If two forms of matter coexist in the same space but they do not interact in any way including inertia, are they separate universes?
How would you even know that matter is there? This looks like a non-falsifiable statement [0].
I think Penrose's ideas warrant more precisely defined language.
On one hand yes, it feels like it should be the same. On the other hand, it is not parsimonious to create another space when there is plenty of room left in this one.
Should we define another universe as something which never has and never will interact with our universe? If so, anything we can observe of seemingly another universe is in actuality part of our universe.
The topic seems like it could use a consensus about what the words it uses should mean and not mean.
Yes, this is how I would interpret the definition of THE UNIVERSE. My personal interpretation of "another universe" would be something like- a multidimensional vacuum populated with matter/energy that currently exists from our inertial frame of reference, and there is nothing linking our own sparsely populated vacuum to this other universe. i.e. if you can observe it, then it's not a different universe. As Penrose states, this is merely an earlier version of the same universe, which existed before the Big Bang.
Here is a different prediction, a timelapse, of the fate of our universe (as told by Brian Cox), which assumes the universe will continue expanding...