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

Understand

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.