Caustics are singularities that occur naturally in optical, hydrodynamic and
quantum waves, giving rise to high amplitude patterns that can be described
using catastrophe theory. In this paper we study caustics in a statistical
field theory setting in the form of the sine-Gordon model that describes a
variety of physical systems including coupled 1D superfluids. Specifically, we
use classical field simulations to study the dynamics of two ultracold 1D Bose
gases (quasi-condensates) that are suddenly coupled to each other and find that
the resulting non-equilibrium dynamics are dominated by caustics. Thermal noise
is included by sampling the initial states from a Boltzmann distribution for
phononic excitations. We find that caustics pile up over time in both the
number and phase difference observables leading to a characteristic non-thermal
`circus tent' shaped probability distribution at long times.Comment: 28 pages, 13 figure