Is our universe the only one?

How far things stretch beyond the Hubble radius no one knows. But some theories suggest they stretch to infinity. If that were true, then all permitted arrangements of matter might exist somewhere. They might even exist in infinite numbers. There might be an infinite number of Earths with readers reading this article on them. In effect, these places, cut off from one another by their own Hubble radii, would be isolated universes as the term is currently understood by science.

That may sound mind-boggling, but it is pedestrian compared with the second type of Tegmark multiverse. The first type assumes the laws of physics are the same everywhere. The second suggests they can vary from one universe to another. Tinkering with physics’s laws would change the nature of reality, so these universes would be different—perhaps very different—from each other.
In the third type of Tegmark multiverse, as in the first, the laws of physics are the same from one to another. In this type, though, the component universes are continually separating from each other as time passes. At every moment within such a multiverse, all possible futures allowed by the uncertainties of quantum mechanics actually happen somewhere, and that somewhere constitutes a new universe.

The final type of multiverse Dr Tegmark proposes is one in which any and all coherent systems of mathematics describe a physical reality of some sort.