Exactly Scale-Invariant Cosmological Perturbations From a Colliding Bubble Braneworld Universe

One of the aims of braneworld cosmology is to identify a "smoking gun" for the existence of large extra dimensions by means of observations of the early universe. Braneworld cosmology has succeeded in reproducing the conventional cosmology at late times in a higher-dimensional context, but finding a unique observational signature has proved elusive. Here we demonstrate that the recently proposed colliding bubble universe predicts an exactly scale-invariant spectrum of scalar cosmological perturbations, with no liberty for the spectrum tilting and bending allowed in conventional slow-roll inflation. This enhanced predictiveness of the colliding bubble braneworld scenario results from the absence of arbitrary or unknown scalar field potentials. The dynamics of the expanding bubble wall branes are completely fixed by symmetry considerations