Towards Axionic Starobinsky-like Inflation in String Theory

TL;DR

This paper demonstrates that Starobinsky-like inflation can arise from string-theory axions in a flux-compactified Type IIB setup with non-geometric fluxes. By introducing a tunable parameter $\lambda$ and accounting for backreaction, the uplifted F-term axion-monodromy potential interpolates between a quadratic and a Starobinsky-like form, enabling three distinct inflationary regimes with observable $r$ values ranging from ~0.13 to ~0.0015. The analysis highlights UV-control trade-offs: larger $\lambda$ improves moduli stabilization but risks UV-scale decoupling, while smaller $\lambda$ yields a flatter potential at the expense of multi-field dynamics. The work provides a string-motivated mechanism for realizing plateau-like inflation from axions, with implications for reheating and connections to low-energy supersymmetry, though further model-building is required for full phenomenological viability. Overall, the study offers a concrete route to embedding Starobinsky-like inflation in string theory through axion monodromy and flux-induced backreaction.

Abstract

It is shown that Starobinsky-like potentials can be realized in non-geometric flux compactifications of string theory, where the inflaton involves an axion whose shift symmetry can protect UV-corrections to the scalar potential. For that purpose we evaluate the backreacted, uplifted F-term axion-monodromy potential, which interpolates between a quadratic and a Starobinsky-like form. Limitations due to the requirements of having a controlled approximation of the UV theory and of realizing single-field inflation are discussed.

Figures (6)

• Figure 1: The potential $V_{\rm back}(\theta)$ shown in \ref{['backpotential']} in units of $M^4_{\rm Pl}/(4\pi)$ for fluxes $h=1$, $q=1$, $\tilde{\mathfrak f}=10$ and $\lambda=10$. For this large value of $\lambda$, the trajectory \ref{['backshift']} correctly describes the full motion in field space.
• Figure 2: The potentials $V_{\rm back}(\Theta)$ and \ref{['back_01']} in units of $M^4_{\rm Pl}/(4\pi)$ for fluxes $h=1$, $q=1$, $\tilde{\mathfrak f}=10$ and $\lambda=60$. The lower (blue) curve is the exact backreacted potential.
• Figure 3: The potentials $V_{\rm back}(\theta)$ and \ref{['back_01']} in units of $M^4_{\rm Pl}/(4\pi)$ for fluxes $h=1$, $q=1$, $\tilde{\mathfrak f}=10$ and $\lambda=10$.
• Figure 4: The potential $V_{\rm back}(\theta)$ in units of $M^4_{\rm Pl}/(4\pi)$ for fluxes $h=1$, $q=1$, $\tilde{\mathfrak f}=10$ and $\lambda=1$.
• Figure 5: The tensor-to-scalar ratio as a function of $\lambda$ for fixed $n_s=0.967$.