NumCosmoMathCblackbody_energy_density

This functions returns the black body energy density divided by $T^4$. Defined as \begin{equation} \frac{\rho_{\text{BL}}}{T^4} = \frac{8\pi^2k_{\text{b}}^4}{15 \left( hc \right)^3}, \end{equation} where $\rho_{\text{BL}}$ is the black body energy density, $T$ is the temperature, $k_{\text{b}}$ is the Boltzmann constant (#ncm_c_kb()), $h$ is the Planck constant (#ncm_c_h()) and $c$ is the speed of light (#ncm_c_c()).