  Assuming that the background geometry is filled with free gas consisting of
matter and radiation and no phase transitions being occurred in the early
Universe, we discuss the thermodynamics of this {\it closed} system using
classical approaches. We find that essential cosmological quantities, such as
Hubble parameter $H$, scale factor $a$ and curvature parameter $k$, can be
derived from this simple model, which on one hand fulfills and entirely obeys
the laws of thermodynamics. On the other hand, the results are compatible with
the Friedmann-Lemaitre-Robertson-Walker model and the Einstein field equations.
The inclusion of finite bulk viscosity coefficient derives to important changes
in all these cosmological quantities. Accordingly, our picture about the
evolution of the Universe and its astrophysical consequences seems to be a
subject of a radical revision. We find that $k$ strongly depends on the
thermodynamics of background matter. The time scale, at which negative
curvature might take place, depends on the relation between the matter content
and the total energy. Using quantum and statistical approaches, we assume that
the size of the Universe is given by the volume occupied one particle and one
photon. Different types of their interactions are taken into account.
Expressions for $H$ and $a$ are introduced. Therefore, the expansion of the
Universe turns to be accessible.
