Radiation relation between deceleration and density
= Radiation relation between deceleration and density
{title2=$q=\Omega$}
For <radiation in cosmology> with zero <cosmological constant>, $p=\rho/3$. The <Friedmann acceleration equation> gives $\ddot a/a=-8\pi G\rho/3$, so the <deceleration parameter> equals the <cosmological density parameter>, regardless of spatial curvature. This is a relation for a pure radiation fluid, not a general identity for a mixture.