The Future of Primordial Black Holes: Open Questions and Roadmap

Table of Links

Abstract and 1 Introduction

2 Some open questions

2.1 What is the abundance of PBHs?

2.2 What is the effect of PBH clustering?

2.3 What fraction of the currently observed GW events can be ascribed to PBHs?

2.4 Are PBHs the Dark Matter?

3 The PBH Roadmap

3.1 High redshift mergers

3.2 Sub-solar PBHs

3.3 Plugging the pair instability gap with PBH?

3.4 PBH eccentricity, 3.5 PBH spin and 3.6 Future gamma-ray telescopes

4 Conclusions and References

Abstract

We discuss some of the the open questions and the roadmap in the physics of primordial black holes. Black holes are the only dark matter candidate that is known to actually exit. Their conjectured primordial role is admittedly based on hypothesis rather than fact, most straightforwardly as a simple extension to the standard models of inflation, or even, in homage to quantum physics, more controversially via a slowing-down of Hawking evaporation. Regardless of one’s stance on the theoretical basis for their existence, the possibility of primordial black holes playing a novel role in dark matter physics and gravitational wave astronomy opens up a rich astrophysical phenomenology that we lay out in this brief overview.

Introduction

Primordial black holes (PBHs) provide an attractive way to resolve the dark matter problem. Unlike essentially all other dark matter particle candidates, BHs actually exist, and there are several robust prospects for determining whether there is a primordial component over a broad mass range.

Theoretically PBHs may populate the universe in a humongous range of masses from 10-20 Mo to 1020 Mo. They could be the dark matter for masses around 10-12 Mo. Moreover even if PBHs are dark matter-subdominant,, they could be responsible for some of the currently observed mergers of BHs in the solar mass range or they may play key roles as rare IMBH (Intermediate Mass BHs in the range (103 - 106)Mo that may seed both SMBH (Supermassive Black Holes observed in the mass range (10° - 101º)Mo and massive galaxies in the early universe.

PBHs formed very early and hence may be ubiquitous at the highest redshifts accessible by direct imaging, that is & ~ 100, when there was little astrophysical competition. This provides us with the possibility of finding truly unique high redshift signatures of PBHs, for example via gravity wave interferometers such as Einstein Telescope and Cosmic Explorer that will probe z ~ 20, for potential sources of EMRI signals.

Well before these telescopes are in action, there will be 21cm dark ages telescopes destined for the lunar far side, such as LUSEE-Night and DSL to probe PBH accretion imprints on diffuse hydrogen absorption against the CMB to z ~ 100, both scheduled for deployment in 2026. And on a similar time-scale we anticipate LIGO-class gravity wave telescopes to make significant inroads on the possible existence of solar mass black holes as well as black holes populating the "forbidden" region in the pair instability gap, at masses ≥ 50 Mo. Such observations would provide further evidence for the existence of PBHs.

We review some of these arguments in this chapter, with particular attention to the open questions, challenges and future opportunities.