ACT, SPT, and chaotic inflation

Abstract

We show that the simplest generalization of the chaotic inflation model $\tfrac12 {m^{2}φ^{2}}$ with nonminimal coupling to gravity $(1+φ) R$ provides a good match to the results of the latest data release of the Atacama Cosmology Telescope, with $r \approx10^{-2}$.

Figures (3)

• Figure 1: The figure shows the latest constraints on $n_{s}$ and $r$ according to ACT (P-ACT-LB) Louis:2025tst. The dashed line at the bottom corresponds to the Starobinsky model. The yellow star at the center of the dark purple area favored by ACT shows the values of $n_{s}$ and $r$ (\ref{['60']}) in the model (\ref{['Jordan']}) for $N_{e}= 60$.
• Figure 2: Figure 2 from Chang:2022tzj (credit R. Flaugher) for the CMB-S4 collaboration shows the predictions of $\alpha$-attractors $\tanh^2 {\varphi/\sqrt {6\alpha}}$ (gray band), the predictions of $\alpha$-attractors for Poincaré disks with discrete $\alpha$ (purple lines), as well as Higgs inflation and the Starobinsky ($R^2$) model (orange circles dots) for $47 < N_e< 57$. Starobinsky and Higgs inflation correspond to $\alpha$-attractors with $\alpha=1$. Note a complete absence of targets at $n_s \gtrsim 0.966$.
• Figure 3: The potential of the model (\ref{['Jordan']}) in the Einstein frame as a function of the canonically normalized inflaton field $\varphi$. Unlike the potential in the Higgs inflation, this potential is not symmetric with respect to the change $\varphi \to -\varphi$. Rather, it is similar to the potential in the Starobinsky model (red dashed line) and E-models of $\alpha$-attractors Kallosh:2013yoa, but it approaches the plateau more slowly.