The Universe has boundary?

Based on what facts do we conclude that the universe has boundary?
We only can not see with our telescopes the things lying far far away from us, where the light has not yet reached us because it is too far away,
we actually don't see and don't know what are those things so far away, however, we concluded that the universe has boundary,
this is not scientific.
what do u think?

 

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Well lets just say there is always something behind a boundary. Yes?

If there is something behind the boundary then why the conclusion of a boundary?

Are you saying this universe into the next universe or are you claiming space stops, ceases to exist?

 

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I do not know because the text book of Physics just says that according to big bang theory, the universe has an expanding boundary.

 

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And the astronomers try to support their hypothesis by telescopic observation.

 

I guess it is just like an ocean habitat expanding to once "dead" ocean. Here they would say the boundary is where the two meet.

 

We don't. We conclude (from best available evidence and models) that the universe has no spatial boundary.

 

Cosmologists say that the universe is actually expanding faster and that whatever it is that keeps it together will perhaps one day be broken free from and the universe will just die away.


"Earlier at the time after big bang it was expanding much faster than the speed of light. This is just a theory. Now it is expanding at a slower rate.
Recently, there has been evidence to show that the speed of which the universe is expanding is accelerating.


If the theory which states it was traveling faster then the speed of light were true then hypothetically there is something which travels faster than light, 'The Universe', hehe, so then everything at one time could have been travelling faster then light, but in relation to what? Now that is a hard one like trying to image infinite space, tricky.

I have recently read that some galaxies move at an alarming rate and others are relatively slow and nowhere near the speed of light. Also in regards with galaxies slowing, some physicists have been lead to believe that the universe itself will eventually come to a stop and no longer expand. But then, at this point, all of the galaxies which were once expanding, will alternatively contract. This is called the Crunch Theory/Phase.

This theory was believed for some period of time, but now, not all physicists believe it will contract/crunch anymore."


http://www.dogpile.com/clickserver/...23=0&40=TBxE0yG5yJYmQQehSYZEeg==&_IceUrl=true

 

The visible universe has a boundary, it's due to the finite age of the universe, the speed of light and space-time expansion. Whether beyond that visible horizon the universe has an 'edge' is unknown and perhaps unknowable.

 

If the universe does have a boundary then where are all the galaxies being carried away to? They are accelerating away from each other and outwardly as well.

 

I've never pursued this much, but this view (which I've read before) doesn't seem consistent with the idea the spatial and temporal dimensions "emerged" from the Big Bang does it?

 

Who knows ...?
But I give you a scenario where the universe is finite, but it does not have boundary.

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To understand me, I have to resort to an analogy between an universe with two spatial dimensions and an universe with three spatial dimensions.
We assume the existence of a two-dimensional universe.
For this universe there are only two dimensions. They can not perceive the third dimension, so for them no third dimension.
But if these two-dimensional universe, is curved in accordance with the third dimension and form the surface of a sphere in three-dimensional universe?
An interesting property of this universe would be, a straight line drawn from one point in any direction, returns to the point of origin. So these universe would be finite.
But if our three-dimensional universe is curved according to a fourth spatial dimension, so that it forms a surface of a hypersphere in an universe with four spatial dimensions?
That would mean that a straight line drawn from a point in any direction, it returns to point of origin.
This universe would be finite.

 

I agree with you, saying the universe has a boundary is like saying the Earth is flat...

 

AlphaNumeric didn't say the Universe has a boundary, he said the visible part of the Universe has a boundary.

I don't know anything about the idea of spatial and temporal dimensions emerging from the Big Bang... I don't think that's part of the Hot Big Bang model at all? Is that from String Theory or M-Theory?

I don't pretend to understand GR well, but my fuzzy concept is that:

• The Hot Big Bang model assumes an existing hot and dense space-time manifold.
• If you extrapolate the model back to it's mathematical conclusion, you reach a singularity of infinite density
• This singularity is not interpreted as a physical conclusion, but as an artefact of the model outside its domain of applicability.

But that's peripheral.
To address your point, I think that a 3-space without spatial boundaries (ie the space of the Universe 'now') is consistent with a 4-space with a timelike boundary in the past (ie the spacetime of the Universe with a beginning).
Again my understanding is lacking, but for what it's worth, I think that there are two broad possibilities, one of which I understand better than the other:

1. The 3-space may be spatially finite (bounded), but without a boundary (like the surface of a sphere, a torus, or a klein bottle). This is consistent with a continuous 4-space with a single point of minimum time (a definite beginning).
2. The 3-space my be spatially infinite. I think (not sure) that this is not consistent with single point of beginning, and I'm not sure how this is addressed in the Hot Big Bang model. I think that a past timelike boundary in an infinite universe must itself be infinite, ie if the Universe is infinite now, then it must have been infinite at the time of the Big Bang.
   I don't understand all the implications of that.

 

Yes, a finite 4D hypersphere would be enclosed by infinite 3D space as its so-called "surface". The 4th dimension amounts to time, which is both the polarity of charge of matter particles as well as the motion of the particles, for time is the difference of space(s).

The ratio of the hypersphere (Distance^4) to spacetime (TimeDistance^3) reduces to Distance/Time, the dimensions of the one and only possible speed of light (c), which is the dimensional equivalent of distance to time.

This arrangement quantizes energy.

 

truly a sane statement. i'm tired of reading that the universe is infinite as a fact when we just don't know. actually it's preposterous to make such a definite claim, no different than religious dogma.

 

I have an uncertain recollection of reading in Kip Thorne's Black Holes and Time Warps that to embed the curved spacetime of GR into a Euclidian space, you need six or seven dimensions altogether.

But note that this doesn't mean that the extra dimensions have to be real, because there doesn't seem to be any reason that a space has to be euclidian.

He also gave a great description of how a spherical geometry could be realised on a flat plane by having rulers that stretched or compressed according to their position.

I wish I had better intuition for multi-dimensional non-euclidian geometry.

 

Pete:

The "common analogy" is what I've read about. "Space itself" being described as expanding is how I've seen the period of cosmic hyper-expansion in apparent violation of c to be explained. I'm certain I've read many times that space itself "came into existence" with the Big Bang; you haven't? It really forces us to consider, what IS space? Is it "something" beyond emptiness? If not, "what" does GR predict is curved in the presence of energy?

 

Suppose the BB and our current visible universe is a natural product of a much larger structure. Has happened before and will happen again and probably happens concurrently at distances from each other we can't begin to understand. If infinite space time does exist, this concept is not unreasonable and actually brings our universe back into context with nature and allows speculation about what kind of structure might support a BB that results in a visible universe.

 

The world is flat and if you sail to far out in the ocean you will fall off the edge

 

Yes, but that analogy doesn't pretend to describe spacetime coming into existence, just how it changed from some particular state to some other state.

Yes, I've read that in pop-sci expositions, but on investigating textbooks and academic publications I've found that the Hot Big Bang model actually presupposes spacetime. The origins of spacetime itself is in the realms of speculative theories, if my amateur understanding is correct.

Yes, this was recognized and acknowledged by Einstein fairly early on, I think, and is an obvious part of all mainstream theories, as far as I know. Spacetime is something real that has real properties, such as geometry.