Authors:
(1) Antonio Riotto, Département de Physique Theorique, Universite de Geneve, 24 quai Ansermet, CH-1211 Geneve 4, Switzerland and Gravitational Wave Science Center (GWSC), Universite de Geneve, CH-1211 Geneva, Switzerland;
(2) Joe Silk, Institut d’Astrophysique, UMR 7095 CNRS, Sorbonne Universite, 98bis Bd Arago, 75014 Paris, France, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore MD 21218, USA, and Beecroft Institute of Particle Astrophysics and Cosmology, Department of Physics, University of Oxford, Oxford OX1 3RH, UK.
Table of Links
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?
3.3 Plugging the pair instability gap with PBH?
3.4 PBH eccentricity, 3.5 PBH spin and 3.6 Future gamma-ray telescopes
3.2 Sub-solar PBHs
The detection of sub-solar BHs in a compact binary merger is regarded as one of the smoking gun signatures of PBHs. Indeed, BHs of astrophysical origin are not expected. The sub-solar mass range may be however populated by other compact objects like neutron stars, white dwarfs, or exotic compact objects [51] (e.g. boson stars [52]).
GW signatures of any population of PBH below the astrophysical production provide a powerful statistical constraint on any solar or subsolar component, as evidenced by the sharp decline observed in 03 events. There could of course be rare events generated by solar or subsolar PBHs that might contribute to DM at the O(10%) level. One key ingredient is therefore to distinguish PBHs from other compact objects, e.g. through the measurements of the tidal disruption and tidal deformability Λ. A recent analysis has shown that for subsolar neutron-star binaries, the O4 and O5 projected sensitivities would allow measuring the effect of tidal disruption on the waveform in a large portion of the parameter space, also constraining the tidal deformability at O(10%) level, thus excluding a primordial origin of the binary. Viceversa, for subsolar PBH binaries, model-agnostic upper bounds on the tidal deformability can rule out neutron stars or more exotic competitors. Assuming events similar to the sub-threshold candidate SSM200308 reported in LVK O3b data are PBH binaries, O4 projected sensitivity would allow ruling out the presence of neutron-star tidal effects at ≈ 3σ C.L., thus strengthening the PBH hypothesis. Future experiments would lead to even stronger (> 5σ) conclusions on potential discoveries of this kind, see Fig. 2.
PBHs of lunar mass, a regime where hints of unaccounted-for microlensing events have been reported by the OGLE survey of the galactic bulge [53], could be sufficiently numerous that nearby binaries might be detectable with future generation detectors [54].
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