= Thermal relic
{title2=$Y=n/s$}
A <thermal relic> is a surviving particle population whose earlier abundance was set by chemical <thermal equilibrium> with the cosmic plasma. After number-changing reactions cease and cosmic <entropy> is conserved, its comoving abundance is described by the yield $Y=n/s$. A relativistically decoupled species can retain an appreciable yield, whereas a species remaining in <thermal equilibrium> after becoming nonrelativistic is subject to <Boltzmann suppression>. Its later mass density, lifetime and momentum distribution determine its cosmological effects.
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