... by

The subscript 0 denotes the values at the present epoch.

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... times

More precisely at redshift $z= 6.3 \times 10^4 ~ (\Omega_{\rm b} h^2)^{-6/5}$, where $\Omega_{\rm b}$ is the baryonic fraction and h=H₀/100 (see Danese & Zotti 1977; Freese et al. 1987).

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... types

Mangano et al. (2002) performed a new analysis that takes into account the quantum electrodynamics correction at finite temperature to the photon and e$^{\pm}$ plasma equation of state. They found $N_\nu = 3.0395$, which is in agreement with the recent estimates from the primordial nucleosynthesis and WMAP results, see Barger et al. (2003).

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... electrons

Which depends on the reactions of recombination and photoionization coupled with the reactions of charge exchange. All of these reaction rates depend on the temperature and density (see Pfenniger & Puy 2003).

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