# B A Question about space and the multiverse

#### Quantum Alchemy

My question is about space and the multiverse. I was reading work by Max Tegmark and he sees the multiverse as Level 1-4. A Level 1 multiverse seems like it's self evident and I was wondering about the evidence against it. It's simple:

Space expands faster than we can observe it so we exist in a pocket or bubble universe that's simply a sphere determined by the speed of light in every direction. So our observable universe is just limited by how far we can see in any direction.

If you go to a galaxy a billion light years away, couldn't you say it's observable universe is a sphere that's determined by the speed of light in every direction from that point and will include space outside of our observable universe.

Wouldn't there be an infinity of the sphere's or pockets as space expands? Does space end because we can't observe it?

Just curious.

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#### phinds

Gold Member
My question is about space and the multiverse. I was reading work by Max Tegmark and he sees the multiverse as Level 1-4. A Level 1 multiverse seems like it's self evident and I was wondering about the evidence against it. It's simple:

Space expands faster than we can observe it so we exist in a pocket or bubble universe that's simply a sphere determined by the speed of light in every direction. So our observable universe is just limited by how far we can see in any direction.

If you go to a galaxy a billion light years away, couldn't you say it's observable universe is a sphere that's determined by the speed of light in every direction from that point and will include space outside of our observable universe.

Wouldn't there be an infinity of the sphere's or pockets as space expands? Does space end because we can't observe it?

Just curious.
Yes, that's the definition of Observable Universe, BUT ... you are mistaken if you think that they are in any way disconnected from each other. Every single point in the universe has its own OU, but so what? That is not a multiverse in the normal sense of that term. [When you get far enough away you ARE causally disconnected, yes, but that's still not what's normally meant by a multiverse] Because every point has its own OU, they are a continuum.

#### Quantum Alchemy

Yes, that's the definition of Observable Universe, BUT ... you are mistaken if you think that they are in any way disconnected from each other. Every single point in the universe has its own OU, but so what? That is not a multiverse in the normal sense of that term. [When you get far enough away you ARE causally disconnected, yes, but that's still not what's normally meant by a multiverse] Because every point has its own OU, they are a continuum.
This is my point. I said I'm talking about Tegmark's level 1 multiverse not the Parallel universes of QM and Schrodinger's Cat or Inflation.

So, yes, they would be connected. All of these observable universes would share the same physics and the same space. The only thing that would determine these observable universes is the speed of light. So a planet in the Andromeda Galaxy has a different observable universe than earth. So this planets observable universe would consist of space that's not in earth's observable universe and vice versa.

Unless space vanishes because we can't observe it. What's the flaw in this line of thinking?

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#### phinds

Gold Member
What's the flaw in this line of thinking?
There is none. I am not familiar w/ the various forms of "multiverse" and have never considered what you are talking about to be one. If someone wants to define it that way, well OK.

#### Bandersnatch

Space expands faster than we can observe it so we exist in a pocket or bubble universe that's simply a sphere determined by the speed of light in every direction. So our observable universe is just limited by how far we can see in any direction.
Just a note here: The first sentence here is not entirely correct - we can see farther than where the recession velocities reach the speed of light. I've noticed Tegmark makes confusing statements in his classification - e.g. here: https://arxiv.org/pdf/0905.1283.pdf he calls what is the particle horizon (how far we can see, i.e. our observable universe; ~46 billion light-years) the Hubble volume ($c/H$; i.e. where expansion reaches the speed of light; ~14.4 billion light years). It's clear that he means the former, since he includes the size. Whereas if somebody then checks the term on the Wikipedia or in a cosmology textbook (e.g. B.Ryden, Introduction to Cosmology) they'll find the latter.
Also, he uses cosmic horizon synonymously with observable universe, which can lead to confusion with cosmic event horizon - which is the maximum possible extent of the observable universe.
Treat every mention of Hubble volume (or cosmic horizon) as the size of the observable universe, and you should be fine.

If you go to a galaxy a billion light years away, couldn't you say it's observable universe is a sphere that's determined by the speed of light in every direction from that point and will include space outside of our observable universe.

Wouldn't there be an infinity of the sphere's or pockets as space expands? Does space end because we can't observe it?
As far as I understand, Tegmark's level 1 calls for observable universes that are entirely non-overlapping. I.e. you need to go to a galaxy 2*the radius of the OU away to momentarily be in another lvl 1 universe. The point being that we are currently completely causally disconnected from any event happening in our next-door neighbour, so in this sense the universes are separate.
However, since the classification is arbitrary, we could very well adopt an overlapping scheme for our own classification (maybe call it level 0 multiverse). Then every one of the infinite number of observers would have their own, at least slightly non-overlapping, lvl 0 universe.
I don't understand why you think this constitutes evidence against Tegmark's lvl 1 multiverse, though.

"A Question about space and the multiverse"

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