  In the context of a toy model we discuss the phenomenon of colliding
five-branes, with two of the extra space dimensions compactified on tori. In
one of the branes (hidden world) the torus is magnetised. Assuming
opposite-tension branes, we argue that the collision results eventually in a
(time-dependent) cosmological vacuum energy, whose value today is tiny, lying
comfortably within the standard bounds by setting the breaking of the
four-dimensional supersymmetry at a TeV scale. The small value of the vacuum
energy as compared with the supersymmetry-breaking scale is attributed to
transient phenomena with relaxation times of order of the Age of the Universe.
An interesting feature of the approach is the absence of a cosmic horizon,
thereby allowing for a proper definition of an S-matrix.As a result of the
string non-criticality induced at the collision,our model does not provide an
alternative to inflation, given that arguments can be given supporting the
occurence of an inflationary phase early after the collision. The physics
before the collision is not relevant to our arguments on the cosmological
constant hierarchy, which are valid for asymptotically long times after it.
