Question: Is our universe the only one? Or is it a multi-universe?

Jacob Leygonie answered on 12 Nov 2019:

I am not a physicist nor an expert in space, but for sure this is a question that many researchers are currently interested in. For instance, you may have heard of Stephen Hawking, one of the most important physicists of the 20th century. He had an article developing a theory (which I don’t know about) which implies the existence of parallel universes. It still remains to show that this theory is true.

At least we can try to naively develop the question. To try and tackle it one should first agree on a reasonable definition of “universe”. Let us say that a universe is the data of things happening in the 3D space as time evolves.

Note that there are already many criticisms made possible about this definition. First, does the time needs to “start” at some point, i.e. with a Big Bang type of phenomenon, or is it allowed to go backward at infinity. Second, the space is not really 3D (remember, 1D is the line, 2D is your sheet of paper or your TV screen, 3D is what we see every day around us), since we know the space becomes curved near blackholes and plays with time in intricate ways. Anyway, let us forget these problems for now, and agree on the (simplistic) definition of universe above.

As we are part of a universe, the question can be rephrased as:
“Is there another universe than the one we are in?”

I like to think that this is true. I like to believe that whenever we make a choice or when History takes a strong direction, then in some other universe we make the other decision and History orientates in the other way.