= Superhorizon adiabatic CDM mode in radiation domination
{title2=$\delta_C=A_R(k)\tau^2,\quad\delta_R=\tfrac43\delta_C$}
During <radiation domination>, $a\propto\tau$ and $8\pi G\rho_Ra^2=3/\tau^2$. The <adiabatic initial conditions> reduce the <CDM density equation in a matter-radiation universe> to $\delta_C''+\delta_C'/\tau-4\delta_C/\tau^2=0$. Its <Euler-Cauchy equation> powers are two and minus two. The growing solution is the displayed one, in the stated CDM-comoving <synchronous gauge>.
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