Cosmic superstrings, metastable strings and ultralight primordial black holes: from NANOGrav to LIGO and beyond

TL;DR

This paper explores how ultralight primordial black holes can reconcile the gravitational wave signals observed by PTA with the non-detection by LIGO, by affecting the GW spectrum across different frequencies.

Contribution

It introduces a model where ultralight primordial black holes modify the GW spectrum, providing a potential explanation for observational tensions and guiding future detection strategies.

Findings

Ultralight primordial black holes can suppress high-frequency GWs.

The combined PTA and LIGO data can constrain properties of primordial black holes.

Future detectors like LISA and ET can further probe these black holes.

Abstract

While topologically stable cosmic strings are disfavoured by the recent observation of nHz stochastic gravitational waves (GW) by Pulsar Timing Arrays (PTA), e.g., NANOGrav, cosmic metastable strings and superstrings are not. However, because the gravitational waves from all classes of strings generally span a wide range of frequencies, they contradict LIGO's non-observation of stochastic gravitational waves at the $f\sim $ 25 Hz band for a substantial string-parameter space favoured by the PTA data. Suppose ultralight primordial black holes ($M_{\rm BH}<10^9$ g) existed in the early universe. In this case, they reduce the amplitude of the GWs at higher frequencies by providing an early matter-dominated phase, alleviating the tension between LIGO observation and PTA data. We show that the recent PTA data complemented by future LIGO-Virgo-KAGRA (LVK) runs plus detectors such as LISA and ET would be able to dapple the properties and further search strategies of such ultralight primordial black holes which are otherwise fairly elusive as they evaporate in the early universe by Hawking radiation.