Why the entropy of spacetime is independent of species of particles --   the species problem
  The Hawking radiation emits all species of particles, but the Bekenstein-Hawking entropy is independent of the number of the species of particles. This is the so-called species problem --- a puzzling problem for a long time. In this paper, we suggest a solution to this problem. A result of the scheme is that the black hole atmosphere has a mass equaling 3/8 mass of a classical Schwarzschild black hole, which agrees with 't Hooft's brick wall model. 
Yu-Zhu Chen, Wen-Du Li, Wu-Sheng Dai
gr-qc