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Galaxies 2016, 4(4), 78; doi:10.3390/galaxies4040078

On Effective Degrees of Freedom in the Early Universe

Department of Physics, Norwegian University of Science and Technology, N-7491 Trondheim, Norway
Academic Editor: Emilio Elizalde
Received: 20 October 2016 / Revised: 8 December 2016 / Accepted: 8 December 2016 / Published: 17 December 2016
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Abstract

We explore the effective degrees of freedom in the early Universe, from before the electroweak scale at a few femtoseconds after the Big Bang until the last positrons disappeared a few minutes later. We look at the established concepts of effective degrees of freedom for energy density, pressure, and entropy density, and introduce effective degrees of freedom for number density as well. We discuss what happens with particle species as their temperature cools down from relativistic to semi- and non-relativistic temperatures, and then annihilates completely. This will affect the pressure and the entropy per particle. We also look at the transition from a quark-gluon plasma to a hadron gas. Using a list a known hadrons, we use a “cross-over” temperature of 214 MeV, where the effective degrees of freedom for a quark-gluon plasma equals that of a hadron gas. View Full-Text
Keywords: viscous cosmology; shear viscosity; bulk viscosity; lepton era; relativistic kinetic theory viscous cosmology; shear viscosity; bulk viscosity; lepton era; relativistic kinetic theory
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Husdal, L. On Effective Degrees of Freedom in the Early Universe. Galaxies 2016, 4, 78.

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