Large Extra Dimensions and Cosmological Problems

TL;DR

This paper explores a brane-world cosmology with a hyperbolic extra-dimensional space, addressing entropy, homogeneity, flatness, and hierarchy problems through topological and geometric effects.

Contribution

It introduces a model combining hyperbolic extra dimensions with brane-world cosmology, providing new insights into entropy generation, smoothing of inhomogeneities, and hierarchy problem solutions.

Findings

Large initial entropy due to hyperbolic geometry.

Smoothing of inhomogeneities explains universe's homogeneity.

Framework accommodates solutions to hierarchy and flatness problems.

Abstract

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 an attractive reformulation of the cosmological entropy problem, in which the large entropy is a consequence of the topology, though we would argue that a final solution of the entropy problem requires a dynamical explanation of the topology of spacetime. Nevertheless, it is reassuring that 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 the initial entropy onto the brane acts to smooth out initial inhomogeneities. This smoothing is then sufficient to account for the current homogeneity of the universe. With only mild fine-tuning, the current flatness of the universe can also then be understood. Finally, recent brane-world approaches to the hierarchy problem can be readily realized within this framework.