Quasi-Palatini formulation of scalar-tensor gravity

The Palatini formulation has been successful in the development of several alternative theories of gravity. It is well understood that the Palatini and metric formulations are equivalent in minimally coupled scalar-tensor models, but nonminimal scalar-tensor models can lead to physically distinct theories depending on the underlying formulation. Once a model has been selected, the choice of formulation is a discrete one, and so promoting it to be continuous is expected to give rise to a wider class of actions. To this end, we propose the “quasi-Palatini” formulation, a method for interpolating between the metric and Palatini formulations for a given model that gives rise to a continuous family of models. We apply the quasi-Palatini formulation to Higgs inflation, induced gravity inflation, and Starobinsky inflation, and demonstrate how this leads to a deformation of the potential with a tangible effect on the observables. We also discuss how the interpolation between different actions can be extended to scalar-torsion and scalar-nonmetricity models.