= Superhorizon conservation of uniform-density curvature
{title2=$\zeta'\simeq0$}
With the convention $\zeta=\Phi-\delta\rho/[3(\bar\rho+\bar P)]$, energy conservation gives $\zeta'=\mathcal H\delta P_{\rm nad}/(\bar\rho+\bar P)+(\nabla\cdot\mathbf v)/3$. The <non-adiabatic pressure perturbation> vanishes for <adiabatic cosmological perturbations>, and regular velocity gradients are suppressed on <superhorizon scales>. Curvature is then conserved to leading gradient order, allowing primordial initial conditions to be propagated between cosmological eras.
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