= Einstein-de Sitter photon flight time
{c}
{title2=$\Delta t=\frac2{3H_0}[(1+z_2)^{-3/2}-(1+z_1)^{-3/2}]$}
For an earlier emission epoch $z_1>z_2$, the matter-only flat model gives $R\propto t^{2/3}$ and $t(z)=2(1+z)^{-3/2}/(3H_0)$. Subtraction gives the displayed elapsed <cosmic time>. It is not the comoving travel <distance> divided by $c$, because expansion changes the spatial scale during flight. An Einstein-de Sitter approximation fixed to the same $H_0$ differs from a matter-era approximation normalized to $H_0\sqrt{\Omega_{m0}}$.
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