Primordial Black Holes as Cosmic Architects: Imprints of an extended mass function

Abstract

Primordial black holes (PBHs) remain compelling dark-matter candidates and possible relics of early-Universe physics, with observable imprints across both cosmological structure formation and gravitational-wave astronomy. We investigate the consequences of a PBH population described by an extended, physically motivated mass function shaped by features in the thermal history of the early Universe. We compute the associated Poisson contribution to the matter power spectrum and propagate its effects to the halo mass function, the cosmic star-formation rate density, and the reionization history. The PBH component produces a moderate enhancement of small-scale power, leading to earlier low-mass halo formation and a correspondingly mild increase in the high-redshift star-formation rate density while remaining consistent with current constraints on the Thomson optical depth. We then evaluate the merger-rate distribution expected for current and future gravitational-wave detectors. In the stellar-mass regime, the predicted PBH binary population occupies the region probed by LIGO-Virgo-KAGRA and can contribute to the observed compact-object merger population. For LISA, the detectable signal is dominated by intermediate-mass binaries with component masses of $10^3$-$10^4\,M_\odot$, with cumulative rates of order a few events per year extending to redshifts as high as $z \sim 50$. The detection of such high-redshift intermediate-mass black hole mergers would constitute a distinctive signature of a primordial origin, providing evidence that PBHs contribute non-negligibly to the dark matter budget and may have served as early seeds for the supermassive black holes observed at later cosmic epochs.