Brane worlds in 5D and warped compactifications in IIB

In computing potentials for moduli in, for instance, type IIB string theory in the presence of fluxes and branes a factorisable ansatz for the ten-dimensional metric is usually made. We investigate the validity of this ansatz by examining the cosmology of a brane world in a five-dimensional bulk and find that it contradicts the results obtained by using a factorisable ansatz. We explicitly identify the problem with the latter in the IIB case. These arguments support our previous work on this question. (C) 2004 Elsevier B.V. All rights reserved.