Tessellating the cosmological dark-matter sheet: origami creases in the
  universe and ways to find them
  Tessellations are valuable both conceptually and for analysis in the study of
the large-scale structure of the universe. They provide a conceptual model for
the 'cosmic web,' and are of great use to analyze cosmological data. Here we
describe tessellations in another set of coordinates, of the initially flat
sheet of dark matter that gravity folds up in rough analogy to origami. The
folds that develop are called caustics, and they tessellate space into stream
regions. Tessellations of the dark-matter sheet are also useful in simulation
analysis, for instance for density measurement, and to identify structures
where streams overlap.
