The Universe has boundary?

[Rav]

I think that an infinite volume of infinite density is also a consistent endpoint for this backward extrapolation.

Someone correct me if I'm wrong, but doesn't quantum mechanics tell us that the density should be finite (albeit unimaginably high) as per the applicable planck unit? I've seen it expressed as 10^23 solar masses compressed into the space of the nucleus of a single atom.

 

[Pete]

Someone correct me if I'm wrong, but doesn't quantum mechanics tell us that the density should be finite (albeit unimaginably high) as per the applicable planck unit? I've seen it expressed as 10^23 solar masses compressed into the space of the nucleus of a single atom.

I'm sure you're right.
My point was that even a naive non-quantum reversal of expansion won't necessarily reduce an infinite universe to a point.

Mind you, I'm talking out my rear... I can't pretend to understand either GR or quantum physics.

 

[funkstar]

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

Oh, go away. You haven't the slightest clue about recursion theory, and, besides, it really, really isn't pertinent.

 

[AlexG]

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

Yes, but you still come up with a finite volume. If something is finite at any point in it's existence, it can't be infinite.

 

[wellwisher]

An infinite universe can't expand, unless someone can explain what is infinity plus 1 to simulate infinity getting larger? Only finite can expand and only up to the limit of infinity.

Space and time could be infinite, but this will require that the mass/energy of the universe be finite. In this case, only the finite mass/energy and its local space-time induction will expand. The local contraction in space-time, due to mass/energy will create a potential with space-time at infinity. That could cause local contracted space-time to expand.

 

[KilljoyKlown]

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.

Please define 'nothing' a bit more. Is that nothing infinite? or are there other point singularities getting ready to expand into universes as we know them?

'unobservable universe' I thought we had that covered already. First I think there's a great deal of visible universe that is unobservable to us from our current point of view. How much that might be, is unknown to us at this time and if our visible universe is part of a larger unobservable structure. How big is that, infinite or finite?

Whatever! If there's any possibility of something more out there than our universe, it needs to have it's own name. Nothing is not doing it for me and unobservable universe applies to a lot of something I'm not talking or thinking about. How can any of us think about the subject if we keep tripping over what to call it?

 

[wellwisher]

How can any of us think about the subject if we keep tripping over what to call it?

One simple consideration is light can travel faster than matter, since matter can not move at C. Therefore, even if space-time expanded in a way that allows the universe to expand faster than C (many points expanding like the balloon analogy) light will always go further and faster than matter, regardless of what space-time does and how it does it. The observeable universe is matter/energy based, while the unobserveable is only energy based, since the matter/energy of the observeable universe will lag behind aspects of pure energy.

 

[chinglu]

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.

I don't think my ideas of recursion are new.

Can you explain why recursion must include Godel numbers of formulas?

Try to take another look at the recursion theorem.

You are very well uninformed.

Let me try to teach you.

The recursion theorem proves given a function f R->R as in LT, exists a function G such that G(n+1) = f(G(n)).

So, we can start wtih some state and G(n+1) must arrive at only one state.

Can you prove this is true?

 

[Rav]

Yes, but you still come up with a finite volume.

An infinite universe is still infinite even if you take away all the space, much like an infinite number line is still infinite even if you take away every number that it not a prime.

 

[Big Chiller]

When we usually use the word universe we are talking about the observable universe but also everything connected to it, similar to it, but undetectable, we don't usually include literally everything especially what's not connected at all and cannot be compared to the observable universe at all.

 

[AlexG]

An infinite universe is still infinite even if you take away all the space, much like an infinite number line is still infinite even if you take away every number that it not a prime.

Yes, it is. but if the volume is finite, the universe is finite. An expanding universe demands that the universe was smaller in the past. You can't say that an infinite universe was smaller in the past. Nor can you say that an infinite universe contracted. expansion and contraction have no meaning if the universe is infinite.

But if it is infinite now, it must have always been infinite, which doesn't agree with cosmological observation.

 

[markl323]

Please define 'nothing' a bit more. Is that nothing infinite? or are there other point singularities getting ready to expand into universes as we know them?

'unobservable universe' I thought we had that covered already. First I think there's a great deal of visible universe that is unobservable to us from our current point of view. How much that might be, is unknown to us at this time and if our visible universe is part of a larger unobservable structure. How big is that, infinite or finite?

Whatever! If there's any possibility of something more out there than our universe, it needs to have it's own name. Nothing is not doing it for me and unobservable universe applies to a lot of something I'm not talking or thinking about. How can any of us think about the subject if we keep tripping over what to call it?

you are clearly confused and ignorant and you have some attitude for whatever reason i don't care to know. so find the answer yourself.

 

[OnlyMe]

Yes, it is. but if the volume is finite, the universe is finite. An expanding universe demands that the universe was smaller in the past. You can't say that an infinite universe was smaller in the past. Nor can you say that an infinite universe contracted. expansion and contraction have no meaning if the universe is infinite.

But if it is infinite now, it must have always been infinite, which doesn't agree with cosmological observation.

Alex, the logic above only applies in a Newtonian sense.

Within GR space is dynamic. Even should space be infinite, that dynamic character would allow for localized expansions and contractions. The same would be true should space be both dynamic and finite. And extending that logic to the universe as a whole, if it is dynamic, in any sense, and we have reason to believe that it is, whether it is finite or infinite, it could be locally expanding or contracting.

All we can know of the world is that part within our ability to observe it. There is almost certainly a great deal beyond those limitations. That part of the universe within our ability to observe it does in many ways appear to be both expanding and in some instances contracting. This is the case wether the universe as a whole, including those portions beyond our ability to observe them, is infinite or contained in some way and ultimately finite.

 

[Rav]

You can't say that an infinite universe was smaller in the past.

When we say that universe is expanding, we are not making the claim that it is getting bigger. Rather we are saying that the distance between gravitationally bound clumps of matter is increasing locally. That's all we can definitively say.

But if it is infinite now, it must have always been infinite, which doesn't agree with cosmological observation.

How so?

 

[OnlyMe]

When we say that universe is expanding, we are not making the claim that it is getting bigger. Rather we are saying that the distance between gravitationally bound clumps of matter is increasing locally. That's all we can definitively say.

Bold emphasis mine.

This is not necessarily accurate. Strictly speaking when we talk about the expansion of space and associated expanding universe, we must also keep in mind that the measuring measuring rods we use to define distances are also being stretched, expanded or lengthened along with space. The distance remains the same. Light takes the same time to travel the distance involved before and after expansion. Only the wavelength of light is affected not its velocity or the distance represented by time and velocity.

 

[AlexG]

When we say that universe is expanding, we are not making the claim that it is getting bigger. Rather we are saying that the distance between gravitationally bound clumps of matter is increasing locally. That's all we can definitively say.

We are saying that the universe is getting bigger. There is more space and more distance between NON-local gravitationally bound objects. Now, if you want to say that what we observe is only a local effect in an infinite universe, you must provide some evidence for this. Current theory holds there to have been an initial time, some 13.7 billion years or so ago. If there is an initial time, then the universe is not infinite.

 

[Rav]

We are saying that the universe is getting bigger. There is more space and more distance between NON-local gravitationally bound objects. Now, if you want to say that what we observe is only a local effect in an infinite universe, you must provide some evidence for this.

That's not what I meant, and it's probably my fault for being lazy with my wording. By local I meant what we can observe. If we had a true picture of what was going on globally, we might actually know whether or not the universe is finite. I mean sure, if the universe is expanding locally, it's most likely expanding locally everywhere, but this doesn't imply that the universe is getting bigger in the sense that you mean it, because you're assuming that it's finite.

Current theory holds there to have been an initial time, some 13.7 billion years or so ago. If there is an initial time, then the universe is not infinite.

I don't see how that follows. The absence of a temporal dimension as we understand it implies that the BB singularity is necessarily finite?

 

[AlexG]

I don't see how that follows. The absence of a temporal dimension as we understand it implies that the BB singularity is necessarily finite?

Yes. If something is infinite, it is infinite in both time directions. If it's not infinite in one direction, then it isn't infinite.

 

[arfa brane]

We usually consider spatial infinity, and perhaps whether time is infinite when considering the volume of the universe. But what about the boundary of the history of the universe, or its energy boundary?

We know if the universe was 'contained' in an infinitely dense, infinitely energetic point at one stage, it must have been spatially finite.

If the universe has multiple, possibly infinite histories, then there can't be a "history boundary" either. The fact that time appears to point into the future implies that the future history is unbounded. The only thing holding it back is that black holes remove a part of this future for an amount of time that exceeds any measurable history at a given time (like say, now).

This suggests that without black holes, the universe would have an unbounded future of possible histories--cosmologists in the future would not recognise the universe we can see today because its history would change. As it is, future cosmologists will have less matter to look at, perhaps a few slowly evaporating black holes (assuming there's an available source of energy to keep them alive and doing astronomy).
They won't be able to extrapolate backwards to a singular history like we can with what we see today.

 

[Rav]

Yes. If something is infinite, it is infinite in both time directions. If it's not infinite in one direction, then it isn't infinite.

But you're talking about time. I'm talking about volume. So I still don't see how the absence of a temporal dimension (as we understand it) means that the associated volume is necessarily finite.