= Comoving-gauge density contrast
{title2=$\Delta=\delta-3\mathcal H(1+w)v$}
For a <perfect fluid> with $P=w\rho$, in <Newtonian gauge in cosmology> the density perturbation on fluid-comoving slices is the displayed combination of the fractional density perturbation and velocity potential. The Einstein energy and momentum constraints combine into $\nabla^2\phi=4\pi Ga^2\bar\rho\Delta$ when scalar anisotropic stress vanishes. This gives a relativistic Poisson constraint valid beyond the subhorizon limit.
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