Energy density ($\rho$)

The energy density is energy per unit volume. A homogeneous particle species with one-particle energy $E(\mathbf{p})$ has

Energy density per unit spectral coordinate, such as $U_{\nu }$ per frequency or $U_{\lambda }$ per wavelength. Conservation of energy in corresponding bins requires $U_{\lambda }=U_{\nu }|d\nu /d\lambda |$; therefore their peak positions need not map by $\lambda =c/\nu$.