  Bucher [Bucher2001] has recently proposed an interesting brane-world
cosmological scenario where the ``Big Bang'' hypersurface is the locus of
collision of two vacuum bubbles which nucleate in a five dimensional flat
space. This gives rise to an open universe, where the curvature can be very
small provided that $d/R_0$ is sufficiently large. Here, d is the distance
between bubbles and $R_0$ is their size at the time of nucleation. Quantum
fluctuations develop on the bubbles as they expand towards each other, and
these in turn imprint cosmological perturbations on the initial hypersurface.
We present a simple formalism for calculating the spectrum of such
perturbations and their subsequent evolution. We conclude that, unfortunately,
the spectrum is very tilted, with spectral index $n_s=3$. The amplitude of
fluctuations at horizon crossing is given by $<(\delta \rho/\rho)^2> \sim
(R_0/d)^2 S_E^{-1} k^2$, where $S_E\gg 1$ is the Euclidean action of the
instanton describing the nucleation of a bubble and k is the wavenumber in
units of the curvature scale. The spectrum peaks on the smallest possible
relevant scale, whose wave-number is given by $k\sim d/R_0$. We comment on the
possible extension of our formalism to more general situations where a Big Bang
is ignited through the collision of 4D extended objects.
