= Comoving particle horizon
{title2=$r_p(t)=c\int_0^t\frac{dt'}{a(t')}$}
The comoving particle horizon is the <particle horizon> expressed in <comoving coordinates>, assuming a finite initial cosmic-time boundary at zero. In a spatially flat <FLRW metric>, it is the distance a photon could have travelled since that boundary. The corresponding <proper distance in cosmology> at time $t$ is $a(t)r_p(t)$. It differs from the future-directed <cosmological event horizon> and from the instantaneous <comoving Hubble radius>.
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