Can anyone solve the problems we have when we say there is just one universe?
In that case this universe must be infinite without a beginning and without an end. It must be macroscopically homogenous and isotropic towards infinity. However, if we extrapolate the situation we find in our observable universe, then it seems almost unbelievable that our observable universe does not belong to a very large (our) universe that really is a black-hole. Such a very large black-hole, with a corresponding low average density, will have an event horizon separating us from a space which must contain something (or even is something where, (in autonomous reality), nothing can not exist). It is not so bad a suggestion, I think, to name that environment a multi-verse in which ours is only one part.
So the question can be, what is more acceptable, is it the described universe I started with, or will it be part of something like an, indeed not observed and not known, multi-verse. I see heroic attempts to introduce super-symmetry, (even in a complex mathematical setting) in order to bring vacuum energy to zero, but that does not help to make me believe that reality can be made out of nothing. To my feeling a multi-verse gives a better view and a better lead to our developing hypotheses and theories than restricting ourselves to a framework we will never really understand. I have much sympathy for physicists like Martin Rees, Gabriele Veneziano, or Paul Steinhardt who are (dare) indeed trying to go in such a direction. It is to my opinion contra productive to ask, “show just one other universe”. Maybe detection of gravitational waves from our cosmic background can be of some help in proceeding. Anyway, reasonable hypotheses and theories on one hand, observations and experiments on the other always have led to better models of our reality, which can only be a tiny part of an autonomous reality we will never completely understand. Maybe there even is a limit to experiments we will be able to set up. In that case, maybe, we can only proceed a bit further by going on with coherent and intuitive thinking and putting our thoughts together? In mathematics, however of great help, one can suggest everything, but that is not physics!