 Angular particle-horizon size in a power-law universe

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.