= Angular particle-horizon size in a power-law universe
{title2=$\theta_p=r^{1-p}/(1-r^{1-p}),\quad r=t_e/t_0$}
Divide the <particle horizon in a power-law universe> at emission by the <angular diameter distance in a flat power-law universe>. The result is the small angular extent of one radial horizon <length>. Matter-only and radiation-only histories have powers $1/3$ and $1/2$, respectively. These are idealized single-era histories; a real <recombination> horizon and distance depend on different portions of the expansion history.
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