One small item of contradiction to the illustrations of bubble universes, as well as part of the text. It's called "boundarylessness". I take it to mean you can't bump your face into a wall of bubble such as the illustrations and a part of the text suggests.
"Boundarylessness", to me means that bubble universes offset. They overlay and inlay into one another (of course not exactly except for those infinities of absolute duplicates at infinite distances apart; in which case there is no difference in and between them from an absolute of just "one and one only"), thus an infinitely flat (infinitely flat (infinite in depth of plane(s), infinite in extent -- "dimensionless point"), smooth, non-local, non-relative, 'naked singularity' of 'Space-verse' Big Universe (U).
"Boundarylessness", herein, also means "horizon": horizon, to horizon, to horizon. In this case it means its there at a distance from you, while at exactly the same time you exist inside it; in the exact middle of it between horizon that always remains constant to you in every direction no matter what direction you travel; no matter how fast you travel. You're inside it, and it is inside you. So you travel through offset bubbles, losing relativity, gaining relativity. Relativity breaking down behind, relativity building ahead. You do it all the time, walking room to room, or any other place to any other place; inter-planet, inter-planetary, inter-stellar, inter-galactic, or inter-universe. Offset to offset, to offset.
Don't let those illustrations throw you. Or, if you must think of them the way they are illustrated, think interconnections by wormholes between them all (a filament connected matrix to infinity rather than loose bubbles floating so loosely and so completely disconnected from each other).... in the place of simply horizon to horizon to horizon, through infinities of horizons looking and being closed up to just one collapsed 'Horizon'. There is more than one way to skin a "Multiverse".