Fig. 6
From: Spacetime perspective of gravitational lensing in perturbed cosmologies
Portions of three wave fronts that have passed through the lens at successive times plotted along the \(\hat{x}\) (horizontal) and \(\hat{z}\) (vertical) axes. As the wave front moves into the past, it is traveling vertically upwards in this plot. The wave front folds over itself creating a cusp singularity. To show the evolution of the wave fronts on one graph, we have collapsed the vertical distance between the wave fronts, as shown by the parallel break lines.
