= Sudden decay of a dominant nonrelativistic relic
{title2=$\rho_\phi\simeq3M_{\rm Pl}^2\Gamma^2$}
The sudden-decay approximation converts a dominant cold relic's rest-mass density into radiation at $H\simeq\Gamma$, neglecting expansion during the conversion. Its pre-decay radiation temperature obeys $T_-^3\simeq3M_{\rm Pl}^2\Gamma^2/(c_1g_\star Ym)$, while the <reheating temperature> is set by the post-decay radiation energy density. Constant relativistic counts and prompt thermalization are additional assumptions.
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