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

Abstract

It is generally assumed 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 a blackbody spectrum in vacuum at low frequencies. In particular, concentrating on the primordial nucleosynthesis era, it has more energy than the blackbody spectrum for frequencies less than 3vpe to 6vpe, where vpe is the electron plasma frequency. [S0031-9007(97)04160-4] PACS numbers: 98.80. ‐ k, 52.25.Gj, 95.30.Qd, 98.70.Vc It is usually assumed in cosmology that the primordial plasma was a homogeneous plasma and that the electromagnetic field was a blackbody spectrum in vacuum. These assumptions are used, for example, in standard big bang nucleosynthesis calculations. Deviations from a blackbody spectrum in vacuum and a homogeneous plasma can affect primordial nucleosynthesis, and in this Letter we concentrate on this epoch. In the epoch of big bang nucleosynthesis, the Universe was a thermal bath of photons, electrons, positrons, baryons, and neutrinos. The usual treatment considers the Universe as a homogeneous plasma in thermal equilibrium and the electromagnetic field as a blackbody spectrum in vacuum. For example, in the energy density calculation, the energy density of the photons is given by the energy density of the blackbody spectrum in vacuum. In this manner, the primordial universe is treated as an ideal gas: collective effects are assumed to be