Abstract
Although the dark matter is usually assumed to be made up of some form of elementary particle, primordial black holes (PBHs) could also provide some of it. However, various constraints restrict the possible mass windows to 1016–1017 g, 1020–1024 g, and 10–103 M⊙. The last possibility is contentious but of special interest in view of the recent detection of black hole mergers by LIGO/Virgo. PBHs might have important consequences and resolve various cosmological conundra even if they account for only a small fraction of the dark matter density. In particular, those larger than 103 M⊙ could generate cosmological structures through the seed or Poisson effect, thereby alleviating some problems associated with the standard cold dark matter scenario, and sufficiently large PBHs might provide seeds for the supermassive black holes in galactic nuclei. More exotically, the Planck-mass relics of PBH evaporations or stupendously large black holes bigger than 1012 M⊙ could provide an interesting dark component.
Highlights
L rays [6], the annihilation line radiation from the Galactic center [7], the reionization of the pregalactic medium [8], and some short-period γ -ray bursts [9]
If primordial black holes (PBHs) make up just 10% of the dark matter, the analytic estimates are reliable and indicate that the constraint from the observed LIGO/Virgo rate is strongest in the mass range 2–160Mࣻ, albeit weakened because of the suppression of mergers
If most of the dark matter is in the form of elementary particles, these will be accreted around any small admixture of PBHs
Summary
PBHs may have been produced during the early Universe through various mechanisms For all such mechanisms, the increased cosmological energy density at early times plays a major role [38], yielding a rough connection between the PBH mass and the horizon mass at formation: M. PBHs may form over an extended period, resulting in a wide range of masses. Even if they formed at a single epoch, their mass spectrum could still extend considerably below the horizon mass because of so-called critical phenomena [39,40,41,42,43], most of the PBH density would still be in the most massive ones
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