Large Extra Dimensions and Cosmological Problems

We consider a variant of the brane-world model in which the universe is the direct product of a Friedmann, Robertson-Walker (FRW) space and a compact hyperbolic manifold of dimension $d\geq2$. Cosmology in this space is particularly interesting. The dynamical evolution of the space-time leads to the injection of a large entropy into the observable (FRW) universe. The exponential dependence of surface area on distance in hyperbolic geometry makes this initial entropy very large, even if the CHM has relatively small diameter (in fundamental units). This provides a possible solution to the cosmological entropy problem. Furthermore, this entropy can be achieved within the holographic limit if the ordinary FRW space is also a compact hyperbolic manifold. In addition, the very large statistical averaging inherent in the collapse of entropy onto the brane acts to smooth out initial inhomogeneities. This smoothing is sufficient to offer an alternative solution of the homogeneity problem of standard cosmology. With only mild fine-tuning, the flatness problem can also be solved. Finally, recent brane-world approaches to the hierarchy problem can be readily realized within this framework.