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

Daniel Frolovsky, Sergei V. Ketov

TL;DR

This work assesses whether single-field inflation models that produce primordial black holes (PBHs) remain compatible with ACT CMB observations. Focusing on a concrete viable $\alpha$-attractor E-model, the authors analyze how bending the inflaton plateau—parametrized by $\theta$—affects $n_s$ and $α_s$ while aiming to generate PBHs in the asteroid-mass range. They show that upward bending to raise $n_s$ inevitably drives $α_s$ negative, which is disfavored by ACT at roughly the $2σ$ level, with tension increasing for heavier PBHs; a potential workaround is to start with a low $q \lesssim 1$ and bend downward to obtain $α_s>0$ while keeping $n_s$ within bounds. The discussion highlights that multi-field scenarios can alleviate the tension by altering the shape of the scalar power spectrum, and future measurements of $r$ and gravitational waves (e.g., LiteBIRD, LISA, DECIGO) will offer crucial tests of these inflationary scenarios.

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 $α_s$, 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 $α$-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 $α_s$. Those values are disfavored by the ACT bounds just above $2σ$ even for PBHs in the asteroid-mass range, while the tension becomes stronger for heavier PBHs.

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

TL;DR

This work assesses whether single-field inflation models that produce primordial black holes (PBHs) remain compatible with ACT CMB observations. Focusing on a concrete viable -attractor E-model, the authors analyze how bending the inflaton plateau—parametrized by —affects and while aiming to generate PBHs in the asteroid-mass range. They show that upward bending to raise inevitably drives negative, which is disfavored by ACT at roughly the level, with tension increasing for heavier PBHs; a potential workaround is to start with a low and bend downward to obtain while keeping within bounds. The discussion highlights that multi-field scenarios can alleviate the tension by altering the shape of the scalar power spectrum, and future measurements of and gravitational waves (e.g., LiteBIRD, LISA, DECIGO) will offer crucial tests of these inflationary scenarios.

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 and its {\it positive} running , 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 -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 through bending lead to negative values of . Those values are disfavored by the ACT bounds just above even for PBHs in the asteroid-mass range, while the tension becomes stronger for heavier PBHs.

Paper Structure

This paper contains 3 sections, 7 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Bending Inflaton Potential
  3. Discussion

Figures (2)

  • Figure 1: The potential in the E-model for various values of $\theta$ of the order $10^{-5}$. The other parameters are tuned to generate PBHs with masses of the order $10^{19}\,\mathrm{g}$.
  • Figure 2: The dependence of $n_s$ upon $\theta$ in $[0, \ldots, 10^{-6}]$ (left), and $\alpha_s$ upon $\theta$ in $[-10^{-6}, \ldots, 10^{-6}]$ and e-folds $N_e$ (right). The other parameters are tuned to generate PBHs with masses of the order $10^{19}\,\mathrm{g}$.