Has ACT measured radiative corrections to the tree-level Higgs-like inflation?

TL;DR

This work investigates whether radiative (1-loop) corrections to a Higgs-like, nonminimally coupled inflation can reconcile inflationary predictions with ACT data, which favor a higher $n_s$ than traditional tree-level Higgs-like or Starobinsky models. By modeling a 1-loop corrected quartic potential in the Jordan frame as $V_{ m eff}(φ)=rac{λ_{ m eff}(φ)}{4}φ^4$ with $λ_{ m eff}(φ)≃λ(M_P)[1+δ\ln(φ/M_P)]$, and transforming to the Einstein frame, the authors analyze both metric and Palatini gravity. They find that positive radiative corrections ($δ>0$) shift the predictions of $n_s$ and $r$, bringing the models into the ACT $1σ$ region, with the Palatini formulation yielding stronger shifts and compatibility at smaller $ξ$. The study also discusses perturbative unitarity, showing that with a free $λ$ the unitarity cutoff can remain above the inflationary scale ($Λ_ξ$ in metric and $Λ_{√ξ}$ in Palatini), suggesting robustness of the results, and highlighting the potential of ACT to detect signatures of radiative corrections in inflation.

Abstract

Starobinsky inflation and nonminimally coupled (NMC) Higgs inflation have been among the most favored models of the early Universe, as their predictions for the scalar spectral index $n_s$ and tensor-to-scalar ratio $r$ fall comfortably within the constraints set by Planck and BICEP/Keck. However, new results from the Atacama Cosmology Telescope (ACT) suggest a preference for higher values of $n_s$, introducing tension with the simplest realizations of these models. In this work, being agnostic about the nature of the inflaton, we show that incorporating one-loop corrections to a quartic NMC inflationary scenario leads to a shift in the predicted value of $n_s$, which brings NMC inflation into better agreement with ACT observations. The effect is even more significant when the model is formulated in the Palatini approach, where the modified field-space structure naturally enhances deviations from the metric case. These findings highlight the importance of quantum corrections and gravitational degrees of freedom in refining inflationary predictions in light of new data.

Figures (3)

• Figure 1: $r$ vs. $n_s$ (upper left panel), $r$ vs. $\xi$ (upper right), $\xi$ vs. $n_s$ (lower left) and $\lambda$ vs. $\xi$ (lower right) for $N = 50$$e$-folds in the metric (continuous) and Palatini formulation (dashed), with $\delta=0.1\%$, $\delta=1\%$ and $\delta=3\%$ in the loop-corrected NMC scenario. The gray (purple) areas represent the 1,2$\sigma$ allowed regions coming from the latest combination of Planck, BICEP/Keck and BAO data BICEP:2021xfz (from Planck, ACT, and DESI ACT:2025tim). For reference, we also plot the predictions of quartic (brown), quadratic (orange), linear (green) and Starobinsky Starobinsky:1980te (black) inflation in metric gravity, and, in the right lower panel, $\lambda_{\rm ew} \simeq 0.13$ (gray dashed line), i.e., the value of the Higgs self-quartic coupling at EW scale. The arrow in the upper left panel denotes the direction of increasing $\xi$.
• Figure 2: $r$ vs. $n_s$ (upper left panel), $r$ vs. $\xi$ (upper right), $\xi$ vs. $n_s$ (lower left) and $\lambda$ vs. $\xi$ (lower right) for $N = 60$$e$-folds in the metric (continuous) and Palatini formulation (dashed), with $\delta=0.1\%$, $\delta=1\%$ and $\delta=3\%$ in the loop-corrected NMC scenario. The gray (purple) areas represent the 1,2$\sigma$ allowed regions coming from the latest combination of Planck, BICEP/Keck, and BAO data BICEP:2021xfz (from Planck, ACT, and DESI ACT:2025tim). For reference, we also plot the predictions of quartic (brown), quadratic (orange), linear (green) and Starobinsky Starobinsky:1980te (black) inflation in metric gravity, and, in the right lower panel, $\lambda_{\rm ew} \simeq 0.13$ ( gray dashed line) i.e. the value of the Higgs self-quartic coupling at EW scale. The arrow in the upper left panel denotes the direction of increasing $\xi$.
• Figure 3: The value of the potential \ref{['eq:Ufinal']} at horizon crossing for $N=50$ as a function of $\xi$ for the metric and Palatini formulations (colors as in Figs. \ref{['Fig:fig1']} and \ref{['Fig:fig2']}). The cutoff scales $\Lambda_\xi$ (solid black line) and $\Lambda_{\sqrt{\xi}}$ (dashed black line) are also shown to indicate the region of validity of our results.