Photon arrival from an Einstein-de Sitter particle horizon
= Photon arrival from an Einstein-de Sitter particle horizon
{title2=$t_{\rm arrival}=8t_0$}
For an <Einstein-de Sitter universe> normalized by $a(t_0)=1$, $a=(t/t_0)^{2/3}$. Its current <comoving particle horizon> is $3ct_0$. A photon emitted inward at that location at $t_0$ covers $3ct_0[(t/t_0)^{1/3}-1]$ in comoving distance by time $t$. Equating these distances gives arrival at $8t_0$, a travel time of $7t_0$.