  Several approaches to the matter creation problem in the context of
cosmological models are summarily reviewed. A covariant formulation of the
general relativistic imperfect simple fluid endowed with a process of matter
creation is presented. By considering the standard big bang model, it is shown
how the recent results of Prigogine et alii \cite{1} can be recovered and, at
the same time their limits of validity are explicited.

Astrophysics