Negative non-Gaussianity as a salvager for PBHs with PTAs in bounce

TL;DR

This paper explores how negative non-Gaussianity in the early universe can enhance primordial black hole formation, resolve overproduction issues, and explain recent PTA gravitational wave signals within bounce cosmology models.

Contribution

It demonstrates that large negative non-Gaussianity can significantly increase PBH abundance and reconcile gravitational wave observations with bounce cosmology scenarios.

Findings

Negative non-Gaussianity enhances PBH abundance.

Large negative $f_{NL}$ mitigates PBH overproduction.

Effective sound speed and $f_{NL}$ explain PTA signals.

Abstract

Non-Gaussianity in the primordial curvature perturbation is a crucial element of the early universe due to its significant impact on the primordial black hole (PBH) production. In this work, we focus on the effects of negative non-Gaussianity on PBH abundance through the lens of the compaction function criterion for PBH formation. Our setup utilizes an effective field theory of non-singular bounce, including the standard slow-roll inflation with an ultra-slow roll phase for amplifying the curvature perturbations to form PBHs. We investigate with two separate values of the non-Gaussianity parameter, $f_{\rm NL}=(-39.95,-35/8)$, found within the ekpyrotic contraction and the matter bounce scenarios, respectively, and show that a negatively large amount of $f_{\rm NL}$ can provide sizeable abundance, $10^{-3}\leq f_{\rm PBH}\leq 1$, and completely mitigates the PBH overproduction issue. We also highlight that the case with the effective sound speed $c_{s}=1$, coupled with $f_{\rm NL}=-39.95$, provides an agreement under $1\sigma$ for the scalar-induced gravitational wave explanation of the latest PTA (NANOGrav15 and EPTA) signal. Lastly, we extract an upper bound on the most negative value of, $f_{\rm NL}\sim -60$, below which we show breaching of the underlying perturbativity constraints on the power spectrum amplitude.