No, it isn't, it's what you get after integrating to all frequencies the Planck distribution for the radiation of a black body in thermodynamic equilibrium, whi

No, it isn't, it's what you get after integrating to all frequencies the Planck distribution for the radiation of a black body in thermodynamic equilibrium, which if you can get a sufficiently black one, you can check out it matches perfectly. I mean perfectly, that's what remote sensing satellites start with for determining surface temperature, after convenient linearization and modeling emissivity. That's what we see in the CMB, again perfectly. It's like this, I'm sorry. (Look at the CMB spectrum —and refs therein—: https://commons.m.wikimedia.org/wiki/File:Cmbr.svg https://commons.m.wikimedia.org/wiki/File:Cmbr.svg)