Non-minimally coupled scalar k-inflation dynamics

In this work, we shall study k-inflation theories with non-minimal coupling of the scalar field to gravity, in the presence of only a higher-order kinetic term of the form similar to const x (XX)-X-mu, with X=1/2 partial derivative(mu)phi partial derivative(mu)phi. The study will be focused in the cases where a scalar potential is included or is absent, and the evolution of the scalar field will be assumed to satisfy the slow-roll or the constant-roll condition. In the case of the slow-roll models with scalar potential, we shall calculate the slow-roll indices and the corresponding observational indices of the theory, and we demonstrate that the resulting theory is compatible with the latest Planck data. The same results are obtained in the constant-roll case, at least in the presence of a scalar potential. In the case that models without potential are considered, the results are less appealing since these are strongly model dependent, and at least for a power-law choice of the non-minimal coupling, the theory is non-viable. Finally, due to the fact that the scalar and tensor power spectra are conformal invariant quantities, we argue that the Einstein frame counterpart of the non-minimal k-inflation models with scalar potential can be a viable theory, due to the conformal invariance of the observational indices. The Einstein frame theory is more involved and thus more difficult to work with it analytically, so one implication of our work is that we provide evidence for the viability of another class of k-inflation models.

Automated summary

This study focuses on k-inflation theories with non-minimal coupling of the scalar field to gravity, investigating the evolution of the scalar field under slow-roll or constant-roll conditions when a scalar potential is present or absent. The results show that the theory is compatible with the latest Planck data in the slow-roll models with scalar potential, but less appealing in models without potential. Additionally, it is suggested that the Einstein frame counterpart of non-minimal k-inflation models with scalar potential may be a viable theory due to the conformal invariance of the observational indices.