Primordial magnetic fields as the birthplace of ultralight black holes

A magnetogenesis model during inflation amplifies curvature perturbations at small scales, producing ultralight primordial black holes across a broad mass range. These black holes evaporate quickly, leaving imprints on the reheating era. The resulting stochastic gravitational wave background — sourced by both the electromagnetic spectrum and the evaporating black holes themselves — carries fingerprints of the underlying magnetic field model parameters, making the scenario testable with future gravitational wave detectors.