Gravitation and Cosmology in Brane-worlds

In this contribution, I review the basic ingredients of the so-called braneworld models. These models are based on the assumption that our universe is a brane: a sub-space embedded in a higher dimensional bulk spacetime. In contrast with the traditional Kaluza-Klein treatment of extra dimensions, ordinary matter fields are confined on the brane. As a consequence, the size of the extra-dimensions in brane-world models is not limited by the usual bound, typically R . (1TeV), based on the non-detection of Kaluza-Klein modes in collider experiments up to energies close to 1 TeV. In brane-worlds, only gravity propagates in the higher-dimensional spacetime and, therefore, the size of the extra-dimensions can be much larger, up to a fraction of a millimeter, corresponding to the current limit of gravity experiments that look for deviations of Newton’s law1. Although there were some precursor works on brane-worlds, the huge interest they have encountered in the last few years is due to recent developments in string/M-theory. This has in turn opened up new avenues to tackle some fundamental problems. For example, the fact that the four-dimensional Planck mass Mp is in this context only a “projection” of the higher-dimensional (fundamental) Planck mass M∗, which can thus be lower than Mp, offers a new perspective on the hierarchy problem and suggests the possibility that quantum gravity might be closer than previously thought. This has been emphasized by Arkani-Hamed, Dimopoulos and Dvali2, who suggested a very simple model