Brane Collisions and Braneworld Cosmology

Braneworld scenario especially of the Randall-Sundrum type 2) have recently attracted much attention. In particular, cosmological models in this scenario have been studied actively 3), 4). It has been shown that braneworld cosmology seems to be consistent with the 4-dimensional conventional cosmology at least on scales much lower than that corresponding to the brane tension, provided bulk Weyl curvature is sufficiently small. However, it is not clear whether braneworld cosmology can predict any evidences for the existence of extra-dimension(s) which are testable in near future observations. Further, it is still not evident whether braneworld models actually have a great advantage over the conventional ones. Under such a current situation, it is an important direction of research to seek for an alternative scenario in which the existence of extra-dimension(s) plays an essential role. As one of such attempts, the present authors recently proposed a new cosmological braneworld model 1) in which an inflation occurs on a boundary brane driven by small mismatch between the bulk vacuum energy and the brane tension, and the nucleation of a true vacuum bubble becomes a trigger of the big-bang in the braneworld. One of the distinctive features in our model is that the bubble nucleation occurs in extra-dimension(s). Not only does such a vacuum bubble coming from extra-dimension heat up the brane universe through the colliding process, but also provide simultaneously an anti-de Sitter bulk of the Randall-Sundrum setup, reducing the effective cosmological constant on the brane to zero. In this article, we shall briefly summarize a colliding brane cosmology. We first give a brief review of colliding brane models so far proposed in the next section. Then, in section 3, we illustrate our brane big-bang model proposed in Ref. 1). In section 4), we discuss problems in our model, some of which may be common in any types of colliding brane models.