PI: Paul Shellard

Since the recent detection of a stochastic gravitational wave background (SGWB) by PTAs (pulsar timing arrays) there has been renewed interest in their potential primordial origin due to violent processes in the early universe. One of these alternatives for generating gravitational waves is through the collapse and demise of networks of domain walls because this process can alleviate the tension between the detected spectrum of gravitational waves and astrophysical models based on the emission from supermassive black hole binaries. Typically, the emission from domain walls is modelled by assuming an instantaneous collapse. We performed a suite of large-scale simulations of networks of domain walls, where we break the $\mathbb{Z}_2$ symmetry by introducing a small bias term, so that one of the two vacuum states is energetically preferred. By varying the size of this bias term we can produce networks that begin to collapse at different characteristic timescales. Throughout our simulations, we compute the spectrum of gravitational waves emitted and analyse the spectral shape. This allowed us to show that a more realistic, prolonged collapse phase leads to an enhancement of the power emitted into gravitational waves and a high frequency tail which becomes less steep for larger biases. These features of the spectrum can potentially be used to distinguish a domain wall origin for the SGWB from other possible sources and to determine the timescale of the network collapse.