= Angular Hubble-radius scale in an Einstein-de Sitter universe
{title2=$\theta_H=[2(\sqrt{1+z}-1)]^{-1}$}
In an <Einstein-de Sitter universe>, $H(z)=H_0(1+z)^{3/2}$ and the radial <comoving distance> is $2H_0^{-1}[1-(1+z)^{-1/2}]$. A physical <Hubble radius> $H(z)^{-1}$ has comoving size $(1+z)/H(z)$. Dividing by the radial distance gives the displayed small angle, in radians. A Hubble diameter has twice this angle; the matter-era particle-horizon radius also equals twice the <Hubble radius>. These distinctions matter when comparing approximate <CMB> angular scales.
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