= Solution
For <radiation>, $P=\rho c^2/3$ and $w=1/3$. The continuity equation gives $\rho\propto R^{-4}$, so the flat, zero-$\Lambda$ background expands as $R\propto t^{1/2}$. Hence
$$
\boxed{q=1.}
$$
<Radiation pressure> doubles the <deceleration parameter> relative to dust at the same density parameter. The printed radiation-filled Einstein-de Sitter wording is interpreted as a flat radiation-only model; the conventional name Einstein-de Sitter normally refers to the dust case.
Back to article page
