What new physics can we extract from inflation using the ACT DR6 and DESI DR2 Observations?

Abstract

We present a comprehensive analysis of inflationary models in light of projected sensitivities from forthcoming CMB and gravitational wave experiments, incorporating data from recent ACT DR6, DESI DR2, CMB-S4, LiteBIRD, and SPHEREx. Focusing on precise predictions in the $(n_s, α_s, β_s)$ parameter space, we evaluate a broad class of inflationary scenarios -- including canonical single-field models, non-minimally coupled theories, and string-inspired constructions such as Starobinsky, Higgs, Hilltop, $α$-attractors, and D-brane models. Our results show that next-generation observations will sharply constrain the scale dependence of the scalar power spectrum, elevating $α_s$ and $β_s$ as key discriminants between large-field and small-field dynamics. Strikingly, several widely studied models -- such as quartic Hilltop inflation and specific DBI variants -- are forecast to be excluded at high significance. We further demonstrate that the combined measurement of $β_s$ and the field excursion $Δφ$ offers a novel diagnostic of kinetic structure and UV sensitivity. These findings underscore the power of upcoming precision cosmology to probe the microphysical origin of inflation and decisively test broad classes of theoretical models.

Figures (8)

• Figure 1: Comparison of inflationary potentials $V(\phi)$ for the models considered in this work. The plateau structure in the Starobinsky and $\alpha$-attractor T-Model potentials facilitates slow-roll inflation with suppressed tensor-to-scalar ratio $r$. The Higgs potential exhibits spontaneous symmetry breaking with characteristic minima. The inset highlights representative features such as plateaus and minima over the inflaton field range, expressed in Planck units.
• Figure 2: Tensor-to-scalar ratio $r$ vs. spectral index $n_s$ for Higgs and Starobinsky inflation with nonminimal coupling $f(\phi) = \xi\phi^2$ and $N_e = 50, 60$, varying $\boldsymbol{\xi}$.
• Figure 3: Scalar spectral index $n_s$ and tensor-to-scalar ratio $r$ for different models with the nonminimal coupling $f(\phi)=\xi\phi^2$, evaluated for $N_e = 50$ and $N_e = 60$.
• Figure 4: Comparison of theoretical predictions from five inflationary models in the $(n_s, \alpha_s)$ plane with forecasted constraints from upcoming CMB experiments. Markers indicate the number of e-folds $N = 50$ (circles) and $N = 60$ (squares).
• Figure 5: Correlation between the running of the spectral index $\alpha_s$ and its running $\beta_s$ for three inflationary models with nonminimal coupling $f(\phi)=\phi$. The color coding represents the nonminimal coupling strength $\xi$ on a logarithmic scale. The inset zooms in on the low-$\beta_s$ region, highlighting the tight clustering of Hilltop and D-Brane models compared to the broad scatter of the T-Model.