$\star$ Photons

The photons are the most abundant particles of the universe. They come mainly from the radiation of the cosmological diffuse bottom. As we saw, it comes from the freezing of the interactions between the ionized matter and the photons which took place when the universe was still sufficiently hot and dense (a few $\rho_\gamma = \sigma \frac{\pi^2}{15 } T^4$, which is equivalent to:

$$\Omega_\gamma = 2.48\times 10^{-5}h^{-2}$$ (1.25)

What represents a density of $1 \ \ { \rm photon}.cm^{-3}$).