  The quasinormal mode of a gravitating and magnetized fluid in a spatially
flat, isotropic and homogeneous cosmological background is derived in the
presence of the fluid sources of anisotropic stress and of the entropic
fluctuations of the plasma. The obtained gauge-invariant description involves a
system of two coupled differential equations whose physical content is analyzed
in all the most relevant situations. The Cauchy problem of large-scale
curvature perturbations during the radiation dominated stage of expansion can
be neatly formulated and its general solution is shown to depend on five
initial data assigned when the relevant physical wavelengths are larger than
the particle horizon. The consequences of this approach are explored.
