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Abstract:

We discuss early cosmology in theories where the fundamental Planck mass is close to the TeV scale. In such theories the standard model fields are localized to a (3 + 1)-dimensional wall with n new transverse sub-millimeter sized spatial dimensions. The topic touched upon include: early inflation that occurs while the size of the new dimensions are still small, the spectrum and magnitude of density perturbations, the post-inflation era of contraction of our world while the internal dimensions evolve to their final "large" radius, and the production of gravitons in the bulk during these two eras. The radion moduli problem is also discussed.

Abstract:

Recently it was proposed that the standard model (SM) degrees of freedom reside on a $(3+1)$-dimensional wall or ``3-brane'' embedded in a higher-dimensional spacetime. Furthermore, in this picture it is possible for the fundamental Planck mass $\mst$ to be as small as the weak scale $\mst\simeq O(\tev)$ and the observed weakness of gravity at long distances is due the existence of new sub-millimeter spatial dimensions. We show that in this picture it is natural to expect neutrino masses to occur in the $10^{-1} - 10^{-4}\ev$ range, despite the lack of any fundamental scale higher than $\mst$. Such suppressed neutrino masses are not the result of a see-saw, but have intrinsically higher-dimensional explanations. We explore two possibilities. The first mechanism identifies any massless bulk fermions as right-handed neutrinos. These give naturally small Dirac masses for the same reason that gravity is weak at long distances in this framework. The second mechanism takes advantage of the large {\it infrared} desert: the space in the extra dimensions. Here, small Majorana neutrino masses are generated by breaking lepton number on distant branes.

Abstract:

The magnitude of the Fayet-Iliopoulos term is calculated for compactifications of Type-I string theory and Horava-Witten M-theory in which then exists a pseudo-anomalous U(1)(x). Contrary to various conjectures, it is found that in leading order in the perturbative expansion around the weakly-coupled M-theory or Type-I limits, a result identical to that of the weakly-coupled E-8 x E-8 heterotic string is obtained. The result is independent of the values chosen for the Type-I string scale or the size of the M-theory 11th dimension, only depending upon Newton's constant and the unified gauge coupling. (C) 1998 Published by Elsevier Science B.V. All rights reserved.