Laboratory observation of caustics, optical simulation of the motion of particles, and cosmology

Laboratory experiments on the observation of caustics when light rays are refracted by a “random” smooth surface are described. The analogy between caustics in geometrical optics and the singularities that arise in irrotational flows in a cold collisionless medium is discussed. The analogy is then extended to a very important cosmological process–the formation of the large-scale structure of the Universe. It is shown that in an expanding Universe the evolution of smooth irrotational perturbations, which grow under the influence of the gravitational force, leads to the formation of structures geometrically similar to generic optical caustics.