= Total comoving visibility radius
{title2=$\chi_{\rm max}=\int_{t_i}^{t_f}c\,dt/a(t)$}
In a spatially flat <FLRW metric>, the supremum of the <comoving radial distance> from which a signal emitted at the initial time can eventually reach a fixed comoving observer is the total <conformal time> interval. At an intermediate time $t$, this equals its <comoving particle horizon> plus the comoving radius of its <cosmological event horizon>, when both integrals converge. This is a visibility bound for emission arbitrarily close to the initial boundary; it does not assert that a transparent universe or a source existed there.
Back to article page
