Brian Greene is a theoretical physicists that studies string theory. He is well known for his PBS special and his book The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory.
His new book published in 2011 is The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos
He talked with NPR’s Fresh Air about the book, which you can listen here.
He says that our universe may be only one of a multitude of universes out there in what is termed the multiverse. And those other universes may be entirely different from our own including having different elementary particles and different physical laws.