Clustering and runaway merging in a primordial black hole dominated universe

Light Primordial Black Holes (PBH) can dominate the universe before the onset of the radiation epoch and may evaporate prior to Big Bang Nucleosynthesis (BBN). The authors show that if this PBH-dominated phase lasts sufficiently long, PBHs can form dense clusters whose mergers exhibit runaway behavior, producing massive black holes that survive beyond BBN. Such relics can significantly alter the PBH mass distribution and yield distinctive observational signatures, thereby constraining regions of parameter space that were previously considered viable.