On the strong coupling scale in Higgs G-inflation

TL;DR

This paper evaluates the strong coupling scale in Higgs G-inflation, showing it remains weakly coupled during observable inflation but cannot be described from the Planck scale, impacting initial condition assumptions.

Contribution

It provides a detailed analysis of the background-dependent strong coupling scale in Higgs G-inflation, including Nambu-Goldstone modes, clarifying the model's validity range.

Findings

The system is weakly coupled during observable inflation.

The strong coupling scale is below the Planck scale.

The model cannot be described from the Planck scale or chaotic initial conditions.

Abstract

Higgs G-inflation is an inflation model that takes advantage of a Galileon-like derivative coupling. It is a non-renormalizable operator and is strongly coupled at high energy scales. Perturbative analysis does not have a predictive power any longer there. In general, when the Lagrangian is expanded around the vacuum, the strong coupling scale is identified as the mass scale that appears in non-renormalizable operators. In inflationary models, however, the identification of the strong coupling scale is subtle, since the structures of the kinetic term as well as the interaction itself can be modified by the background inflationary dynamics. Therefore, the strong coupling scale depends on the background. In this letter, we evaluate the strong coupling scale of the fluctuations around the background in the Higgs G-inflation including the Nambu-Goldstone modes associated with the symmetry breaking. We find that the system is sufficiently weakly coupled when the scales which we now observe exit the horizon during inflation and the observational predictions with the semiclassical treatment are valid. However, we also find that the inflaton field value at which the strong coupling scale and the Hubble scale meet is less than the Planck scale. Therefore, we cannot describe the model from the Planck scale, or the chaotic initial condition.