Cosmic simulations of axion: probing dark matter and gravitational waves

Recently, pulsar timing array experiments observed a signature of the stochastic gravitational wave (GW) background around the nanohertz frequency range, but the first direct evidence of particle dark matter in the laboratory is still missing. Topological defects in the early universe, such as cosmic strings and domain walls, can emit GW radiation and particles that may act as dark matter, thus providing an opportunity to bridge observable gravitational wave signatures with the phenomenology of particle physics. Based on high-resolution real-time simulations of the early Universe, we demonstrate for the first time that axion-like particles of topological defects origin can simultaneously account for dark matter and the stochastic GW background observed in recent pulsar timing array experiments. Our simulations incorporate the evolution of cosmic strings and string-wall networks following both a high-scale Peccei-Quinn phase transition and a subsequent low-scale QCD phase transition. Our findings reveal that these string-wall networks generate observable GW signatures in the nanohertz to millihertz frequency band, which corresponds to axion-like particle dark matter masses ranging from keV to GeV.