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Bucher [Bucher2001] has recently proposed an interesting brane-worldcosmological scenario where the ``Big Bang'' hypersurface is the locus ofcollision of two vacuum bubbles which nucleate in a five dimensional flatspace. This gives rise to an open universe, where the curvature can be verysmall provided that $d/R_0$ is sufficiently large. Here, d is the distancebetween bubbles and $R_0$ is their size at the time of nucleation. Quantumfluctuations develop on the bubbles as they expand towards each other, andthese in turn imprint cosmological perturbations on the initial hypersurface.We present a simple formalism for calculating the spectrum of suchperturbations and their subsequent evolution. We conclude that, unfortunately,the spectrum is very tilted, with spectral index $n_s=3$. The amplitude offluctuations 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 theinstanton describing the nucleation of a bubble and k is the wavenumber inunits of the curvature scale. The spectrum peaks on the smallest possiblerelevant scale, whose wave-number is given by $k\sim d/R_0$. We comment on thepossible extension of our formalism to more general situations where a Big Bangis ignited through the collision of 4D extended objects.

Cosmological perturbations in the 5D big bang