Cosmological horizon entry
= Cosmological horizon entry
{title2=$k\simeq aH,\quad k/(aH)\text{ increases through one}$}
During a decelerating expansion the <comoving Hubble radius> grows. A fixed comoving <wavenumber> then passes from a <superhorizon scale> to a shorter scale when $k\simeq aH$. In <radiation domination>, $aH=1/\tau$, so this occurs at $k\tau\simeq1$. Here horizon entry refers to <Hubble radius> crossing; a true causal horizon instead depends on a conformal-time integral.