Gravitational waves from the Axiverse

Models with axion like particles (ALPs) often predict the formation of a string-domain wall network in the early universe. We study how such networks of defects appear in the context of string theory, and discuss the conditions for their long-term stability. In a scenario with several axions, we show how a bias term in the potential arises naturally from the effects of multiple instantons, leading to the eventual decay of the domain walls. We find that the annihilation of the network leads to the generation of a stochastic gravitational wave background (SGWB) with a spectrum that has characteristic contributions from both walls and strings. The unique shape of the spectrum provides an opportunity to probe string theory axions at existing and upcoming observatories. The extinction of the network is also accompanied by the production of different axion mass eigenstates. In a region of the parameter space, the lightest eigenstate can be long lived and make up the dark matter in the universe.