= Zeldovich pancake thickness after shell crossing
{c}
{title2=$1+\delta=\prod_{i=1}^3(1-b\lambda_i)^{-1}$}
For the <Zeldovich approximation> $\mathbf x=\mathbf q-b\nabla_q\psi$, let $\lambda_i$ be the eigenvalues of the potential Hessian, with this sign convention. Before <shell crossing>, <mass conservation> gives the displayed density. The first positive eigenvalue reaches $b\lambda_i=1$ and collapses one direction, creating a sheetlike <cosmological caustic>. Extrapolating straight trajectories after crossing lets particles pass through without self-consistent gravitational trapping, so the predicted multistream sheet spreads excessively. Collisionless gravitational evolution keeps matter more tightly bound; the linear displacement extrapolation cannot determine its late thickness reliably.
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