Gravitational-wave signatures of primordial black hole clusters and the imprint of an early dense-core collapse
Abstract
Primordial black holes (PBHs) are expected to be born essentially non-spinning and, if the primordial curvature fluctuations possess non-Gaussian tails, strongly clustered. Any spin measured in a PBH population must therefore be dynamically generated. Using direct N-body simulations with \textsc{nbody6++gpu}, modified to treat nearly parabolic dynamical-capture mergers, we quantify the conditions under which an initially non-spinning PBH cluster develops the spin, mass and stochastic-background signatures that gravitational-wave observations are now sensitive to. We find a sharp tension: producing a root-mean-square spin χrms∼0.1 − 0.2 in the 30 − 50 M⊙ range requires initial Plummer radii of order 10−5 pc, whereas survival over cosmological times requires parsec-scale clusters. This tension is resolved naturally if PBHs form with a compact, short-lived core embedded in an extended halo, and we therefore simulate the two regimes as independent blocks. The core block ($4000$ PBHs, Plummer radii 3×10−6 − 10−4 pc) collapses in days to years, producing a burst of mergers whose orbits are strongly non-circular (e→1) and of low dimensionless orbital angular momentum, so that remnant spins are systematically below the quasi-circular value χ≃0.7. Redshifted from their formation epoch, the core mergers deposit a stochastic background peaking in the 10−6--10−4 Hz band accessible to LISA. The picture predicts a specific division of labour between observables, spins set in the primordial collapse, detectable mergers set by the long-term halo, and a two-component stochastic background, and, in particular, that both the spin and the mass data favour a primordial mass function truncated near 50 M⊙.
Related
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BibTeX
@misc{barcelosastre2026gravitationalwave,
title = {{Gravitational-wave signatures of primordial black hole clusters and the imprint of an early dense-core collapse}},
author = {Marc Barceló-Sastre and Juan García-Bellido},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.01331},
eprint = {2609.01331},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.01331},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Gravitational-wave signatures of primordial black hole clusters and the imprint of an early dense-core collapse
AU - Marc Barceló-Sastre
AU - Juan García-Bellido
PY - 2026
DA - 2026/09/01
PB - arXiv
DO - 10.48550/arxiv.2609.01331
UR - https://arxiv.org/abs/2609.01331
AB - Primordial black holes (PBHs) are expected to be born essentially non-spinning and, if the primordial curvature fluctuations possess non-Gaussian tails, strongly clustered. Any spin measured in a PBH population must therefore be dynamically generated. Using direct N-body simulations with \textsc{nbody6++gpu}, modified to treat nearly parabolic dynamical-capture mergers, we quantify the conditions under which an initially non-spinning PBH cluster develops the spin, mass and stochastic-background signatures that gravitational-wave observations are now sensitive to. We find a sharp tension: producing a root-mean-square spin χ_rms∼0.1 − 0.2 in the 30 − 50 M_⊙ range requires initial Plummer radii of order 10^−5 pc, whereas survival over cosmological times requires parsec-scale clusters. This tension is resolved naturally if PBHs form with a compact, short-lived core embedded in an extended halo, and we therefore simulate the two regimes as independent blocks. The core block ($4000$ PBHs, Plummer radii 3×10^−6 − 10^−4 pc) collapses in days to years, producing a burst of mergers whose orbits are strongly non-circular (e→1) and of low dimensionless orbital angular momentum, so that remnant spins are systematically below the quasi-circular value χ≃0.7. Redshifted from their formation epoch, the core mergers deposit a stochastic background peaking in the 10^−6--10^−4 Hz band accessible to LISA. The picture predicts a specific division of labour between observables, spins set in the primordial collapse, detectable mergers set by the long-term halo, and a two-component stochastic background, and, in particular, that both the spin and the mass data favour a primordial mass function truncated near 50 M_⊙.
ER -