Hybrid metric-Palatini Higgs inflation

TL;DR

This paper extends Higgs inflation within hybrid metric-Palatini gravity, exploring inflationary predictions, unitarity, and UV completions, revealing conditions for effective field theories and potential for observable cosmological signatures.

Contribution

It introduces a novel hybrid metric-Palatini Higgs inflation model with detailed inflationary and unitarity analysis, and proposes a UV extension with an additional scalar field.

Findings

Slow-roll inflation achieved with observationally consistent spectral index.

Tensor-to-scalar ratio varies with non-minimal couplings, maxing at ~10^{-3}.

UV cutoff is lower than Planck scale, but can be improved with extra scalar fields.

Abstract

We propose an extension of the Higgs inflation to the hybrid metric-Palatini gravity, where we introduce non-minimal couplings between Higgs and both the metric-type and the Palatini-type Ricci scalars. We study the inflationary phenomenology of our model and find that slow-roll inflation can be realized in the large-field regime, giving the observationally favored predictions. In particular, the scalar spectral index exhibits an attractor behavior to $n_{\mathrm{s}}\sim 0.964$, while the tensor-to-scalar ratio can take an arbitrary value depending on the non-minimal coupling parameters, with the metric-Higgs limit $r\sim10^{-3}$ being the maximum. We also investigate the unitarity property of our model. As the ultraviolet (UV) cutoff as a low-energy effective field theory (EFT) of this model is significantly lower than the Planck scale due to a strong curvature of field-space, we consider a possible candidate of UV-extended theories with an additional scalar field introduced so as to flatten the field-space in five-dimension. While the field-space can be flatten completely and this approach can lead to a weakly-coupled EFT, we gain an implication that Planck-scale EFT can be only realized in the limit of metric-Higgs inflation. We also discuss generalizations of the model up to mass-dimension four.