Can we distinguish astrophysical from primordial black holes via the stochastic gravitational wave background?

TL;DR

This paper explores how the stochastic gravitational wave background can be used to differentiate between astrophysical and primordial black holes by analyzing their merger rate evolution across cosmic history.

Contribution

It proposes a joint estimation method of hybrid merger rates from gravitational wave data to distinguish black hole origins, considering cosmic merger rate evolution.

Findings

Current data provide weak constraints on black hole merger rates.

Future detector sensitivities could enable population distinction.

Estimated parameters suggest potential for differentiating black hole origins.

Abstract

One of the crucial windows for distinguishing astrophysical black holes from primordial black holes is through the redshift evolution of their respective merger rates. The low redshift population of black holes of astrophysical origin is expected to follow the star formation rate. The corresponding peak in their merger rate peaks at a redshift smaller than that of the star formation rate peak ($z_p \approx 2$), depending on the time delay between the formation and mergers of black holes. Black holes of primordial origin are going to be present before the formation of the stars, and the merger rate of these sources at high redshift is going to be large. We propose a joint estimation of a hybrid merger rate from the stochastic gravitational wave background, which can use the cosmic history of merger rates to distinguish between the two populations of black holes. Using the latest bounds on the amplitude of the stochastic gravitational wave background amplitude from the third observation run of LIGO/Virgo, we obtain weak constraints at $68\%$ C.L. on the primordial black hole merger rate index $2.56_{-1.76}^{+1.64}$ and astrophysical black hole time delay $6.7_{-4.74}^{+4.22}$ Gyr. We should be able to distinguish between the different populations of black holes with the forthcoming O5 and A+ detector sensitivities.