Universality classes, thermodynamics of group entropies, and black holes.

Conventional Boltzmann-Gibbs statistical mechanics successfully describes systems with weak to moderate correlations, where the number of accessible configurations W (N ) grows exponentially with the number of degrees of freedom N . However, this framework breaks down for systems with strong correlations or long-range interactions, for which the configuration space exhibits nonexponential growth. While numerous generalized entropies have been proposed to address this limitation, a coherent link to classical thermodynamic laws has remained elusive. Here, we propose group entropies as a unifying framework, defining universality classes of entropies through the asymptotic scaling of W (N ), each yielding an extensive entropy. We show that this approach provides the basis for a consistent thermodynamic formulation beyond the Boltzmann-Gibbs paradigm. In particular, by expressing these entropies in terms of thermodynamic state variables we demonstrate their consistency with classical thermodynamics, in close analogy to the emergence of the Clausius entropy from the Boltzmann-Gibbs formalism. Focusing on the zeroth thermodynamic law, we identify the empirical temperature and, by using Carathéodory's formulation of the second law, we derive the associated absolute temperature. As an application we analyze black-hole thermodynamics using the extensive group entropy class corresponding to stretched-exponential behavior of W (N ) and derive a negative specific heat, known to be a hallmark property of black holes. This result holds for the stretched-exponential entropies associated with both the Bekenstein-Hawking and Barrow entropy scalings.