Cosmological perturbations in the 5D Big Bang

evolution. We conclude that, unfortunately, the spectrum is very tilted, with spectral index ns = 3. The amplitude of fluctuations at horizon crossing is given by < (δρ/ρ) >∼ (R0/d)2S−1 E k, where SE 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 ∼ d/R0. 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