Stability analysis for the background equations for inflation with
  dissipation and in a viscous radiation bath
  The effects of bulk viscosity are examined for inflationary dynamics in which
dissipation and thermalization are present. A complete stability analysis is
done for the background inflaton evolution equations, which includes both
inflaton dissipation and radiation bulk viscous effects. Three representative
approaches of bulk viscous irreversible thermodynamics are analyzed: the Eckart
noncausal theory, the linear and causal theory of Israel-Stewart and a more
recent nonlinear and causal bulk viscous theory. It is found that the causal
theories allow for larger bulk viscosities before encountering an instability
in comparison to the noncausal Eckart theory. It is also shown that the causal
theories tend to suppress the radiation production due to bulk viscous
pressure, because of the presence of relaxation effects implicit in these
theories. Bulk viscosity coefficients derived from quantum field theory are
applied to warm inflation model building and an analysis is made of the effects
to the duration of inflation. The treatment of bulk pressure would also be
relevant to the reheating phase after inflation in cold inflation dynamics and
during the radiation dominated regime, although very little work in both areas
has been done, the methodology developed in this paper could be extended to
apply to these other problems.
