Primordial Black Holes as Generators of Cosmic Structures

TL;DR

Primordial black holes could explain dark matter, seed galaxy formation, and produce detectable gravitational waves, offering a unified framework for understanding cosmic structure and black hole origins.

Contribution

This paper explores how primordial black holes can simultaneously serve as dark matter, seed galaxy formation, and generate gravitational wave backgrounds, highlighting their multifaceted cosmological roles.

Findings

PBHs can account for dark matter in certain mass ranges.

PBHs can seed supermassive black holes and early galaxy formation.

Gravitational wave background from PBHs spans LIGO to LISA frequencies.

Abstract

Primordial black holes (PBHs) could provide the dark matter in various mass windows below $10^2 M_{\odot}$ and those of $30 M_{\odot}$ might explain the LIGO events. PBHs much larger than this might have important consequences even if they provide only a small fraction of the dark matter. In particular, they could generate cosmological structure either individually through through the `seed' effect or collectively through the `Poisson' effect, thereby alleviating some problems associated with the standard CDM scenario. If the PBHs all have a similar mass and make a small contribution to the dark matter, then the seed effect dominates on small scales, in which case PBHs could seed the supermassive black holes in galactic nuclei or even galaxies themselves. If they have a similar mass and provide the dark matter, the Poisson effect dominates on all scales and the first bound clouds would form earlier than in the usual scenario, with interesting observational consequences. If the PBHs have an extended mass spectrum, which is more likely, they could fulfill all three roles - providing the dark matter, binding the first bound clouds and generating galaxies. In this case, the galactic mass function naturally has the observed form, with the galaxy mass being simply related to the black hole mass. The stochastic gravitational wave background from the PBHs in this scenario would extend continuously from the LIGO frequency to the LISA frequency, offering a potential goal for future surveys.