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 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 2.2 What is the effect of PBH clustering? Another open issue worth mentioning is the clustering of PBHs. It may significantly affect the merger rates of coalescing binaries and, consequently, the gravitational wave signal measured by the LIGO/Virgo collaboration and by future experiments. Indeed, even if isolated PBH binaries are not significantly affected by encounters with a third PBH, PBH binaries residing in PBH clusters might suffer such interactions, thus modifying the expected merger rate. If PBHs cluster or not is therefore an important question to address. A significant PBH clustering might also modify constraints arising from microlensing observations and from the cosmic microwave background. Clustering might also be relevant in determining PBH spins. While the gravitational collapse of a spherical over density during radiation domination generates nearly spinless PBHs [6], they may acquire a large spin in the presence of accretion [7]. Indeed, if PBHs cluster they can more easily merge to form binaries at late times, so that the resulting PBH coming from such merger will have a non-zero value even when no accretion is present. PBHs, even if rare, are potentially important for seeding galaxy and even quasar formation at very high redshift [11]. These would be intermediate mass PBHs that enhanced local growth by gravitational instability of cold dark matter. in such a scenario, both massive galaxies and SMBH would form at epochs much earlier than in the standard model of structure formation. Moreover SMBH formation would precede or be contemporaneous with massive galaxy formation, as actually favored by recent JWST observations [12]. This paper is available on arxiv under CC BY 4.0 DEED license. 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. Authors: 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 Abstract and 1 Introduction Abstract and 1 Introduction 2 Some open questions 2 Some open questions 2.1 What is the abundance of PBHs? 2.1 What is the abundance of PBHs? 2.2 What is the effect of PBH clustering? 2.2 What is the effect of PBH clustering? 2.3 What fraction of the currently observed GW events can be ascribed to PBHs? 2.3 What fraction of the currently observed GW events can be ascribed to PBHs? 2.4 Are PBHs the Dark Matter? 2.4 Are PBHs the Dark Matter? 3 The PBH Roadmap 3 The PBH Roadmap 3.1 High redshift mergers 3.1 High redshift mergers 3.2 Sub-solar PBHs 3.2 Sub-solar PBHs 3.3 Plugging the pair instability gap with PBH? 3.3 Plugging the pair instability gap with PBH? 3.4 PBH eccentricity, 3.5 PBH spin and 3.6 Future gamma-ray telescopes 3.4 PBH eccentricity, 3.5 PBH spin and 3.6 Future gamma-ray telescopes 4 Conclusions and References 4 Conclusions and References 2.2 What is the effect of PBH clustering? Another open issue worth mentioning is the clustering of PBHs. It may significantly affect the merger rates of coalescing binaries and, consequently, the gravitational wave signal measured by the LIGO/Virgo collaboration and by future experiments. Indeed, even if isolated PBH binaries are not significantly affected by encounters with a third PBH, PBH binaries residing in PBH clusters might suffer such interactions, thus modifying the expected merger rate. If PBHs cluster or not is therefore an important question to address. A significant PBH clustering might also modify constraints arising from microlensing observations and from the cosmic microwave background. Clustering might also be relevant in determining PBH spins. While the gravitational collapse of a spherical over density during radiation domination generates nearly spinless PBHs [6], they may acquire a large spin in the presence of accretion [7]. Indeed, if PBHs cluster they can more easily merge to form binaries at late times, so that the resulting PBH coming from such merger will have a non-zero value even when no accretion is present. PBHs, even if rare, are potentially important for seeding galaxy and even quasar formation at very high redshift [11]. These would be intermediate mass PBHs that enhanced local growth by gravitational instability of cold dark matter. in such a scenario, both massive galaxies and SMBH would form at epochs much earlier than in the standard model of structure formation. Moreover SMBH formation would precede or be contemporaneous with massive galaxy formation, as actually favored by recent JWST observations [12]. This paper is available on arxiv under CC BY 4.0 DEED license. This paper is available on arxiv under CC BY 4.0 DEED license. available on arxiv

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