| Sumario: | Cosmological scales leave the horizon during in?ation and reenter it after big bang nucleosynthesis. Throughout the superhorizon era it is very useful to de?ne a primordial cosmological curvature perturbation, which is conserved if and only if pressure throughout the Universe is a unique function of energy density (the adiabatic pressure condition) [1–7].
Observation directly constrains the curvature perturbation at the very end of the superhorizon era, a few Hubble times before cosmological scales start to enter the horizon, when it apparently sets the initial condition for the subsequent evolution of all cosmological perturbations. The observed curvature perturbation is almost Gaussian with an almost scale-invariant spectrum.
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