Why do we live in 3+1 dimensions?
  In the context of string theory we argue that higher dimensional Dp-branes unwind and evaporate so that we are left with D3-branes embedded in a (9+1)-dimensional bulk. One of these D3-branes plays the role of our Universe. Within this picture, the evaporation of the higher dimensional Dp-branes provides the entropy of our Universe. 
Ruth Durrer (1), Martin Kunz (1) and Mairi Sakellariadou (1,2) ((1)   University of Geneva, (2) University of Athens)