While reading the cold spot links in the thread about the opaque part of the universe, I ran into this on Wikipedia:
I was taught that the universe is everything that exists. What definition are they using that they can have a different group of everything that excludes things we know exist?
Are they saying this alternate universe is made up of something other than matter, energy and space?
If it is made up of those particles, wouldn’t it (by definition) be our universe?
There are a few theories about how there can be multiple universes. They are considered to be part of a multiverse.
But you see, that’s self contradictory with the definition given above. If they contain matter/energy/space, by definition they are part of this universe. So what new definition of universe are they using?
It’s just semantics. The definition is not really the correct one for a multiverse scenario.
“The universe” or “our universe” would be the set of all energy and matter we can observe or possibly reach with a starship, given infinite time. “Other universes” are some additional energy and matter that cannot be observed or reached directly.
Or you could argue that “existing” means “observable or reachable”.
If any of the multiverse theories are true, then that definition is wrong.
But they’re only theories; until me manage to prove a multiverse theory (not likely any time soon, if ever), that definition works OK.
There are a few multiverse theories, and they differ somewhat.
Like what Heracles said.
It depends on what the definition of “is” is.
(or, in our case, on what the definition of “exists” is)
The universe can be thought of as everything that we can interact with either now or within the possible future (lightcone). Of course, this makes the universe a personal definition; your cosmic event horizon is not the same as your compatriot in Galaxy UDFy-38135539. All things that are outside of your future lightcone are, by that definition, no longer part of your universe, including other domains that exist beyond or separate from the seed event of our universe. The addition of non-local and non-causal connectivity via entanglement is a complication to that topologically simple geometric definition, but it wouldn’t be the first time that quantum mechanics has thrown a monkey wrench in the works.
Stranger