ρ α a^-3
a α Time^2/3
∴ ρ α Time^-2 
a α °T^-1
∴ ρ α °T³

°T  = temperature ( ≡ T)†
Time = time (≡ t)†
ρ = density
a = scale factor
α = is proportional to

ρ α a^-4
a α Time^1/2
ρ α Time^-2
a α °T^-1
∴ ρ α °T⁴