Relaxation versus collision times in the cosmological radiative era

We consider the Lemaitre-Tolman-Bondi metric with an inhomogeneous viscous fluid source satisfying the equation of state of an interactive mixture of radiation and matter. Assuming conditions;prior to the decoupling era, we a pply extended irreversible thermodynamics (EIT) to this mixture. Using the full transport equation of EIT we show that the relaxation time of shear vi scosity can be several orders of magnitude larger than the Thomson collisio n time between photons and electrons. A comparison with the 'truncated' tra nsport equation for these models reveals that the latter cannot properly de scribe the decoupling of matter and radiation.