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

TL;DR

The paper examines how recent ACT observations challenge single-field inflation models, especially those predicting primordial black holes, by analyzing updated CMB constraints and exploring potential model adjustments.

Contribution

It provides a detailed analysis of the tension between ACT data and single-field inflation models, including a case study of the $eta$-attractor E-model and potential modifications to reconcile them.

Findings

ACT data favors higher $n_s$ and positive $eta_s$, constraining models.

Bending the inflaton potential to increase $n_s$ results in disfavored negative $eta_s$.

Tensions are more severe for models predicting heavier primordial black holes.

Abstract

The data release from the Atacama Cosmology Telescope (ACT) imposes stronger constraints on primordial black holes (PBHs) formation in single-field inflation models versus the Planck data. In particular, the updated Cosmic Microwave Background (CMB) radiation measurements favour a {\it higher} scalar spectral index $n_s$ and its {\it positive} running $\alpha_s$, which put the single-field models under scrutiny. Even in the absence of PBHs production, the new data constrain many single-field models of inflation. To explore this tension, we study PBHs formation in a concrete viable $\alpha$-attractor E-model. We investigate an impact of bending of the inflaton potential plateau toward reconciling the model with the ACT bounds on the CMB observables. We find that attempts to increase $n_s$ through bending lead to negative values of $\alpha_s$. Those values are disfavored by the ACT bounds just above $2\sigma$ even for PBHs in the asteroid-mass range, while the tension becomes stronger for heavier PBHs.