He asserts that the idea that we live in a “multiverse” is not just a theory, it is rather a consequence of our current understanding of theoretical physics, and says the distinction is crucial. We have not waved our hands and said: “Let there be a multiverse”. Instead the idea that the universe is perhaps one of infinitely many is derived from current theories like quantum mechanics and string theory.
String theory is the most promising avenue to be able to unify quantum mechanics and gravity. It states that all fundamental particles are made up of one-dimensional strings and can describe all known forces of nature at once: gravity, electromagnetism and the nuclear forces. However, for string theory to work mathematically, it requires at least ten physical dimensions. Since we can only observe four dimensions: height, width, depth (all spatial) and time (temporal), the extra dimensions of string theory must therefore be hidden somehow. To be able to use the theory to explain the physical phenomena we see, these extra dimensions have to be “compactified” by being curled up in such a way that they are too small to be seen. Perhaps for each point in our large four dimensions, there exist six extra indistinguishable directions?
A problem, or some would say, a feature, of string theory is that there are many ways of doing this compactification –10500 possibilities is one number usually touted. Each of these compactifications will result in a universe with different physical laws – such as different masses of electrons and different constants of gravity.
Our study of cosmology shows that the very early universe underwent a period of accelerated expansion called inflation. A consequence of inflation is that there must be other parts of the universe that are still accelerating - eternally inflating. Some parts can therefore end up becoming other universes, which could become other universes etc. generating an infinite number of universes.
Testing the theory.
The universes predicted by string theory and inflation live in the same physical space and can overlap or collide. Indeed, they inevitably must collide, leaving possible signatures in the cosmic sky which we can try to search for. These signatures are actively being pursued by scientists. Some are looking for it directly through imprints in the cosmic microwave background. However, no such signatures are yet to be seen. Others are looking for indirect support such as gravitational waves, which are ripples in space-time as massive objects pass through. Such waves could directly prove the existence of inflation, which ultimately strengthens the support for the concept of the multiverse.