The Hidden Link between Pulsar Timing Arrays and LIGO

In June 2023, Pulsar Timing Array (PTA) collaborations announced the groundbreaking discovery of a Stochastic Gravitational Wave (GW) Background at nano-Hertz frequencies. The origin of this signal remains uncertain, potentially arising from supermassive black hole binaries or from exotic phenomena in the early universe, such as first-order phase transitions, domain walls, or curvature peaks. Intriguingly, recent studies suggest that if the PTA GW signal has a primordial origin, then a non-negligible fraction of dark matter could be composed of solar-mass primordial black holes. These primordial black holes are currently under investigation by the LIGO-VIRGO-KAGRA interferometers. This highlights a fascinating connection between the nano-Hertz and kilo-Hertz GW observatories, both of which are probing the same class of early universe scenarios: violent exotic phenomena occurring around the epoch of quark confinement.