Post-freeze-out relic abundance
= Post-freeze-out relic abundance
{title2=$Y_\infty$}
For $dY/dx=-\lambda Y^2/x^2$ after freeze-out, the exact limit is $Y_\infty=[Y_f^{-1}+\lambda/x_f]^{-1}$. The frequently used estimate $x_f/\lambda$ additionally neglects the initial inverse abundance.