Identification of primordial black hole mergers at cosmological distances*

The abundance of primordial black holes (PBHs), which may form from the collapse of primordial overdensities right after the Big Bang, is still uncertain. One of the smoking gun evidence can be the gravitational wave (GWs) emitted from binary black hole (BBH) mergers of PBHs at redshifts $z\gtrsim 30$, where the formation of astrophysical black holes is unlikely. Future ground-based GW detectors, Cosmic Explorer ICE) and Einstein Telescope (ET), will be able to observe equal-mass BBH mergers with total mass of $\mathcal{O}(10-100)~\msun$ at such distances. We simulate BBHs of different masses, mass ratios and orbital orientations to investigate whether the redshift measurement of a single BBH source can be precise enough to establish its primordial origin. With a network of one ET in Europe, one 40-km CE in the US and one 20-km CE in Australia, we show that one can constrain $z>30$ at up to 97\% credibility for BBHs with total masses between $20~\msun$ and $40~\msun$ merging at $z \geq 40$ one.

We then assess the dependence of this result on the Bayesian redshift priors used for the analysis, specifically on the relative abundance of the BBH mergers originated from the first stars, and the primordial BBH mergers.