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
$1\,M_ødot$ could provide much of the dark matter in galactic halos, this
being allowed by the Large Magellanic Cloud 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 $1\,M_ødot$, and this would allow
PBHs to explain much of the evidence in a unified way.
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