We propose a novel method to probe primordial gravitational waves by means of primorial black holes (PBHs). When the amplitude of initial tensor perturbations generated during inflation on comoving scales much smaller than those relavant to Cosmic Microwave Background is very large, it induces scalar perturbations due to second-order effects. If the amplitude of resultant scalar perturbations becomes too large, then too many PBHs are produced to a level that is inconsistent with a variety of existing observational constraints on the abundance of PBHs. This consideration leads to upper bounds on the amplitude of primordial tensor perturbations. The resultant PBH upper bounds turn out be tighter than other bounds obtained from Big Bang Nucleosynthesis, Cosmic Microwave Background, LIGO and Virgo.
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