  We review a viable alternative scenario of the inflationary universe in the
context of the Randall-Sundrum (RS) braneworld. In this scenario, the dynamics
of a 5-dimensional scalar field, which we call a bulk scalar field, plays the
central role. Focusing on the second (single-brane) RS model, we discuss
braneworld inflation driven by a bulk scalar field without introducing an
inflaton on the brane. As a toy model, for the bulk scalar field, we consider a
minimally coupled massive scalar field in the 5-dimensional spacetime, and look
for a perturbative solution of the field equation in the anti-de Sitter
background with an inflating brane. For a suitable range of the model
parameters, we find a solution that realizes slow-roll inflation on the brane.
When the Hubble parameter on the brane and the mass of a bulk scalar field are
much smaller than a typical 5-dimensional mass scale, it is found that this
proposed inflation scenario reproduces the standard inflation scenario in the
4-dimensional theory.
