Merger rates in primordial black hole clusters without initial binaries. (arXiv:1911.03483v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Korol_V/0/1/0/all/0/1">Valeriya Korol</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mandel_I/0/1/0/all/0/1">Ilya Mandel</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Miller_M/0/1/0/all/0/1">M. Coleman Miller</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Church_R/0/1/0/all/0/1">Ross P. Church</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Davies_M/0/1/0/all/0/1">Melvyn B. Davies</a>
Primordial black holes formed through the collapse of cosmological density
fluctuations have been hypothesised 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
mergers of primordial black holes in small clusters of $sim 30$ 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 are at least an
order of magnitude lower than previously suggested, and are thus insufficient
to allow for gravitational-wave detections of such sources. Conversely,
gravitational-wave observations cannot constrain the contribution of primordial
black holes to the dark matter density in this scenario.
Primordial black holes formed through the collapse of cosmological density
fluctuations have been hypothesised 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
mergers of primordial black holes in small clusters of $sim 30$ 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 are at least an
order of magnitude lower than previously suggested, and are thus insufficient
to allow for gravitational-wave detections of such sources. Conversely,
gravitational-wave observations cannot constrain the contribution of primordial
black holes to the dark matter density in this scenario.
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