Natural TeV-Scale Gravity and coupling constant unification, in   Heterotic M-Theory, with the usual hidden and visible sectors swapped
  I consider a class of Grand Unified models, in which E8 is broken to SU(3)xSU(2)xSU(2)xSU(2)xU(1)_Y, then to SU(3)xSU(2)_{diag}xU(1)_Y. The breaking of (SU(2))^3 to SU(2)_{diag} reduces the SU(2) coupling constant, at unification, by a factor of 1/\sqrt{3}, so that the ratio of the SU(3) and SU(2)_{diag} coupling constants, at unification, is equal to the ratio observed at about 1 TeV. By choosing a suitable alignment of U(1)_Y, and introducing a generalization of the CKM matrix, the U(1)_Y coupling constants of the observed fermions, at unification, can also be arranged to have the ratios, to the SU(3) coupling constant, that are observed at about 1 TeV. This suggests a model of Heterotic M-Theory, with a standard embedding of the spin connection in one of the E8s, but with the visible sector now having the E8 that is unbroken at unification. The universe is pictured as a thick pipe, where the long direction of the pipe represents the four extended dimensions, the circumference represents a compact six-manifold, and the radial direction represents the eleventh dimension. The inner radius of the pipe is about 10^{-19} metres, and the outer radius of the pipe is about 10^{-14} metres. We live on the inner surface of the pipe. The low-energy generation structure and the high mass of the top quark follow from the breaking pattern, and gravity and the Yang-Mills interactions are unified at about 1 TeV. Parity breaking must occur spontaneously in four dimensions, rather than being inherited from ten dimensions. The stability of the proton might be correlated with the entries in the CKM matrix. 
Chris Austin