= Cosmic coincidence problem
Nonrelativistic matter dilutes as $a^{-3}$, while a <cosmological constant> has constant <energy density>. The ratio of <vacuum energy> density to <matter density> consequently scales as $a^3$, so comparable contributions occur during a limited epoch. The <cosmic coincidence problem> asks why observers find these very different components comparable near their own epoch. This timing puzzle is distinct from the <cosmological constant problem>, which concerns the tiny absolute vacuum-energy scale.
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