String Theory: Level 1 Parallel Universes
In string theory, the idea of Level 1 parallel universes basically says that space is so big that the rules of probability imply that surely, somewhere else out there, are other planets exactly like Earth. In fact, an infinite universe would have infinitely many planets, and on some of them, the events that play out would be virtually identical to those on our own Earth.
We don’t see these other universes because our cosmic vision is limited by the speed of light — the ultimate speed limit. Light started traveling at the moment of the big bang, about 14 billion years ago, and so we can’t see any further than about 14 billion light-years (a bit farther, since space is expanding). This volume of space is called the Hubble volume and represents our observable universe.
The existence of Level 1 parallel universes depends on two assumptions:
The universe is infinite (or virtually so).
Within an infinite universe, every single possible configuration of particles in a Hubble volume takes place multiple times.
In regard to the first assumption, inflation theory predicts that the universe is actually far larger than our Hubble volume. Recall that eternal inflation implies that universes are constantly being created and destroyed by quantum fluctuations, which means that space is actually infinite under the most extreme application of this theory.
The regions created in an eternal inflation model trigger every single set of initial conditions, leading to the second assumption. This means that there’s another region of space that consists of a Hubble volume that has the exact same initial conditions as our universe. If it has exactly the same initial conditions, then such a region would evolve into a Hubble volume that resembles ours exactly.
If Level 1 parallel universes do exist, reaching one is virtually (but not entirely) impossible. For one thing, we wouldn’t know where to look for one because, by definition, a Level 1 parallel universe is so far away that no message can ever get from us to them or them to us. (Remember, we can only get messages from within our own Hubble volume.)
In theory, however, you could get in a spaceship that can travel at nearly the speed of light, point it in a direction, and head off. Time for you would slow, but the universe would continue to age as you moved throughout the entire expanse of the universe looking for your twin. If you’re lucky, and dark energy is weak enough that eventually gravity causes cosmic expansion to end, you might eventually be able to get to your twin’s planet.