The effective Nambu-Goto description of ($2+1$)-dimensional domain walls predicts singular behavior of its world sheet resulting in swallowtail bifurcations. This phenomenon is intimately related to the formation of cusps, which emerge in different forms that we identify and classify. We describe in detail how swallowtail bifurcations generically arise in the collision of wiggles on straight domain wall strings, as well as in the collapse of closed loops, even for smooth initial conditions. Remarkably, by means of accurate lattice simulations, we find that these distinctive swallowtail features are reproduced in the field theory evolution of sufficiently thin walls, typically emitting a significant fraction of their initial energy in the process. These results suggest that such singular evolutions could potentially have important implications for the observable signatures associated with the collapse of domain wall networks in $3+1$ dimensions in the early universe.

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• Cosmic strings & domain walls