It doesn't come easy's Journal: Universe evolution favors 3 and 7 dimensions

In case anyone was wondering why we live in a universe with 3 infinitely long directional dimensions, Andreas Karch (University of Washington assistant professor of physics) and his collaborator, Lisa Randall (of Harvard) says it's because the natural evolution of universes (or more specifically the branes described in M-Theory) favor the eventual formation of a universe where you end up with either 3 or 7 infinite physical directions (the remaining dimensions shrink to a minuscule size). Other numbers of physical dimensions are possible, just not favored. An interesting note, the good professor implies that our universe actually contains many regions with different numbers of spatial dimensions at the same time; we just happen to live in a region that ended up with 3 spatial dimensions.

The announcement also implies that our universe is infinitely large and has big bangs happening inside it somewhere all the time. On the other hand, it has also been theorized that two branes colliding might have created what we call the universe.

Karch and Randall detail their work in the October edition of Physical Review Letters, published by the American Physical Society.