CMB as thermal radiation from cosmic dust grains in equilibrium with the redshifted starlight

This paper suggests that the cosmic microwave background radiation (CMB) arises from thermalized cosmic dust grains both in the interstellar medium (ISM) and in the intergalactic medium (IGM) in electron density zones equal to 0.5 el/m3. Otherwise, the thermalization is characterized by higher temperatures in the observed range 15-27 K. Considering that a black body spectrum requires an absorber/re-emitter to take place, starting from the radiative transfer equation, the calculation method applied determines the diameter of the targets in equilibrium with the redshifted starlight necessary to produce a thermal radiation at 2.725 K. The calculated value of the diameter can only correspond to cosmic dust grains in nature with their specific mass density and to nothing else. It is fully consistent with the observed size range of extra-terrestrial cosmic dust within 10-1000 μm. Furthermore, the carbon composition of the grains is capable to describe a black body behaviour as well as the metal composition is in a position to feature the polarization CMB map under the influence of galactic magnetic fields. The results of this inquiry are valid for any observer in the Cosmos.