Hierarchical Axion Inflation

TL;DR

Two-axion inflation with a simple hierarchy of sub-Planckian decay constants generates an effectively trans-Planckian decay constant $f_{\mathrm{eff}}$ by integrating out a heavy mode, avoiding alignment tuning. In the light-direction limit the potential reduces to $V \approx \tfrac{1}{2} m_1^2 \phi_1^2$, yielding $n_s\simeq 1-\frac{2}{N_e}$ and $r\simeq \frac{8}{N_e}$, while the paper discusses concrete string-theoretic embeddings in Type IIB with moduli stabilization that can realize the required hierarchies and scales with trans-Planckian field ranges after canonical normalization. The approach combines features of KNP and Dante’s Inferno to achieve large-field natural inflation with minimal tuning using only non-perturbative effects and a small number of axions, and outlines viable pathways for embedding in a string-theory framework through KKLT/LVS-type stabilization and axions from $C_4$ and $C_2$.

Abstract

We propose a new field theory mechanism for generating an effective trans-Planckian decay constant from sub-Planckian ones. Using the minimal two axions and a hierarchy between two axion decay constants is sufficient for realizing inflation through non-perturbative effects only and with minimal tuning. The inflationary motion is kept entirely within a sub-Planckian domain. We outline possible strategies of embedding the model in a string theory setup.

Figures (2)

• Figure 1: The axionic potential in the $(r,\theta)$ coordinate system for the decay constants given by the second line of Table \ref{['tab:1']} in units of $\Lambda^4$ for $\Lambda_1=\Lambda=2$.
• Figure 2: The axionic potential of Fig. \ref{['fig:Vrtheta']} in the rotated coordinates $(\phi_1,\phi_2)$.