Does the Universe Have an Edge or Boundary?

[arroy_0205]

Does it make sense to say that the universe has an edge/boundary?

Neutrinos are moving all the time with practically no interaction i.e., unimpeded. Where are these reaching after a very long time (t->infinity)?

 

[Sciurus]

Well, it has an edge in the sense that if the universe has a finite age, there is a radius beyond which events can have no effect on us. Or as another way to look at it, as you look at stars further and further away, you are looking at events further and further in the past. There would be some distance at which you would be looking at the start of the universe, and you can look back no further and therefore at things no farther away, so that is "our" local edge.

I don't know of anything that would impede the expansion, as time continues to increase to infinity or that would stop a neutrino. But I'm no expert, so if someone knows differently, I'd be interested to find out.

 

[bapowell]

The global geometry/shape/topology of the universe is currently unknown. If the universe is closed then it is homeomorphic to a sphere -- finite, but no edges, no boundaries. If it is flat or open, then there may or may not be edges.

But in reference to Sciurus' post, it's important to distinguish between the edge of the universe and the edge of our observable patch.

 

[_PJ_]

If the universe is closed then it is homeomorphic to a sphere

No. A sphere is a 3D object bound by its surface.
A more precidse analogy would be this 2D surface of a sphere as representing spacetime.
This is also inadequate though, since a sphere's surface has exact and equivalent curvature on every point.

I don't know of anything that would impede the expansion, as time continues to increase to infinity or that would stop a neutrino. But I'm no expert, so if someone knows differently, I'd be interested to find out.

Current standard model as well as inflation theories agree that there has been expansion of the universe greater than 'c', this suggests, that even the oldest photons would not reach any 'boundary' or limit to the universe.

 

[bapowell]

No. A sphere is a 3D object bound by its surface.
A more precidse analogy would be this 2D surface of a sphere as representing spacetime.
This is also inadequate though, since a sphere's surface has exact and equivalent curvature on every point.

The word sphere, by itself, makes no implication of dimensionality. To be more specific, the spatial geometry of a closed universe is homeomorphic to a 3-sphere ($S^3$). Here, the hypersurfaces of constant time are 3D surfaces with the topology of a sphere.

With regards to 'exact' and 'equivalent curvature', I think you mean 'constant' curvature. Yes, this is true. These important results apply to homogeneous and isotropic universes -- properties that our universe possesses only on a global scale. But this is why I say global geometry.

Current standard model as well as inflation theories agree that there has been expansion of the universe greater than 'c', this suggests, that even the oldest photons would not reach any 'boundary' or limit to the universe.

You are making a common misconception. Inflation did not cause the universe to expand at a speed 'greater than c'. Expansion is not a speed. Even in spacetimes with non-accelerated expansion, there are regions of the universe receding from Earth at speeds surpassing c -- they are located at the Hubble radius. The important difference between inflation and non-accelerated expansion is that during inflation the Hubble radius is increasing at a rate slower than the background spacetime. However, inflation makes no claims about the global geometry or topology of the universe. I personally don't like the idea, but there very well could be edges to the universe outside our causal horizon -- inflation or no inflation -- we simply don't know.

 

[ZeroKelvin]

Hi
Sorry for my bad English, I hope, however, it can communicate with you all.
I'm a fan of cosmological and physical sciences.
I would like to express my thoughts to know what you think.
I would like to add my opinion in this regard, TD.


I think that the universe has a limit.
This limit is determined by its energy.
The space grows by the amount of energy available.
I believe that there is a close relationship between energy and space.
I think that the energy emitted by the phenomenon of the Big bang has generated space, a space finished only partly visible to us.
We know its real dimensions only when we manage to understand the phenomenon of the Big bang.(how, where, when, why)


Cordially