Past exam of the relativistic Astrophysics and Cosmology course of the University of Cambridge / 2016 / Paper 1 / 2 / b

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 $R\propto t^{1/2}$. HenceRadiation 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.