Cosmological Lagrangian displacement
= Cosmological Lagrangian displacement
{title2=$\boldsymbol\psi(\mathbf q,t)$}
The cosmological Lagrangian displacement sends an initially labelled matter element from its <Lagrangian coordinate> $\mathbf q$ to $\mathbf x(\mathbf q,t)=\mathbf q+\boldsymbol\psi(\mathbf q,t)$. Its <derivative>[time derivative] determines the <peculiar velocity> through $\mathbf v=a\dot{\boldsymbol\psi}$.