How do you relate the number densities of neutrinos and photons soon after the positron–electron annihilation?

Physics Asked on February 13, 2021

I found this equation:

$n_nu = frac{3}{4} frac{g_nu T_nu^3}{g_gamma T_gamma^3} n_gamma$

Where does it come from?

cosmic microwave background cosmological inflation cosmology neutrinos

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