  We revisit the issue of the stability in the Dvali-Gabadadze-Porrati model,
by considering the nucleation of bubbles of the conventional branch within the
self-accelerating branch. We construct an instanton describing this process in
the thin wall approximation. On one side of the bubble wall, the bulk consists
of the exterior of the brane while on the other side it is the interior. The
solution requires the presence of a 2-brane (the bubble wall) which induces the
transition. However, we show that this instanton cannot be realized as the thin
wall limit of any smooth solution. Once the bubble thickness is resolved, the
equations of motion do not allow O(4) symmetric solutions joining the two
branches. We conclude that the thin wall instanton is unphysical, and that one
cannot have processes connecting the two branches, unless negative tension
bubble walls are introduced. This also suggests that the self-accelerating
branch does not decay into the conventional branch nucleating bubbles. We
comment on other kinds of bubbles that could interpolate between the two
branches.
