Was The Electromagnetic Spectrum A Blackbody Spectrum In The Early Universe?

It is assumed, in general, that the electromagnetic spectrum in the Primordial Universe was a blackbody spectrum in vacuum. We derive the electromagnetic spectrum, based on the Fluctuation-Dissipation Theorem that describes the electromagnetic fluctuations in a plasma. Our description includes thermal and collisional effects in a plasma. The electromagnetic spectrum obtained differs from the blackbody spectrum in vacuum at low frequencies. In particular, concentrating on the primordial nucleosynthesis era, it has more energy for frequencies less than 3 to $6{\omega }_{pe}$, where ${\omega }_{pe}$ is the electron plasma frequency.