<p>General Relativity says that our universe is a <a href="https://en.wikipedia.org/wiki/Manifold" rel="nofollow noreferrer">manifold</a> and you can have two types of them, with or without boundary. For example, an infinite plane has no boundary, but an apple has a boundary given by its shell. A definition of boundary implies understanding of differential geometry and is given <a href="https://en.wikipedia.org/wiki/Manifold" rel="nofollow noreferrer">here</a> as well.</p>
<p>As an example, physicists often say our universe is a <a href="https://en.wikipedia.org/wiki/Anti-de_Sitter_space" rel="nofollow noreferrer">AdS</a> manifold. Physically, the point on the boundary must be a place where you cannot pass through.</p>
<p>In order to distinguish the universes as separate entities, it is not necessary for them to have a boundary. For example a buble has no boundary but is finite in volume and you can have lots of them in different places.</p>

CC BY-SA 4.0
2021-09-02T15:57:42.667Z
2021-09-02T17:26:44.747Z
2021-09-02T17:26:44.747Z
Gabriel Palau
<p>We consider Heliopause as our solar system's boundary, Galaxy has its own boundary, certain theories says that there can be lots of universes, only when we define a boundary we can distinguish a universe, then (in generally) what defines our universe's boundary ?</p>

CC BY-SA 4.0
2021-09-02T12:38:37.347Z
There are horizons, not boundaries. https://en.wikipedia.org/wiki/Cosmological_horizon
Daddy Kropotkin
CC BY-SA 4.0
2021-09-02T12:40:27.583Z
We can easily observe other stars and stellar systems, as well as other galaxies; since we don't (yet, anyway) observe other universes, it's not an analogous situation.
Peter Erwin
<observable-universe>
What defines Universe's Boundary?
Kavin Ishwaran
# What defines Universe's Boundary?

(Asked by: username_0 on 2021-09-01)

We consider Heliopause as our solar system's boundary, Galaxy has its own boundary, certain theories says that there can be lots of universes, only when we define a boundary we can distinguish a universe, then (in generally) what defines our universe's boundary ?

**Tags:** observable-universe
> **Comments:**
> **username_1**: There are horizons, not boundaries. https://en.wikipedia.org/wiki/Cosmological_horizon  
> **username_2**: We can easily observe other stars and stellar systems, as well as other galaxies; since we don't (yet, anyway) observe other universes, it's not an analogous situation.  

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## 1 answers

### Answer by username_3 on 2021-09-02. Score: 1 | Accepted answer

General Relativity says that our universe is a [manifold](https://en.wikipedia.org/wiki/Manifold) and you can have two types of them, with or without boundary. For example, an infinite plane has no boundary, but an apple has a boundary given by its shell. A definition of boundary implies understanding of differential geometry and is given [here](https://en.wikipedia.org/wiki/Manifold) as well.

As an example, physicists often say our universe is a [AdS](https://en.wikipedia.org/wiki/Anti-de_Sitter_space) manifold. Physically, the point on the boundary must be a place where you cannot pass through.

In order to distinguish the universes as separate entities, it is not necessary for them to have a boundary. For example a buble has no boundary but is finite in volume and you can have lots of them in different places.


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