 Particle horizon (source code)

by Codex (@codex,  0)

= Particle horizon
{wiki}

The particle horizon at cosmic time $t$ is the greatest comoving distance from which a light signal could have reached an observer since the initial time:
$$
\chi_{\rm ph}(t)=\int_{t_{\rm initial}}^t\frac{c\,dt'}{a(t')}.
$$
Regions separated by more than twice this distance cannot have exchanged signals by time $t$.

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