Question about the Bulk

[Vinni]

Is the bulk the manifold container with 12 dimensions? Also are parallel universes those that operate across different dimensions? For instance our universe operates in dimensions 1, 2, 3, but a parallel universe could operate in dimensions 4,5,6?

[bapowell]

In standard braneworld models of cosmology, parallel universes can be thought of as separate branes evolving in a higher dimension space. So, for example, in such a set-up our universe would be a 3-brane (3 extended spatial dimensions). We can imagine a 4th spatial dimension orthogonal to the 3 dimensions of our brane. We could then in principle fill the 4th dimension with many other 3-branes (think of stacking cd's on a spool -- the cd's are 3-branes and the spool axis is the 4th orthogonal dimension). If each 3-brane could support a universe, then this construct would provide an example of parallel universes.

To answer your first question, the 'bulk' is the term used to describe the dimensions orthogonal to the brane of interest. The dimensionality depends on the model.

[Naty1]

And a bit more here with some links:

http://en.wikipedia.org/wiki/Brane_cosmology

In general the bulk is not accessible, not observable, to us. In string theory, for example, forces such as the electromagnetic and nuclear forces are represented via vibrating strings whose ends are attached to branes....so they are 'stuck' in our three dimensional space.

Gravity, however, may be unattached strings which can "float" off into the bulk and it may be that is why it is so weak in our observable three dimensions of space...it diffuses into extra dimensions we can't so far observe. So one open question is whether the gravity from parallel universes might somehow be detectable in ours.