Astronomy and Astrophysics – Astrophysics
Scientific paper
2001-08-08
Astronomy and Astrophysics
Astrophysics
4 pages, 1 figure
Scientific paper
In the framework of the standard, hot big bang cosmological model the dynamics of the early evolution of the universe is controlled by the energy density of relativistic particles, among which neutrinos play an important role. In equilibrium, the energy density contributed by one flavor of relativistic neutrinos is 7/8 of that of the cosmic background radiation (CBR) photons. As the universe expands and cools, neutrinos decouple and their subsequent contribution to the energy density is modified by the relative heating of the CBR photons when electron-positron pairs annihilate. The small corrections to the post-annihilation energy density of the standard model neutrinos due to incomplete decoupling and finite-temperature QED effects are reviewed (correcting an error in the literature) and extended to account for possible additional relativistic degrees of freedom whose presence might modify the predictions of primordial nucleosynthesis and of the predicted CBR anisotropies.
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