Gravitational Waves from Primordial Black Holes: Connecting Low-Frequency Scalar-Induced Signatures to High-Frequency Binary Mergers

Formation of primordial black holes (PBHs) requires a significant enhancement of curvature perturbations. This enhancement generates a twofold gravitational wave (GW) signature: a low-frequency stochastic background of scalar-induced GWs (SIGWs) and a distinct high-frequency signal from subsequent PBH binary mergers. Assuming a monochromatic PBH mass function, we use PBH abundance constraints on the primordial curvature power spectrum to evaluate the stochastic SIGW background. We also compute the stochastic GW background from mergers of the corresponding PBH binaries, incorporating merger-rate suppression effects to obtain realistic estimates. Furthermore, we derive a model-independent correspondence between the characteristic frequencies of these two signals. This unified framework links these otherwise distinct GW channels, enabling the same primordial fluctuations to be probed across widely separated frequency bands.