Primordial black holes as a novel probe of primordial gravitational waves

TL;DR

This paper introduces a new method to constrain primordial gravitational waves by analyzing the overproduction of primordial black holes caused by large tensor perturbations, providing complementary bounds to existing methods.

Contribution

It presents a novel approach linking primordial black hole overproduction to limits on primordial gravitational wave amplitudes on small scales.

Findings

Derived upper bounds on tensor perturbations from PBH overproduction.

Compared PBH-based bounds with existing constraints from other observations.

Showed PBHs can serve as a new probe for primordial gravitational waves.

Abstract

We propose a novel method to probe primordial gravitational waves by means of primordial black holes (PBHs). When the amplitude of primordial tensor perturbations on comoving scales much smaller than those relevant to Cosmic Microwave Background is very large, it induces scalar perturbations due to second-order effects substantially. If the amplitude of resultant scalar perturbations becomes too large, then PBHs are overproduced to a level that is inconsistent with a variety of existing observations constraining their abundance. This leads to upper bounds on the amplitude of initial tensor perturbations on super-horizon scales. These upper bounds from PBHs are compared with other existing bounds.