Orbital eccentricities in primordial black holes binaries

TL;DR

Measuring eccentricities in primordial black hole binaries can distinguish them from astrophysical sources, providing evidence for PBHs as dark matter if detectable eccentricities are observed by LIGO and the Einstein Telescope.

Contribution

This work demonstrates that eccentricity measurements can identify primordial black hole binaries and predicts detection rates for current and future gravitational-wave observatories.

Findings

LIGO could detect roughly one eccentric PBH merger in six years.

The Einstein Telescope could observe about ten such events in ten years.

Eccentricity signatures can differentiate PBH mergers from astrophysical origins.

Abstract

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. Here we discuss the features of the gravitational-wave signals that indicate this eccentricity and forecast the sensitivity of LIGO and the Einstein Telescope to such effects. We show that if PBHs make up the dark matter, then roughly one event should have a detectable eccentricity given LIGO's expected sensitivity and observing time of six years. The Einstein Telescope should see $O(10)$ such events after ten years.