Merger rates in primordial black hole clusters without initial binaries

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Primordial black holes formed through the collapse of cosmological density fluctuations have been hypothesized as contributors to the dark matter content of the Universe. At the same time, their mergers could contribute to the recently observed population of gravitational-wave sources. We investigate the scenario in which primordial black holes form binaries at late times in the Universe. Specifically, we re-examine the mergers of primordial black holes in small clusters of ∼30 objects in the absence of initial binaries. Binaries form dynamically through Newtonian gravitational interactions. These binaries act as heat sources for the cluster, increasing the cluster's velocity dispersion, which inhibits direct mergers through gravitational-wave two-body captures. Meanwhile, three-body encounters of tight binaries are too rare to tighten binaries sufficiently to allow them to merge through gravitational-wave emission. We conclude that in the absence of initial binaries, merger rates of primordial black holes in the considered scenario are at least an order of magnitude lower than previously suggested, which makes gravitational-wave detections of such sources improbable.


Enheter & grupper
Externa organisationer
  • Leiden University
  • University of Birmingham
  • Monash University
  • Swinburne University of Technology
  • University of Maryland
  • University of Copenhagen

Ämnesklassifikation (UKÄ) – OBLIGATORISK

  • Astronomi, astrofysik och kosmologi


Sidor (från-till)994-1000
Antal sidor7
TidskriftMonthly Notices of the Royal Astronomical Society
Utgåva nummer1
StatusPublished - 2020
Peer review utfördJa