Stochastic gravitational waves associated with the formation of primordial black holes

TL;DR

This paper explores how non-Gaussian primordial fluctuations can lead to stochastic gravitational waves associated with primordial black hole formation, potentially evading current observational constraints.

Contribution

It demonstrates that non-Gaussian initial conditions can alter gravitational wave predictions, relaxing constraints from pulsar timing arrays.

Findings

Non-Gaussian fluctuations can evade pulsar timing constraints.

Induced gravitational waves can be larger or smaller than black hole binary backgrounds.

Primordial black hole formation is linked to stochastic gravitational wave signals.

Abstract

Primordial black hole (PBH) mergers have been proposed as an explanation for the gravitational wave events detected by the LIGO collaboration. Such PBHs may be formed in the early Universe as a result of the collapse of extremely rare high-sigma peaks of primordial fluctuations on small scales, as long as the amplitude of primordial perturbations on small scales is enhanced significantly relative to the amplitude of perturbations observed on large scales. One consequence of these small-scale perturbations is generation of stochastic gravitational waves that arise at second order in scalar perturbations, mostly before the formation of the PBHs. These induced gravitational waves have been shown, assuming gaussian initial conditions, to be comparable to the current limits from the European Pulsar Timing Array, severely restricting this scenario. We show, however, that models with enhanced fluctuation amplitudes typically involve non-gaussian initial conditions. With such initial conditions, the current limits from pulsar timing can be evaded. The amplitude of the induced gravitational-wave background can be larger or smaller than the stochastic gravitational-wave background from supermassive black hole binaries.