The radion in brane cosmology

We consider the homogeneous cosmological radion, which we define as the int erbrane distance in a two-brane and Z(2) symmetrical configuration. In a co ordinate system where one of the brane is at rest, the junction conditions for the second (moving) brane give directly the (nonlinear) equations of mo tion for the radion. We analyse the radion fluctuations and solve the nonli near dynamics in some simple cases of interest. (C) 2001 Elsevier Science B .V. All rights reserved.