The Universe has boundary?

[Rav]

How can Big Bang occur in every point in infinite volume?

How can the universe be expanding metrically if it didn't?

If the universe is indeed infinite (and the fact that leading cosmologists are now convinced [by the available evidence] that it's flat suggests that it is) then if you run the clock of metric expansion back towards the BB, no matter how close you get the universe is still necessarily infinite. In other words, regardless of how little space there is, the universe is still an infinite volume. Take just one more step and you arrive at a 'singularity' that is an infinite volume of energy without any space at all.

 

[AlexG]

How can the universe be expanding metrically if it didn't?

If the universe is indeed infinite (and the fact that leading cosmologists are now convinced [by the available evidence] that it's flat suggests that it is) then if you run the clock of metric expansion back towards the BB, no matter how close you get the universe is still necessarily infinite. In other words, regardless of how little space there is, the universe is still an infinite volume. Take just one more step and you arrive at a 'singularity' that is an infinite volume of energy without any space at all.

The universe being flat will result in indefinite expansion. It does not imply that the universe is infinite. If the universe is infinite, it means it has always been infinite in the past, and our expanding universe says that this is not so.

 

[KilljoyKlown]

What do you mean "in context with nature"?
How do you tell if something is in context with nature or not?

Well to say that there was no spacetime before the BB is not exactly nature friendly.

I'm not trying to say there was no BB, but I am saying I believe it all happened within a much larger structure as a natural event that has happened many times before and will continue happening off into the distant future. The only information I have about the larger structure is that it contains the visible universe that we know and love.

 

[Rav]

The universe being flat will result in indefinite expansion. It does not imply that the universe is infinite.

At the very least we can say that a flat universe can be infinite, but you're right. My previous comment should read like this: "and the fact that leading cosmologists are now convinced [by the available evidence] that it's flat suggests that it is might be". To me the "suggestion" that it's infinite emerges from recent advances in cosmology being consistent with what I at least tentatively consider to be the sound metaphysical idea that "somethingness" is necessarily infinite. But obviously such a discussion is more suited to the philosophy sub-forum.

If the universe is infinite, it means it has always been infinite in the past, and our expanding universe says that this is not so.

I don't see how an expanding universe is at odds with an infinite universe.

 

[Pete]

The universe being flat will result in indefinite expansion. It does not imply that the universe is infinite. If the universe is infinite, it means it has always been infinite in the past, and our expanding universe says that this is not so.

That doesn't sound right at all.
How can a flat universe be finite? What defines its borders?
And how does an expanding universe imply a finite universe?

 

[Pincho Paxton]

Imagine that you have a sea.. it goes out forever, but the water is invisible. In that sea you squirt in some washing up liquid. the washing up liquid is also invisible. Now you put a straw in the sea, and blow into the straw. You get some bubbles. the bubbles are the visible universe, however the sea is infinite. The sea is the whole universe. The boundary to the end of the visible universe is the final set of bubbles.

 

[Pete]

Well to say that there was no spacetime before the BB is not exactly nature friendly.

Firstly, mainstream cosmology doesn't say that space and time started with the big bang - that's a pop-sci extrapolation (see earlier in the thread).
The nature of the universe before the hot, dense, state know as the big bang is an open question.

Secondly... I don't understand what you mean by "nature friendly." It sounds like a "warm fuzzies" kind of attitude, which is no way to distinguish between what's real and what isn't.

I'm not trying to say there was no BB, but I am saying I believe it all happened within a much larger structure as a natural event that has happened many times before and will continue happening off into the distant future. The only information I have about the larger structure is that it contains the visible universe that we know and love.

Well, there might be some cyclic nature to the universe... but what leads to you to believe it?
Similarly, what leads you to believe that the larger universe is infinite?
Does it give you a warm fuzzy sense of rightness?
If so, I don't think that's a reliable guide - our intuitions just aren't that useful in a context so far removed from our daily experience.

Why not consider the possibilities without committing yourself to a particular belief?

 

[Pete]

The only problem I have with the BB theory is that there was nothing before the BB, I disagree with that hypothesis. There has to be some space in which the big bang occur, otherwise it's pretty much you say your car is driving and parking on the road, however the road, the place where your car is driving and parking, does not exist.

Well, the BB model does not say that there was nothing before - it starts with an existing hot, dense spacetime, and describes what happened from there.

As for the universe expanding into something...
For a closed universe, I think you can describe a flat higher dimensional space in which the universe is embedded and expanding, just like the 2D surface of a balloon expands in a 3D space.

But, that would only be an abstract mathematical space. The actual universe is like the 2D surface of the balloon - everything we see and interact with is on the balloon surface.

There does not seem to be any reason to think that that the hypothetical surrounding space is anything more than a mathematical abstraction.

 

[Pincho Paxton]

Why would we be on the surface of the balloon? We are supposed to be inside the balloon, and the expansion is the entire collective expansion inside the balloon.

 

[Pete]

Hi Pincho,
In the balloon analogy for universe expansion, the universe is the surface of the balloon, not the interior.

 

[chinglu]

Firstly, mainstream cosmology doesn't say that space and time started with the big bang - that's a pop-sci extrapolation (see earlier in the thread).
The nature of the universe before the hot, dense, state know as the big bang is an open question.

Secondly... I don't understand what you mean by "nature friendly." It sounds like a "warm fuzzies" kind of attitude, which is no way to distinguish between what's real and what isn't.


Well, there might be some cyclic nature to the universe... but what leads to you to believe it?
Similarly, what leads you to believe that the larger universe is infinite?
Does it give you a warm fuzzy sense of rightness?
If so, I don't think that's a reliable guide - our intuitions just aren't that useful in a context so far removed from our daily experience.

Why not consider the possibilities without committing yourself to a particular belief?

By the recursion theorem, you can show a recursive process for this expansion.

May I see that please?

 

[markl323]

But there has to be space beyond the BIG Bang because there wouldn't be space where the big bang could expand, the same as the balloon can't expand if there is no space outside the balloon.

space expands and creates more space and it goes. it doesn't require already existing space to expand.

 

[markl323]

i think the "flat" universe implies the that the "patch" of space that has expanded to our visible universe today was "flat." but this doesn't mean that it couldn't have been part of a huge sphere. meaning, if you have a sphere that is big enough, and you take a patch of space on the surface and look at it, it will look "flat." so the answer is we don't know if our visible universe is just a patch of a much bigger sphere or not. most leading scientists think our visible universe is part of a larger structure. but they don't know if it's a sphere or not.

http://en.wikipedia.org/wiki/Inflation_(cosmology)

"For cosmology in the global point of view, the observable universe is one causal patch of a much larger unobservable universe."

one of the theories for Big Bang states that in the beginning, there is nothing, not even space. then due to quantum fluctuations, this nothing "cracks" and matter gets separated from antimatter, the super-force breaks down to smaller forces (strong nuclear force, weak nuclear force, gravity, etc.) and space inflated, then expanded. if this is how it happened then i think the global universe (as opposed to our visible universe) should have a spherical shape and a boundary that is much further away than the visible "boundary" we can observe. and our visible universe is just a small patch of this sphere.

 

[KilljoyKlown]

i think the "flat" universe implies the that the "patch" of space that has expanded to our visible universe today was "flat." but this doesn't mean that it couldn't have been part of a huge sphere. meaning, if you have a sphere that is big enough, and you take a patch of space on the surface and look at it, it will look "flat." so the answer is we don't know if our visible universe is just a patch of a much bigger sphere or not. most leading scientists think our visible universe is part of a larger structure. but they don't know if it's a sphere or not.

http://en.wikipedia.org/wiki/Inflation_(cosmology)

"For cosmology in the global point of view, the observable universe is one causal patch of a much larger unobservable universe."

one of the theories for Big Bang states that in the beginning, there is nothing, not even space. then due to quantum fluctuations, this nothing "cracks" and matter gets separated from antimatter, the super-force breaks down to smaller forces (strong nuclear force, weak nuclear force, gravity, etc.) and space inflated, then expanded. if this is how it happened then i think the global universe (as opposed to our visible universe) should have a spherical shape and a boundary that is much further away than the visible "boundary" we can observe. and our visible universe is just a small patch of this sphere.

Sounds like you are still talking about a finite universe even if it's a lot bigger than our visible universe? My question then becomes if our universe is finite, regardless of it's size it still has to be somewhere. I want to know what that somewhere is called? Even if we are only talking theories, it should have a name. I suppose someone is going to confuse the issue again and say it's the universe. Please don't, for the purpose of this question we have already defined the universe as being finite.

 

[Rav]

The problem with a finite universe is the question of what defined the precise quantity of it.

 

[AlexG]

That doesn't sound right at all.
How can a flat universe be finite? What defines its borders?
And how does an expanding universe imply a finite universe?

If the universe is infinite, it must have always been infinite, which means that it was infinite in the past.

An expanding universe implies that at one time, the universe was smaller, and if you extrapolate into the past, at one time, it was a point. If it isn't infinite in the past, it's not infinite.

 

[markl323]

Sounds like you are still talking about a finite universe even if it's a lot bigger than our visible universe? My question then becomes if our universe is finite, regardless of it's size it still has to be somewhere. I want to know what that somewhere is called? Even if we are only talking theories, it should have a name. I suppose someone is going to confuse the issue again and say it's the universe. Please don't, for the purpose of this question we have already defined the universe as being finite.

according to that theory yes, the universe should be finite because it came out of nothing then inflated and expanded.

anything outside of the visible universe is called the unobservable universe. this is the only name i have seen used to refer to it. there may be other names. there is no name for what is outside of this unobservable universe because there is nothing there. you can google Dr. Kaku for his books. they are very easy for laymen to understand. just ignore all the futuristic stuffs if you are not into it or think he's full of it.

 

[Pete]

An expanding universe implies that at one time, the universe was smaller

No, that only works for a finite universe.
An infinite expanding universe was always infinite.

and if you extrapolate into the past, at one time, it was a point.

Only if you extrapolate beyond the domain of applicability. And even then, you don't necessarily get a point.
I think that an infinite volume of infinite density is also a consistent endpoint for this backward extrapolation.

 

[Rav]

An expanding universe implies that at one time, the universe was smaller, and if you extrapolate into the past, at one time, it was a point. If it isn't infinite in the past, it's not infinite.

The whole point of our decades long search for a quantum theory of gravity is about resolving absurdities like point-like singularities.

 

[AlphaNumeric]

By the recursion theorem, you can show a recursive process for this expansion.

Chinglu, I know the 'recursion theorem' is your new favourite phrase but it doesn't have any relevance here or the other places you have brought it up.

Rather than trying to crowbar in buzzwords you don't understand in order to appear like you're competent please keep things directly relevant to the discussion. Discussions about relativity do not need to involve Godel numbers or computability.