[Paper Review] Observational Evidence for Primordial Black Holes: A Positivist Perspective

B. J. Carr, Sébastien Clesse+3 more|arXiv (Cornell University)|Jun 6, 2023

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy427 references23 citations

TL;DR

This review aggregates observational indications for primordial black holes (PBHs) across lensing, dynamical, accretion, and gravitational-wave data, arguing for a unified PBH scenario with an extended mass function.

ABSTRACT

We review numerous arguments for primordial black holes (PBHs) based on observational evidence from a variety of lensing, dynamical, accretion and gravitational-wave effects. This represents a shift from the usual emphasis on PBH constraints and provides what we term a positivist perspective. Microlensing observations of stars and quasars suggest that PBHs of around $1\,M_{\odot}$ could provide much of the dark matter in galactic halos, this being allowed by the Large Magellanic Cloud microlensing observations if the PBHs have an extended mass function. More generally, providing the mass and dark matter fraction of the PBHs is large enough, the associated Poisson fluctuations could generate the first bound objects at a much earlier epoch than in the standard cosmological scenario. This simultaneously explains the recent detection of high-redshift dwarf galaxies, puzzling correlations of the source-subtracted infrared and X-ray cosmic backgrounds, the size and the mass-to-light ratios of ultra-faint-dwarf galaxies, the dynamical heating of the Galactic disk, and the binary coalescences observed by LIGO/Virgo/KAGRA in a mass range not usually associated with stellar remnants. Even if PBHs provide only a small fraction of the dark matter, they could explain various other observational conundra, and sufficiently large ones could seed the supermassive black holes in galactic nuclei or even early galaxies themselves. We argue that PBHs would naturally have formed around the electroweak, quantum chromodynamics and electron-positron annihilation epochs, when the sound-speed inevitably dips. This leads to an extended PBH mass function with a number of distinct bumps, the most prominent one being at around $1\,M_{\odot}$, and this would allow PBHs to explain many of the observations in a unified way.

Motivation & Objective

• Evaluate observational evidence suggesting PBHs contribute to dark matter and cosmological structure formation.
• Assess how PBH clustering, lensing, dynamics, and gravitational waves fit into a unified PBH scenario.
• Discuss mass function shapes and formation epochs that could explain diverse cosmic observations.

Proposed method

• Synthesize diverse observational channels (microlensing, dynamics, accretion, GW) within a unified PBH framework.
• Discuss Poisson clustering and seed effects to model PBH fluctuations and cluster formation.
• Outline mathematical relations linking PBH mass, abundance, and clustering to observable signals (e.g., isocurvature fluctuations, clustering scales).
• Argue for an extended PBH mass function with features tied to early Universe phase transitions.

Experimental results

Research questions

• RQ1 Can PBHs provide a substantial fraction of dark matter without violating existing constraints when the mass function is extended?
• RQ2 How does PBH Poisson clustering influence structure formation, microlensing signals, and gravitational-wave event rates?
• RQ3 What early-Universe epochs and sound speed dips favor PBH formation at specific mass scales (e.g., QCD, electroweak, e+e− annihilation) and how do these relate to observations?
• RQ4 Can a unified PBH scenario explain high-redshift galaxy formation, CIB–XRB correlations, UFDGs properties, and LVK detections together?

Key findings

• PBHs with an extended mass function around solar masses could account for substantial dark matter in galactic halos, compatible with LMC microlensing under certain mass-function assumptions.
• Poisson fluctuations from PBHs lead to early, large-scale clustering and may seed non-linear structures at high redshift, affecting cosmic backgrounds and early galaxy formation.
• PBH clustering can explain features of ultra-faint dwarf galaxies and may influence Galactic disk heating and PBH merger rates observed by LVK.
• A unified PBH framework can potentially address multiple cosmological puzzles, including SMBH seeding and early galaxy formation, even if PBHs form a small DM fraction.
• The most prominent PBH mass feature is around 1 solar mass, arising from specific early-Universe epochs with sound-speed dips, supporting a multi-bump extended mass function.
• PBH clusters evolve through relaxation, heating, and possible evaporation, with dynamical timescales constraining present-day cluster sizes and survivability.