Primordial black holes versus their impersonators at gravitational wave observatories

Abstract

The detection of primordial black holes (PBHs) would mark a major breakthrough, with far-reaching implications for early universe cosmology, fundamental physics, and the nature of dark matter. Gravitational wave observations have recently emerged as a powerful tool to test the existence and properties of PBHs, as these objects leave distinctive imprints on the gravitational waveform. Notably, there are no known astrophysical processes that can form sub-solar mass black holes, making their discovery a compelling signal of new physics. In addition to PBHs, we consider other exotic compact object (ECO) candidates-such as strange quark stars and boson stars-which can produce similar gravitational signatures and potentially mimic PBHs. In this work, we employ the Fisher matrix formalism to explore a broad parameter space, including binary masses, spins, and a variety of nuclear and quark matter equations of state. Our goal is to assess the ability of next-generation gravitational wave detectors-specifically Cosmic Explorer and the Einstein Telescope-to distinguish PBHs from ECOs, stellar BHs and neutron stars. We compute the maximum luminosity distances at which confident ($\geq 3\sigma$) detections of sub-solar masses or tidal effects are possible, providing quantitative benchmarks for PBH identification or exclusion under various observational scenarios. Our results indicate that next-generation detectors will be capable of probing sub-solar mass PBHs out to cosmological distances of $z \sim 3$. For heavier objects with masses up to $M \lesssim 2 M_\odot$, we show that PBHs can be distinguished from neutron stars via their lack of tidal effects up to redshifts of $z \sim 0.2$.