  Much work has been devoted to the phenomenology and cosmology of the
so-called braneworld universe, where our (3+1)-dimensional universe lies on a
brane surrounded by a (4+1)-dimensional bulk spacetime that is essentially
empty except for a negative cosmological constant and the various modes
associated with gravity. For such a braneworld cosmology, the difficulty of
justifying some preferred initial conditions inevitably arises. The various
proposals for inflation restricted to the brane only partially explain the
homogeneity and isotropy of the resulting braneworld universe because the
homogeneity and isotropy of the bulk must be assumed. We propose a mechanism by
which a brane surrounded by AdS space arises naturally so that the homogeneity
and isotropy of both the brane and the bulk are guaranteed. We postulate an
initial false vacuum phase of (4+1)-dimensional Minkowski or de Sitter space
subsequently decaying to a true vacuum of anti-de Sitter space, assumed
discretely degenerate. This decay takes place through bubble nucleation. When
two bubbles of the true AdS vacuum collide, a brane (or domain wall) inevitably
forms between the two AdS phases. We live on this brane. The SO(3,1) symmetry
of the collision geometry ensures the three-dimensional spatial homogeneity and
isotropy of the universe on the brane as well as of the bulk. In the
semi-classical limit, this symmetry is exact. We sketch how the leading quantum
corrections translate into cosmological perturbations.
