Zel'dovich approximation

The Zel'dovich approximation is a model of the formation of the large scale structure of the universe applying Lagrangian perturbation theory. It was published in 1970 by Soviet physicist Yakov Zel'dovich (also known for the Sunyaev-Zel'dovich effect). Zel'dovich pancakes are ellipsoid/pancake-shaped high-density volumes which his model produces and he suggested were seeds of large scale structures such as superclusters.