Relaxation Principle in (n+1) Dimensions
  Why do we live in a (3+1) dimensional universe? In this paper, I review the "relaxation principle" first introduced by Karch and Randall (arXiv:hep-th/0506053v2) and generalize its ideas to an arbitrary (n+1) dimensions. This is done by referring to the Friedman equation and the scaling solution to derive the energy densities of the non-interacting and interacting branes, and then formulating their dimensionalities. I also demonstrate that the largest interacting d-brane in n spatial dimensions is always the (n-2)-brane, and that such dimensionality constraint "relaxed" our universe from nine dimensions to three dimensions. 
Choong Min Um
hep-th