\[D=\frac{1}{3}\,\alpha^{2/3} {\mathrm{H}_{\mathrm{P}}}\,\Big[\frac{c}{\varkappa \rho}\, g\, \beta\,(1-\beta) {\nabla_{\mathrm{\!ad}}}({\nabla_{\mathrm{\!rad}}}-{\nabla_{\mathrm{\!ad}}}) \Big]^{1/3} ,\]