Which field and potential drove inflation?
In plain words
Different versions of inflation predict slightly different tilts in the strength of density ripples with size. New data shifted the measured tilt and put the most favored simple model under pressure.
Precise statement
Identify the inflaton potential class from ($n_s, r$, running $\alpha_s$, non-Gaussianity). ACT DR6 combined with Planck and DESI gives $n_s = 0.974 \pm 0.003$ versus Planck alone $0.965 \pm 0.004$, which puts Starobinsky $R^2$ inflation at about $2\sigma$ for $N = 60$ e-folds. An answer is the potential class (plateau, monomial, hilltop, multi-field) singled out at high confidence, with the reheating history it requires.
What would settle it
Joint measurement of $n_{s}$ to 0.002 and of r or a bound $r < 0.001$, with reheating constraints.
Status in the literature
Unverified note
The 2025 ACT DR6 shift in $n_s$ prompted modified plateau models with higher-curvature terms or altered reheating.