You've got the whole internet in front of you. It's simple enough. Here's a quote:

But there are unanswered questions, and Rees raised two critical ones: "First, is our Big Bang the only one? And, second, if there are many Big Bangs, are they all governed by the same laws of physics?" 

The "fascinating option," said Rees, is that different physical laws govern the other universes — "space may be different, gravity may be different, atoms may be different. This would mean that reality would consist of all these universes, governed by different laws, and only some tiny subset of them would be governed by laws that would allow complexity to evolve. Most universes would be sterile because, for example, gravity would be too strong to allow complex structures, or atoms would not be stable."

If, indeed, many Big Bangs generate an immense variety of physical laws, then, Rees said, only science fiction can describe all that might happen.

"These inflationary bubble or pocket universes expand at speeds approaching the speed of light," Vilenkin noted. "So we cannot possibly travel to other universes. For practical purposes, each of these inflationary bubble universes is a separate, self-contained unit — and they can in principle have different physical properties."

How could different laws in different universes be generated? Leonard Susskind, a physicist at Stanford University in California and one of the originators of string theory, gives one answer: Multiple universes (the innumerable pocket universes) are populated by possible laws of physics emerging from possible structures of string theory. This theory postulates that reality at its most fundamental level consists of miniscule, one-dimensional "strings," whose size is nearly the smallest possible Planck length, ~10^-35- m, or 100 million trillion times smaller than a proton. Their vibrations in multiple dimensions of space-time (10, 11 or 26 dimensions, depending on the specific string theory) give rise to all the laws of physics, the theory says. String theory seems almost impossible to test experimentally, but the elegance and the beauty of its explanatory powers convince its adherents.