Jeans Instability in a Universe with Dissipation
  The problem of Jeans gravitational instability is investigated for static and
expanding universes within the context of the five and thirteen field theories
which account for viscous and thermal effects. For the five-field theory a
general dispersion relation has been derived with the help of relevant
linearized perturbation equations, showing that the shear viscosity parameter
alters the propagating modes for large and small wavelengths. The behavior of
density and temperature contrasts are analyzed for the hard-sphere model in
detail. In the small wavelengths regime, increasing the amount of shear
viscosity into the system forces the harmonic perturbations to damp faster,
however, in the opposite limit larger values of shear viscosity lead to smaller
values of density and temperature contrasts. For the hyperbolic case, the
dispersion relation becomes a polynomial in the frequency with two orders
higher in relation to the five-field theory, indicating that the effects
associated with the shear viscosity and heat flux are non-trivial. The profile
of Jeans mass in terms of the temperature and number density is explored by
contrasting with several data of molecular clouds. Regarding the dynamical
evolution of the density, temperature, stress and heat flux contrasts for a
universe dominated by pressureless matter, we obtain also damped harmonic waves
for small wavelengths. In the case of large wavelengths, the density and
temperature contrasts grow with time (due to the Jeans mechanism) while the
stress and heat flux contrasts heavily decay with time.
