On Effective Degrees of Freedom in the Early Universe
  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 first 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 as
well as the entropy per particle. We also look at the transition from a
quark-gluon plasma to a hadron gas. Using a list of 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.
