WHAT IS ENTROPY

In the absence of experimental facts to support the study of the origi ns of the Universe and black holes, reliance must be shifted to theory , especially thermodynamics. To superstring theories, dealing with the unification of the four known forces during the evolution of the earl y Universe, and black holes, the end product of gravitational collapse , have been attributed rather peculiar thermodynamic properties such a s lack of concavity, with the consequence of negative heat capacities, non-extensivity. It has been proposed that superadditivity rather tha n concavity constitutes the essence of the second law. Here we refute such claims and show that concavity determines the natural evolution o f thermodynamic processes.