Bare and effective fluid description in brane world cosmology
  An effective fluid description, for a brane world model in five dimensions,
is discussed for both signs of the brane tension. We found several cosmological
scenarios where the effective equation differs widely from the bare equation of
state. For universes with negative brane tension, with a bare fluid satisfying
the strong energy condition, the effective fluid can cross the barrier $\omega
_{eff} =-1$.
