The shape of the first collapsed objects.

Abstract

In the early seventies, there was a conjecture (based on the Zel'dovich approximate solution) that the first collapse of a self-gravitating dustlike medium (appropriate approximation for nonbaryonic dark matter) results in the formation of a ``pancake'' object that is a thin surface. Recent works cast doubt on the Zel'dovich conjecture, suggesting that the first collapse might be pointlike or filamentlike rather than pancakelike. Our $N$-body simulations show first pancake collapse. We can reject with 97% confidence the Bayesian prior that the other kinds of collapse are more or equally probable.