Are single-field models of inflation and PBH production ruled out by ACT observations?

The data release from the Atacama Cosmology Telescope (ACT) imposes stronger constraints on primordial black hole (PBH) formation in single-field inflation models than the Planck data. In particular, the updated Cosmic Microwave Background (CMB) radiation measurements favour a $higher$ scalar spectral index $n_s$ and its $positive$ running $α_s$. Even in the absence of PBH production, the updated constraints together place many single-field models under tension with the CMB data. To explore this tension, we study PBH formation in an $α$-attractor E-model. We investigate an impact of bending of the inflaton potential plateau toward reconciling the model with the new bounds for CMB observables. We find that attempts to increase $n_s$ by upward bending lead to negative values of $α_s$ that are disfavoured by the new bounds. A possible model-building approach to resolve the tension is proposed.