Radion cosmology in theories with universal extra dimensions

Abstract

We discuss the cosmology of models with universal extra dimensions, where the Standard Model degrees of freedom live in a (4+n)-dimensional brane, with n compact and small extra spatialdimensions. In these models, the simplest way to obtain theconventional four-dimensional Planck scale starting with a lowstring scale is to also have some larger extra dimensions, whereonly gravity propagates. In such theories, dimensional reductiongenerically leads to at least two radion fields, one associatedwith the total volume of the extra spatial dimensions and theother with the ratio of the sizes of the small and large extradimensions. In this paper, we discuss the impact of the radionfields on cosmology. We emphasize various aspects of radionphysics such as radion coupling to the Standard Model fields,bare and dressed radion masses during inflation, dynamicalstabilization of radions during and after inflation, radiondecay lifetime and its late dominance in the thermal history ofthe universe as well as its quantum fluctuations duringinflation. We argue that models where the radion plays the roleof an inflaton or the inflaton is a brane scalar field run intoproblems. We then present a successful inflation model with bulkscalar fields that seems to have all the desired properties. Wealso briefly discuss the possibility of radions as cold dark matter candidate.