Let’s spend a bit of time on the concept of what a multiverse is. Most people are familiar with the concept of our universe. It consists of everything around us including all of the stars and galaxies that we can see as well as what is beyond the visible horizon, that part of space and time that has not had time for the light emitted from it to reach us.
The theory of the Big Bang says that our universe began as a very hot and dense dimensionless point some 13.7 billion years ago (2). A very small fraction of time later it went through a tremendous expansion or inflationary phase which smoothed it out. This inflation spread the tiny variations of density that were on sub atomic scales out to the size of what eventually became galactic super-cluster size. The universe literally expanded faster than the speed of light leaving most of it so far away that we can not see it. As time goes by light from an ever increasing sphere around us has time to reach us. This visible bubble is estimated to have a radius of about 46 billion light years (3). What lies beyond that can not be seen and can have no effect on us (perhaps not true – see note).
One of the questions that bother both physicists and philosophers is where did this singularity come from and what was it embedded in? Was there something before it? There are number of theories including the cyclic universe that is born in a singularity, expands for a very long time but eventually collapses back upon itself in a Big Crunch which results in another singularity. M-theory or Brane cosmology has our universe as a membrane embedded in a space with many more dimensions than ours (4). I think that a fairly simple mechanism for creating a new universe exists in our own universe. When a black hole has its moment of creation in our universe a bit of the space time continuum pinches off and a new universe comes into being.
(2) http://en.wikipedia.org/wiki/Big_Bang
(3) http://en.wikipedia.org/wiki/Observable_universe
(4) http://en.wikipedia.org/wiki/Multiverse
(note) Quantum mechanics also includes the concept of entanglement (spooky action at a distance). At the temperatures and densities that exist prior to inflation particles that make up our universe are not likely to exist. But if pairs of entangled very high energy particles existed they or their descendants may still be able to act upon each other even though they are separated by a distance greater than the visible bubble. Interesting thought.