  The standard Friedmann universe embedded in a five dimensional and constant
curvature bulk is examined without any a priori junction condition between the
brane and the bulk. A geometrical explanation for the accelerated expansion of
the universe is derived by using a minimum set of assumptions consistent with
the brane-world program. It is shown that the extrinsic curvature of the brane
can be associated to the dark energy which presumably drives the universe
expansion.
