Higgs inflation, unitarity, and emergence of scalaron
The Higgs inflation introduces a large non-minimal coupling between the Ricci scalar and Higgs that causes tree-level unitarity violation well below the Planck scale. After reviewing the unitarity issue during and after inflation, we show that the unitarity can be restored by summing over vacuum polarization-type diagrams. The scattering amplitude develops a pole after the resummation, which we identify as the scalar component of the metric, or the scalaron. It suggests that the Higgs inflation is actually a multi-field inflation model, i.e. Higgs and the scalaron, and does not suffer from the unitarity issue. We also show that this phenomenon can be understood in the language of the non-linear sigma model (NLSM), with the scalaron identified as the sigma field that linearizes the NLSM.