An illustration of multiple, independent Universes, causally disconnected from one another in an ever-expanding cosmic ocean, is one depiction of the Multiverse idea. In a region where the Big Bang begins and inflation ends, the expansion rate will drop, while inflation continues in between two such regions, forever separating them. (OZYTIVE / PUBLIC DOMAIN)

One Universe Is Not Enough

We don’t need the many-worlds of quantum mechanics to have more Universes than we know what to do with.

“The Universe is all there is, or was, or will be.” That seems like a reasonable statement to make, doesn’t it? It certainly matches with our conception of the word Universe, which implies that this is all of space and all the matter and energy within it. We certainly live within the Universe, and can see an enormous amount of it: some 46 billion light years in all directions. After 13.8 billion years since the hot Big Bang, and the fabric of space expanding for all that time, this is the absolute limit of how far away we can see.

But what lies beyond that? Is there more Universe like our own? The answer is yes, there ought to be. But there ought to be something even more than that: a larger spacetime structure that has an enormous, countlessly large number of Universes embedded within it. If our best theories are correct, our one Universe is not enough. Here’s why.

If you look farther and farther away, you also look farther and farther into the past. The earlier you go, the hotter and denser, as well as less-evolved, the Universe turns out to be. The earliest signals can even, potentially, tell us about what happened prior to the moments of the hot Big Bang. (NASA / STSCI / A. FEILD (STSCI))

Imagine you went all the way back to the start of the Universe as-we-know-it: the beginning of the hot Big Bang. What would it look like? You’d…