Constraining ${\hat{I}}^2$-Exponential Inflation with the latest ACT observations

Abstract

Recent observations from the Atacama Cosmology Telescope (ACT), especially when combined with DESI baryon acoustic oscillation data, indicate a scalar spectral index $n_s$ higher than the value reported by \textit{Planck} 2018, placing tension on universal inflationary attractor models. Motivated by this discrepancy, we investigate the inflationary predictions of the ${\hat{I}}^2$-exponential potential, $V(\ddot{I}•)=V_0{\left(1-\hat{I}»{\hat{I}}^2\ddot{I}•/M_p\right)}^{1/{\hat{I}}^2}$ considering both minimally and non-minimally coupled realizations. This potential generalizes standard exponential inflation and naturally arises in braneworld scenarios. We derive analytical expressions for the slow-roll parameters and inflationary observables using a perturbative expansion in the non-minimal coupling $\hat{I}¾$, and validate these results through numerical calculations. In the minimally coupled case, the model predicts $n_s\simeq 0.976$ and $r\simeq 0.035$ for $N=50$ and moderate values of β, remaining compatible with ACT+DESI constraints at the 1σlevel while yielding a spectral tilt larger than the universal attractor prediction. Introducing a small non-minimal coupling significantly improves agreement with observations by suppressing the tensor-to-scalar ratio while preserving the enhanced scalar tilt. For $N=60,\hat{I}»\sim 0.3-0.5$, and ${\hat{I}}^2\sim O(1-5)$, the non-minimally coupled model yields $n_s\simeq 0.974-0.976$ and $r\lesssim 0.03$, comfortably consistent with ACT, DESI, and BICEP/Keck bounds. Our results show that the ${\hat{I}}^2$-exponential potential, especially when implemented with a non-minimal coupling, exhibits good agreement with the latest CMB observations. Our inflationary predictions of the non-minimal model of $n_s$ and $r$ confirming the leading-order contributions in $\hat{I}¾$ are sufficient to capture the essential features of both $r$ and $n_s$ in observationally relevant regimes.

V1: many pages, many figures, abstract shortened, version accepted for publication in EPJC