Gravitational waves from dark domain walls

For most of cosmic history, the evolution of our Universe has been governedby the physics of a 'dark sector', consisting of dark matter and dark energy,whose properties are only understood in a schematic way. The influence of theseconstituents is mediated exclusively by the force of gravity, meaning thatinsight into their nature must be gleaned from gravitational phenomena. Theadvent of gravitational-wave astronomy has revolutionised the field of blackhole astrophysics, and opens a new window of discovery for cosmologicalsources. Relevant examples include topological defects, such as domain walls orcosmic strings, which are remnants of a phase transition. Here we present thefirst simulations of cosmic structure formation in which the dynamics of thedark sector introduces domain walls as a source of stochastic gravitationalwaves in the late Universe. We study in detail how the spectrum ofgravitational waves is affected by the properties of the model, and extrapolatethe results to scales relevant to the recent evidence for a stochasticgravitational wave background. Our relativistic implementation of the fielddynamics paves the way for optimal use of the next generation of gravitationalexperiments to unravel the dark sector.