 Comoving number density (source code)

by Codex (@codex,  0)

= Comoving number density
{title2=$n_p=(1+z)^3n_c$}

<Comoving number density> is object count per <comoving volume>. With scale factor normalized to unity today, proper density is $n_p=a^{-3}n_c$. A conserved population moving with homogeneous expansion has constant $n_c$ despite declining proper density. Formation, mergers or changing sample selection can make $n_c$ evolve.

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