When we look out at the Universe today, it simultaneously tells us two stories about itself. One of those stories is written on the face of what the Universe looks like today, and includes the stars and galaxies we have, how they’re clustered and how they move, and what ingredients they’re made of. This is a relatively straightforward story, and one that we’ve learned simply by observing the Universe we see.
But the other story is how the Universe came to be the way it is today, and that’s a story that requires a little more work to uncover. Sure, we can look at objects at great distances, and that tells us what the Universe was like in the distant past: when the light that’s arriving today was first emitted. But we need to combine that with our theories of the Universe — the laws of physics within the framework of the Big Bang — to interpret what occurred in the past. When we do that, we see extraordinary evidence that our hot Big Bang was preceded and set up by a prior phase: cosmic inflation. But in order for inflation to give us a consistent with what we observe, there’s an unsettling appendage that comes along for the ride: a multiverse. Here’s why physicists overwhelmingly claim that a multiverse must exist.
Source: bigthink