Amplitude of the rest-mass density contrast, | δ |, in units of , as a function of the Universe scale factor, R, for (blue solid line), 100 (red solid line), and 1000 (green solid line). The vertical dashed line marks the onset of matter-dominated era, in which our (linear) perturbative analysis holds. We observe that for z< 1090, polytropic DM perturbations conform with the Meszaros effect (i.e., δ ~ R) to high accuracy.