Induced Gravitational Waves and Primordial Black Holes

Abstract

The recent detection of very low-frequency stochastic gravitational wave background (SGWB) by Pulsar Timing Array collaborations like NANOGrav and IPTA has initiated many studies to understand the possible cosmological origin of such a signal. Amongst other candidates, the existence of primordial black holes (PBHs) in the early universe has also been pointed out as a very promising channel to generate such a signal. The most studied mechanism in this context is the formation of near-solar mass primordial black holes which leads to an amplification of SGWB in the relevant frequency range. This particular mechanism suffers from the overproduction of PBHs which can be overcome if we consider the PBHs to form during an extended non-standard reheating phase, instead of the standard radiation era. On the other hand, the resonant amplification of SGWB due to the domination of very low mass PBHs (10-10^5 g) before BBN can also effectively explain the NANOGrav data. We compare these different channels of SGWB generation with Bayesian analysis and find both these scenarios are statistically favored when individually compared with the astrophysical supermassive black hole binary merger model.