Hierarchical pancaking: why the Zel’dovich approximation describes coherent large-scale structure in N-body simulations of gravitational clustering

To explain the rich structure of voids, clusters, sheets, and filaments seen in the Universe, we present evidence for the convergence of the two classic approaches to gravitational clustering, the "pancake" and "hierarchical" We compare models by examining agreement between individual structures -- the ``pancakes'' characteristic of the Zel'dovich Approximation (ZA) which also appear in hierarchical N-body simulations. We find that we can predict the orientation and position of N-body simulation objects rather well, even in an N-body simulation with initial power spectrum P(k) proportional to k**3. We found a modified ZA worked well even in this extreme hierarchical case, implying that very low-amplitude long waves dominate over local clumps. In this case the correlation length of the initial is extremely small, calling into question the adhesion--based interpretation of superpancakes. The coherence length of the potential grew substantially in those cases in which it was short, explaining the coherent motion of clumps into second-generation pancakes. We show that the nonlinear gravitational potential strongly resembles the smoothed initial potential. This explains why ZA with smoothed initial conditions reproduces large-scale structure so well, and probably why our Universe has a coherent large-scale structure.