How do you define a "universe" (in the context of multiverse)? How do you define a universe (in the context of multiverse)?
The traditional definition of universe is something like
"The Universe is all of spacetime and everything that exists therein".
But in multiverse theory one talks about "universes" and this raises the question of how one can in principle distinguish between two different universes.  
A similar question is asked here and the top answer says "In addition, as can be shown by some holographic arguments, it is not really meaningful to talk about things that could exist outside our own cosmic horizon (or universe)." But here is an article in the Quanta Magazine which says
"If the universe that we inhabit had long ago collided with another universe, the crash would have left an imprint on the cosmic microwave background (CMB)"
So it seems that at least in some multiverse theories, different universes can send and receive physical information (light or gravitational force) between each other. So the question is
If light or gravitational force can travel between two universes then how would you know both are not part of the same universe (other than by a priori assumption)?
 A: As far as I know, "a" Universe is caracterised by fundemental constants such as the speed of light, Newton's gravitationnal constant, the Planck constant and so on. 
You could distinguish between two different universe from the variation of these values I think. 
A: It is a pseudoproblem of definition. Some people define universe as everything that could ever possible exist, to them the word multiverse is an oxymoron. But those who like to use the idea of a multiverse, use it encompassing different things depending on context. For instance, Max tegmark defines 4 different levels of universes/multiverses: 
Level I: Beyond our cosmological horizon
Level II: Universes with different physical constants
Level III: Many-worlds interpretation of quantum mechanics
Level IV: Ultimate ensemble of every mathematically consistent structure 
but you can define your own.  
A: Brian Greene talks of many more types of multiverses...holographic, extra dimensional, simulated etc.
It is also possible our universe is only a construct of our consciousness in which case consciousness would vary in a different universe..but that is in the realm of philosophy.
Higgs field variation- a basic energy field value creating the constants and the laws as we know it. A different value of underlying Higgs field will give different universes with different set of laws and constants. 
A: I'm not sure what is the best possible definition, but I know a simple criteria to validate any such definition:
If a set of observations or events $e$ can occur consistently to all observers $O(e)$ in the future light cone of $C$, where $C$ is the common event whose past cone encompasses all the elements $e$, I would expect that $e$ and $O(e)$ must belong to the same universe, and belonging to the same universe must be an equivalence class (it would be weird if is not an equivalence class)
Being an equivalence class means that all inequivalent observers and events remain separate over all unitary evolution and unable to interact. At least this is what would be expected from Quantum Mechanics
A: If we defined a Universe as any encapsulated system of energy and mass that is governed by consciousness AKA (intelligence awareness)
Then the smallest universe would be the a living cell. Other notable Universes would be every plant,animal, etc. This would make you yourself a Universe
If you are a universe made up of 70 trillion cells ( smaller universes ) then what you call " Self" is in essence a God of your own personal universe.
Obviously this is a metaphoric excersize...but...try it on experientially. It motivates you to take responsibility the harmony of your inner world environment and to create outer world harmony with all the other universes that you encounter.
Im personally massaging this concept and having consistent related epiphanies as i stroll down this path of possibility!
