Testing Primordial Black Holes as Dark Matter through LISA

TL;DR

This paper explores how LISA can detect gravitational waves from primordial black holes around 10^{-12} solar masses, providing a potential test for PBHs as dark matter through GW two-point correlator measurements.

Contribution

It demonstrates that LISA can test PBH dark matter models by measuring GW two-point correlators, while showing the bispectrum remains unobservable due to propagation effects.

Findings

LISA can detect GWs from light PBHs as dark matter.

The GW bispectrum is suppressed and unobservable.

The scenario provides a testable link between PBHs and GWs.

Abstract

The idea that primordial black holes (PBHs) can comprise most of the dark matter of the universe has recently reacquired a lot of momentum. Observational constraints, however, rule out this possibility for most of the PBH masses, with a notable exception around $10^{-12} M_\odot$. These light PBHs may be originated when a sizeable comoving curvature perturbation generated during inflation re-enters the horizon during the radiation phase. During such a stage, it is unavoidable that gravitational waves (GWs) are generated. Since their source is quadratic in the curvature perturbations, these GWs are generated fully non-Gaussian. Their frequency today is about the mHz, which is exactly the range where the LISA mission has the maximum of its sensitivity. This is certainly an impressive coincidence. We show that this scenario of PBHs as dark matter can be tested by LISA by measuring the GW two-point correlator. On the other hand, we show that the short observation time (as compared to the age of the universe) and propagation effects of the GWs across the perturbed universe from the production point to the LISA detector suppress the bispectrum to an unobservable level. This suppression is completely general and not specific to our model.