= De Sitter curvature mode function
{c}
{title2=$u_k(\tau)$}
For approximately constant $H$ and $\epsilon$, a vacuum-normalized curvature mode is $u_k(\tau)=H(1+ik\tau)e^{-ik\tau}/\sqrt{4\epsilon M_{\mathrm{Pl}}^2k^3}$. Its derivative is $u_k^\prime=Hk^2\tau e^{-ik\tau}/\sqrt{4\epsilon M_{\mathrm{Pl}}^2k^3}$. Ordered unequal-time <Wick contractions> use the complex conjugate at the earlier vertex, consistently with the <vacuum prescription for inflationary in-in integrals>.
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