  We derive a new class of exact solutions of Einstein's equations providing a
physically plausible hydrodynamical description of cosmological matter in the
radiative era ($10^6 K > T > 10^3 K$), between nucleosynthesis and decoupling.
The solutions are characterized by the Lema\^{\i}tre-Tolman -Bondi metric with
a viscous fluid source, subjected to the following conditions: (a) the
equilibrium state variables satisfy the equation of state of a mixture of an
ultra-relativistic and a non-relativistic ideal gases, where the internal
energy of the latter has been neglected, (b) the particle numbers of the
mixture components are independently conserved, (c) the viscous stress is
consistent with the transport equation and entropy balance law of Extended
Irreversible Thermodynamics, with the coefficient of shear viscosity provided
by Kinetic Theory for the `radiative gas' model. The fulfilment of (a), (b) and
(c) restricts initial conditions in terms of an initial value function,
$\Delta_i^{(s)}$, related to the average of spatial gradients of the
fluctuations of photon entropy per baryon in the initial hypersurface.
Constraints on the observed anisotropy of the microwave cosmic radiation and
the condition that decoupling occurs at $T=T_{_D}\approx 4\times 10^3$ K yield
an estimated value: $|\Delta_i^{(s)}|\approx 10^{-8}$ which can be associated
with a bound on promordial entropy fluctuations. The Jeans mass at decoupling
is of the same order of magnitude as that of baryon dominated perturbation
models ($\approx 10^{16} M_\odot$)
