  The effective Friedmann equation describing the evolution of the brane
Universe in the cosmology of the Randall-Sundrum model includes a dark (or
mirage or Weyl) radiation term. The brane evolution can be interpreted as the
motion of the brane in an AdS-Schwarzschild bulk geometry. The energy density
of the dark radiation is proportional to the black hole mass. We generalize
this result for an AdS bulk space with an arbitrary matter component. We show
that the mirage term retains its form, but the black hole mass is replaced by
the covariantly defined integrated mass of the bulk matter. As this mass
depends explicitly on the scale factor on the brane, the mirage term does not
scale as pure radiation. For low energy densities the brane cosmological
evolution is that of a four-dimensional Universe with two matter components:
the matter localized on the brane and the mirage matter. There is conservation
of energy between the two components. This behaviour indicates a duality
between the bulk theory and a four-dimensional theory on the brane. The
equation of state of the generalized dark radiation is that of a conformal
field theory, with an explicit breaking of the conformal invariance through the
pressure of the bulk fluid. Accelerated expansion on the brane is possible only
if there is negative pressure on the brane or in the bulk, or if the integrated
mass of the bulk fluid is negative.
