Solution
= Solution
For radiation $P_r/c^2=\rho_r/3$, so $d\log\rho_r=-4\,d\log a$. Normalization at $a=1$ gives \b[$\rho_r=\rho_{r0}a^{-4}$].
= Solution
For radiation $P_r/c^2=\rho_r/3$, so $d\log\rho_r=-4\,d\log a$. Normalization at $a=1$ gives \b[$\rho_r=\rho_{r0}a^{-4}$].