" solution. The explicit examples we consider range from the metric and Palatini induced gravity quintic potential with a Coleman-Weinberg correction factor to Starobinsky, metric and Palatini nonminimal Higgs, second order pole, and several nontrivial Palatini models.