Does the Big Bang necessarily mean we’re part of a multiverse?

A multitude of universes split off a single bit of inflating Universe in this diagram.

Andrei Linde

For most of its history, the idea of a multiverse was the domain of science fiction and some rare speculation from physicists. In recent years, though, the idea that our Universe may be just one among many has gained traction in two different areas. The string theorists think it may help explain why, if there are a huge number of possible universes, we ended up in one with the properties we see around us. Meanwhile, cosmologists are realizing that inflation, which is the only way we know of to get from the Big Bang to our Universe, necessarily implies the creation of other universes.

Both of these threads of thought made appearances at two events held by the World Science Festival earlier this month, and different approaches to them were presented by various theorists—including two who think that we shouldn't be bothering with the multiverse at all.

The (contested) triumph of inflation

We'll start with inflation, which is the leading theory to explain why our Universe looks the way it does. The Universe is, on a large scale, homogeneous (matter is roughly equally distributed throughout). This implies that the Universe started small—small enough that different regions of it could equilibrate. But the Universe isn't perfectly smooth, as galaxies form clusters and other groupings. So how do you get from the small, equilibrated Universe to the existence of structure?