  The low energy regime of cosmological BPS-brane configurations with a bulk
scalar field is studied. We construct a systematic method to obtain
five-dimensional solutions to the full system of equations governing the
geometry and dynamics of the bulk. This is done for an arbitrary bulk scalar
field potential and taking into account the presence of matter on the branes.
The method, valid in the low energy regime, is a linear expansion of the system
about the static vacuum solution. Additionally, we develop a four-dimensional
effective theory describing the evolution of the system. At the lowest order in
the expansion, the effective theory is a bi-scalar tensor theory of gravity.
One of the main features of this theory is that the scalar fields can be
stabilized naturally without the introduction of additional mechanisms,
allowing satisfactory agreement between the model and current observational
constraints. The special case of the Randall-Sundrum model is discussed.
