I just finished my re-read of the trilogy. At the end of Death's End, while they're in Universe 647, it's mentioned how they receive transmissions from greater universes from the super-membrane. This would imply either a multiverse theory, or something akin to it.
I'm confused about the implications.
Is there a singular, "Grand Universe"? One which contains ALL universes across this super-membrane?
OR
Are there many greater universes that may or may not be disconnected from each other?
For example: when the Returners send their message concerning the missing matter required to get a Big Crunch, is this applying to some Grand Universe that affects every single possible universe, or just the greater universe that our characters are from?
I hope my confusion makes sense. If not, I can elaborate.
EDIT: There a few lines, but this is the one that I am mostly thinking about:
Universe 647's monitoring systems regularly intercepted natural phenomena and undecoded signals from other external universes on the supermembrane, though they never received communications from their own original universe.