On the single-event-based identification of primordial black hole mergers at cosmological distances

Abstract

The existence of primordial black holes (PBHs), which may form from the collapse of matter overdensities shortly after the Big Bang, is still under debate. Among the potential signatures of PBHs are gravitational waves (GWs) emitted from binary black hole (BBH) mergers at redshifts z 30, where the formation of astrophysical black holes is unlikely. Future ground-based GW detectors, Cosmic Explorer and Einstein Telescope, will be able to observe equal-mass BBH mergers with total mass of O(10 − 100) M at such distances. In this work, we investigate whether the redshift measurement of a single BBH source can be precise enough to establish its primordial origin. We simulate BBHs of different masses, mass ratios and orbital orientations. We show that for BBHs with total masses between 20 M and 40 M merging at z ≥ 40 one can infer z > 30 at up to 97% credibility, with a network of one Einstein Telescope, one 40-km Cosmic Explorer in the US and one 20-km Cosmic Explorer in Australia. A smaller network made of one Einstein Telescope and one 40-km Cosmic Explorer in the US measures z > 30 at larger than 90% credibility for roughly half of the sources than the larger network. 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.

Stochastic gravitational-wave background as a tool to investigate multi-channel astrophysical and primordial black-hole mergers

“…Another example for distinguishing between channels is that in the case the GC channel dominates the SGWB, one would expect to be able to identify a cusp in Ω GW in LISA frequency band . Other possible ways to discriminate among different channels is the measurement of the merger bias at 3G detectors through the study of the cross-correlation with the large scale structure (Cusin et al 2017(Cusin et al , 2018Scelfo et al 2018;Mukherjee & Silk 2020a,b;Calore et al 2020;Yang et al 2021), the study of the time evolution of the high redshift merger rates (De Luca et al 2021b;Ng et al 2021), and the reconstruction of the spectral shape pre-merger via small band searches.…”

Stochastic Gravitational-Wave Backgrounds: Current Detection Efforts and Future Prospects

“…Note that this remains subject to the theoretically uncertain rate of such events that the nature provided us with; let us cite several investigations on the matter [255,[324][325][326]. Attribution of a merged black hole binary to primordial and not stellar origin based on redshift alone will not be statistically strong even with 40-kilometer Cosmic Explorer, 20-kilometer Cosmic Explorer South, and Einstein Telescope [327]. This means that even in the scenario where primordial black holes are abundant, studies of these events will require a combination of fitting for their ensemble properties as well as cross-correlation-based searches for the stochastic background that are sensitive to subthreshold signals.…”

How to assess the primordial origin of single gravitational-wave events with mass, spin, eccentricity, and deformability measurements

“…The time delay between Pop III star formation and BBH mergers was studied using population synthesis models, and found to be around O(10) Myr [71,[74][75][76][77][78][79][80][81][82]. This means that we can conservatively assume BBHs from Pop III remnants to merge below z ≈ 30, and consider merger redshifts z 30 to be smoking guns for primordial binaries [32,69,83].…”