Radiation-dominated universe
= Radiation-dominated universe
{title2=$a(t)\propto t^{1/2}$}
A <radiation-dominated universe> has expansion controlled mainly by <radiation in cosmology>. When spatial curvature and the <cosmological constant> are negligible, the <Friedmann equation> and density scaling $\rho\propto a^{-4}$ give the displayed <scale factor>, with <Hubble parameter> $H=1/(2t)$.