Primordial black holes as biased tracers

TL;DR

This paper investigates the large-scale clustering of primordial black holes (PBHs) as dark matter candidates, using bias theory and CMB constraints, and finds that PBHs as dark matter are highly constrained or excluded under certain conditions.

Contribution

It introduces a bias-based framework to analyze PBH clustering and derives new constraints on PBH dark matter scenarios considering non-Gaussianity and CMB observations.

Findings

PBH clustering behaves as matter isocurvature perturbation if PBHs make up dark matter.

Constraints exclude PBH-DM scenarios with certain single-field origins even with small non-Gaussianity.

Different source fields for PBHs and CMB perturbations relax some constraints.

Abstract

Primordial black holes (PBHs) are theoretical black holes which may be formed during the radiation dominant era and, basically, caused by the gravitational collapse of radiational overdensities. It has been well known that in the context of the structure formation in our Universe such collapsed objects, e.g., halos/galaxies, could be considered as bias tracers of underlying matter fluctuations and the halo/galaxy bias has been studied well. Employing a peak-background split picture which is known to be a useful tool to discuss the halo bias, we consider the large scale clustering behavior of the PBH and propose an almost mass-independent constraint to the scenario that dark matters (DMs) consist of PBHs. We consider the case where the statistics of the primordial curvature perturbations is almost Gaussian, but with small local-type non-Gaussianity. If PBHs account for the DM abundance, such a large scale clustering of PBHs behaves as nothing but the matter isocurvature perturbation and constrained strictly by the observations of cosmic microwave backgrounds (CMB). From this constraint, we show that, in the case a certain single field causes both CMB temperature perturbations and PBH formations, the PBH-DM scenario is excluded even with quite small local-type non-Gaussianity, $|f_\mathrm{NL}|\sim\mathcal{O}(0.01)$, while we give the constraints to parameters in the case where the source field of PBHs is different from CMB perturbations.