Orbital eccentricities in primordial black holes binaries

Understand

It was recently suggested that the merger of $\sim30\,M_\odot$ primordial black holes (PBHs) may provide a significant number of events in gravitational-wave observatories over the next decade, if they make up an appreciable fraction of the dark matter.

• Here we show that measurement of the eccentricities of the inspiralling binary black holes can be used to distinguish these binaries from those produced by more traditional astrophysical mechanisms.
• These PBH binaries are formed on highly eccentric orbits and can then merge on timescales that in some cases are years or less, retaining some eccentricity in the last seconds before the merger.
• This is to be contrasted with massive-stellar-binary, globular-cluster, or other astrophysical origins for binary black holes (BBHs) in which the orbits have very effectively circularized by the time the BBH enters the observable LIGO window.