= Equal-time two-point function of a de Sitter scalar
{title2=$P_{\delta\phi}(k,\tau)=H^2(1+k^2\tau^2)/(2k^3)$}
For a light scalar in the <Bunch-Davies vacuum> with metric perturbations neglected, the mode $e^{-ik\tau}(1-i/(k\tau))/\sqrt{2k}$ gives the displayed equal-time spectrum. The dimensionless power tends to $H^2/(4\pi^2)$ on <superhorizon scales>. The exact massless infinite-volume correlation has an infrared divergence; a finite-volume or finite-duration infrared regulator, and smearing for coincident products, makes the correlators well defined.
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