Caustics in the sine-Gordon model from quenches in coupled one-dimensional Bose gases

Caustics are singularities that occur naturally in optical, hydrodynamic and quantum waves, giving rise to high amplitude patterns can be described using catastrophe theory. In this paper we study caustics a statistical field theory setting the form of sine-Gordon model describes variety physical systems including coupled 1D superfluids. Specifically, use classical simulations dynamics two ultracold Bose gases (quasi-condensates) suddenly each other find resulting non-equilibrium dominated by caustics. Thermal noise is included sampling initial states from Boltzmann distribution for phononic excitations. We pile up over time both number phase difference observables leading characteristic non-thermal `circus tent' shaped probability at long times.