Zeldovich approximation
= Zeldovich approximation
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The Zeldovich approximation maps a fluid element from its <Lagrangian coordinate> $\mathbf q$ to the comoving position $\mathbf x=\mathbf q+D(t)\mathbf s(\mathbf q)$, where $D$ is the <linear growth factor>. It follows the initial gravitational displacement along a straight comoving trajectory until trajectories cross.