"Reichenbach and Davies are only two of the many authors, coming from philosophy and physics, who approach to the problem of the arrow of time in cosmology in terms of entropy. This approach rests on two assumptions: that it is possible to define entropy for a complete instantaneous cross-section of the universe, and that there is an only time for the universe as a whole. However, both assumptions involve difficulties. In the first place, to confidently transfer the concept of entropy from the field of thermodynamics to cosmology is a very dangerous move. The definition of entropy in cosmology is still a very controversial issue, even more than in thermodynamics: there is not a consensus among physicists regarding how to define a global entropy for the universe. In fact, it is usual to work only with the entropy associated with matter and radiation because there is not yet a clear idea about how to define the entropy due to the gravitational field. But even leaving aside this problem, if the entropy of the universe is considered as an entropy out of equilibrium, there are different definitions for it (cf. Mackey, 1989). In the second place, when general relativity comes into play, time becomes a dimension of a four-dimensional structure: it is not yet acceptable to conceive time as a background parameter which, as in pre-relativistic physics, is used to mark the evolution of the system. Therefore, the problem of the arrow of time in cosmology cannot be posed, from the beginning, in terms of the entropy gradient between the two ends of a linear and open time."