Ok, let's take the standard twin paradox, Alice leaves on a trip in her rocketship near the speed the light, and comes back to Earth some time later to find herself 5 years younger than her twin, Bob. Now they go out to lunch and strike up a conversation as to how old the universe is. Alice says it's ~13.8 billion years old and is such and such a diameter across. Bob says, no no no, the universe is ~13.8 byo + 5 years and has an accordingly larger diameter. Which one is correct? How do we reconcile this? I mean, if each are living in a different-sized universe, how are they able to have lunch together? For instance, how are the physical properties of the restaurant not affected, etc.?