Trapped surfaces in a hadronic fluid
  Pion propagation in a hadronic fluid with a non-homogeneous relativistic flow is studied in terms of the linear sigma model. The wave equation turns out to be equivalent to the equation of motion for a massless scalar field propagating in a curved spacetime geometry. The metric tensor depends locally on the soft pion dispersion relation and the four-velocity of the fluid. For a relativistic flow in curved spacetime the apparent and trapping horizons may be defined in the same way as in general relativity. An expression for the analog surface gravity is derived. 
Neven Bilic, Dijana Tolic
gr-qc hep-ph hep-th