Explaining PTA data with inflationary GWs in a PBH-dominated universe

We show that an ultralight primordial black hole (PBH) dominated phase makes blue-tilted inflationary gravitational waves (BGW) compatible with the recent detection of an nHz stochastic GW background by pulsar-timing arrays (PTAs), for high reheating temperatures. This PBH-dominated phase suppresses the BGW spectrum via entropy dilution, making it consistent with strong constraints from LIGO O3 and BBN. A future observation of such GW spectral features at mid or high frequencies can strongly constrain the possible mass range of PBHs. Since PBHs can exhibit various other unique signatures, they can be distinguished from spectra generated by other types of matter domination or exotic physics. Therefore, while important for studying GWs in the nHz range, the recent PTA result also sets the stage for testing and constraining various well-studied mechanisms following a PBH domination, using low-frequency measurements and correlated observations of unique high-frequency GW spectral features.