Question Estimate the current number of cosmic microwave background (CMB) photons there are now, per cubic meter, given that the mean photon energy of a blackbody distribution is about 2.70kT. Method http://en.wikipedia.org/wiki/Planck_function" [Broken] gives the energy density (i.e. energy per unit volume) per unit wavelength interval. The http://en.wikipedia.org/wiki/Stefan-Boltzmann_law" [Broken] is derived by integrating Planck's law, to give the total energy density (over all wavelengths). The photon number density (photons per cubic metre) can be found by (a) taking the total energy density (from the Stefan-Boltzmann Law) and dividing it by (b) the typical photon energy. Problem It looks like the Stefan–Boltzmann law should help, but it requires a temperature, T, whereas I'm given the mean photon energy in kT. Planck's law includes a term for kT, but results in the energy density per unit wavelength. I'm given the mean photon energy, 2.70kT. Is this necessarily the same as the typical photon energy? I'm obviously missing a crucial next step, can anyone help?