Brane-world models emerging from collisions of plane waves in 5D
  We consider brane-world models embedded in a five-dimensional bulk spacetime with a large extra dimension and a cosmological constant. The cosmology in $5D$ possesses "wave-like" character in the sense that the metric coefficients in the bulk are assumed to have the form of plane waves propagating in the fifth dimension. We model the brane as the "plane" of collision of waves propagating in opposite directions along the extra dimension. This plane is a jump discontinuity which presents the usual ${\bf Z}_2$ symmetry of brane models. The model reproduces the {\em generalized} Friedmann equation for the evolution on the brane, regardless of the specific details in $5D$. Model solutions with spacelike extra coordinate show the usual {\em big-bang} behavior, while those with timelike extra dimension present a {\em big bounce}. This bounce is an genuine effect of a timelike extra dimension. We argue that, based on our current knowledge, models having a large timelike extra dimension cannot be dismissed as mathematical curiosities in non-physical solutions. The size of the extra dimension is small today, but it is {\em increasing} if the universe is expanding with acceleration. Also, the expansion rate of the fifth dimension can be expressed in a simple way through the four-dimensional "deceleration" and Hubble parameters as $- q H$. These predictions could have important observational implications, notably for the time variation of rest mass, electric charge and the gravitational "constant". They hold for the three $(k = 0, + 1, - 1)$ models with arbitrary cosmological constant, and are independent of the signature of the extra dimension. 
J. Ponce de Leon