Black holes and universality classes of critical points
  We argue that there exists an infinite class of conformal field theories in diverse dimensions, having a universal ratio of the central charge c to the normalized entropy density c'. The universality class includes all conformal theories which possess a classical gravity dual according to the AdS/CFT correspondence. From the practical point of view, the universality of c/c' provides an explicit test which can be applied to determine whether a given critical point may admit a dual description in terms of classical gravity. 
Pavel Kovtun and Adam Ritz