Viscosity in a Lepton-Photon Universe
  We look at viscosity production in a universe consisting purely of leptons
and photons. This is quite close to what the Universe actually look like when
the temperature was between $10^{10}$ K and $10^{12}$ K ($1$ -- $100$ MeV). By
taking the strong force and the hadronic particles out of the equation, we can
examine how the viscous forces behave with all the 12 leptons present. By this
we study how shear- and (more interestingly) bulk viscosity is affected during
periods with particle annihilation. We use the theory given by Hoogeveen et.
al. from 1986, replicate their 9-particle results and expanded it to include
the muon and tau particles as well. This will impact the bulk viscosity
immensely for high temperatures. We will show that during the beginning of the
lepton era, when the temperature is around 100 MeV, the bulk viscosity will be
roughly 100 million times larger with muons included in the model compared to a
model without.
