### Question --- What was the approximate temperature of the universe at recombination, and why did recombination happen then? ### Answer --- > [!key-idea] Short answer: > - $z_{\rm rec} = 1100$ at [[Cosmological Timeline#Recombination|Recombination]] > - $T = T_{0}(1+z) \simeq 2.7\,\pu{K}(1100) \simeq 3000\,\pu{K}$ (see [[Blackbody Radiation]]) ![[Cosmological Timeline#Recombination]]