Signatures of non-gaussianity in the isocurvature modes of primordial black hole dark matter

TL;DR

This paper demonstrates that even tiny local non-Gaussianities in primordial fluctuations strongly constrain primordial black holes as dark matter candidates, using large-scale perturbation effects and Planck data.

Contribution

It introduces a method linking large-scale perturbations to PBH abundance constraints, highlighting the impact of non-Gaussianity on PBH dark matter viability.

Findings

PBHs are excluded as dark matter for very small non-Gaussianity levels.

Constraints on $f_{NL}$ and $g_{NL}$ are nearly as strong, even at minimal non-Gaussianity.

Detection of non-Gaussianity on CMB scales would tighten PBH dark matter constraints.

Abstract

Primordial black holes (PBHs) are black holes which may have formed very early on during the radiation dominated era in the early universe. We present here a method by which the large scale perturbations in the density of primordial black holes may be used to place tight constraints on non-gaussianity if PBHs account for dark matter (DM). The presence of local-type non-gaussianity is known to have a significant effect on the abundance of primordial black holes, and modal coupling from the observed CMB scale modes can significantly alter the number density of PBHs that form within different regions of the universe, which appear as DM isocurvature modes. Using the recent \emph{Planck} constraints on isocurvature perturbations, we show that PBHs are excluded as DM candidates for even very small local-type non-gaussianity, $|f_{NL}|\approx0.001$ and remarkably the constraint on $g_{NL}$ is almost as strong. Even small non-gaussianity is excluded if DM is composed of PBHs. If local non-Gaussianity is ever detected on CMB scales, the constraints on the fraction of the universe collapsing into PBHs (which are massive enough to have not yet evaporated) will become much tighter.