High energy particle collisions near the bifurcation surface
  We consider generic nonextremal stationary dirty black holes. It is shown that in the vicinity of any bifurcation surface the energy of collision of two particles in the centre of mass frame can grow unbound. This is a generic property that, in particular, includes collisions near the inner black hole horizon analyzed earlier by different methods. The similar results are also valid for cosmological horizons. The case of the de Sitter metric is discussed. 
O. B. Zaslavskii
gr-qc astro-ph.HE hep-th