  Cosmological perturbations with wavelengths smaller than Hubble radius can be
handled in the context of Newtonian theory with very high accuracy. The
application of this Newtonian approximation, however, is restricted to
nonrelativistic matter and cannot be used for relativistic matter. Recently, by
modifying the continuity equation, Lima, et. al., extended the domain of
applicability of Newtonian cosmology to radiation dominated phase. We adopted
this continuity equation to re-examine linear cosmological perturbation theory
for a two fluid universe with uniform pressure. We study the evolution
equations for density contrasts and their validity in different epochs and on
scales larger than Hubble radius and compare the results with the full
relativistic approach. The comparison shows the high accuracy of this
approximation.
