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Physics, Astronomy & Cosmology / Re: Universe related questions
« on: 25/05/2024 06:02:08 »1. Does Universe has to meet some prerequisite before it can be considered a Universe?The answer to all three questions depends on one's definition of 'universe'. There is no one physics definition of the word.
The universe is by definition "all that there is".Well, that's one definition, and one that renders the word 'muliiverse' meaningless since there cannot be another universe that is something beyond 'all there is'. I'm not saying the definition is wrong. It certainly answers the 3 questions, but the answer to 2 for instance would be 'no' because supposedly what we see is quite a lot, so 'all there is' cannot be something that isn't a lot.
But under a different definition where this small thing can be its own universe, one of many, sure, there can be a little one, even if there's probably not a meaningful way to compare the size of one to the size of another.
I find a pragmatic definition of 'universe' to be a relational one: all that has a causal effect on a given thing, say Earth as we know it. That's sort of pretty close to what is usually referred to as the visible universe, a finite size thing. Since most people consider a star 60 billion light years away to, well, 'exist', most people would probably not agree with that pragmatic definition which says that mildly distant star does not in fact exist relative to Earth.
There is also an epistemological definition: The universe is everything we know about, and all the rest (the little universe say) is a different universe.