Higgs Inflation and Naturalness

Inflation based on scalar fields which are non-minimally coupled to gravity has been proposed as a way to unify inflation with weak-scale physics, with the inflaton being identified with the Higgs boson or other weak-scale scalar article. These models require a large non-minimal coupling xi ~ 10^{4} to have agreement with the observed density perturbations. However, it has been suggested that such models are unnatural, due to an apparent breakdown of the calculation of Higgs-Higgs scattering via graviton exchange in the Jordan frame. Here we argue that Higgs inflation models are in fact natural and that the breakdown does not imply new physics due to strong-coupling effects or unitarity breakdown, but simply a failure of perturbation theory in the Jordan frame as a calculational method. This can be understood by noting that the model is completely consistent when analysed in the Einstein frame and that scattering rates in the two frames are equal by the Equivalence Theorem for non-linear field redefinitions.