“…We also discuss the importance of the equivalent frames describing the same physics, because some symmetries and critical problems can be made more clear in one frame than in the other. In the sigma-model frame for Higgs inflation where the conformal symmetry is manifest, the Higgs kinetic term can be recast into a non-linear model type [17,19], so the unitarity problem can be seen clearly from the non-canonical form of the Higgs kinetic terms [8,10]. Moreover, in the presence of the R 2 term in the Higgs inflation, the dual-scalar field for the R 2 term, called the sigma field, appears to respect the conformal symmetry and internal symmetries in the sigma-model frame, so unitarity becomes manifest [17,19].…”