Radion Dynamics in BPS Braneworlds

We examine the moduli dynamics of a specific class of supergravity-inspired BPS braneworlds, clarifying the role of bulk scalar fields in brane collisions. The model contains as a special case the Randall-Sundrum model both with and without a free, massless bulk scalar field. Its low-energy effective theory is derived with a moduli space approximation (MSA) and agrees with the corresponding results derived elsewhere. Rather than stabilising the radion, we look at cosmological evolution of the system stimulated by breaking the BPS condition on the branes. We examine in detail the range of validity of the MSA in both the RS and BPS case, paying particular attention to the divergences that can arise during a collision of the branes. In the absence of perturbations such an event is finite in the RS model, and accurately described by the low-energy effective theory. We demonstrate, however, that a collision is divergent in the BPS case even with an exact FRW geometry.