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Offline Murchie85

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The Boundary?
« on: 27/03/2010 00:48:36 »
Hey guys,

Im new to the forum, so I guess  the best way to start is with a post! ;)

Ok so space and the known universe as I understand it was formed from the big bang, is appox 14 billion light years in diameter and thus about 14 billion years old (So far so good?). Of course there are a bunch of theories predicting how the nature of the universe such as inflation theory, m/string theory.

What I would like to know is what defines the external boundary of space and the universe? For example if the universe is 14 billion years in age, what lies beyond the void? Does it have potential for matter to exist there and thus just the same like the space here? Or is it a physical/non physical thing that prevents light from going further? I have read journals positing that there is no edge and light would end up at the other side like a spaceship going out one side of the screen in a computer game, although I believe that might have been in connection with string theory. So what is the current astronomical/physics view on this?

-Adam


 

Offline Murchie85

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« Reply #1 on: 27/03/2010 10:44:55 »
Also excuse my grammar, maybe next time I will post earlier in the day :P
 

Offline graham.d

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« Reply #2 on: 27/03/2010 10:51:03 »
First of all nobody knows the answer to your questions but there are quite a few theories. The size of the universe has different definitions. Your initial statement is trying to give an idea of what would be called the "observable universe" based on the idea that we cannot see beyond a point where the velocity of recession due to the Hubble expansion would exceed the velocity of light. This is thought, by some theories, to be true but is rather more complicated than your description.

The theory regarding objects going out one side and appearing on the other is the idea that the universe is a "closed manifold" or compact hypershere (an analogy to a spere in 4 dimensions). There are many other models and we have not got enough precise observations to rule out too many models - although I think the original "steady state" theory has been put to bed. The closed manifold would have no "outside".

This is a huge subject. It is worth reading up the large amount of data on it on the web.
 

Offline LeeE

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« Reply #3 on: 27/03/2010 18:39:34 »
Rather unintuitively, although the Universe is believed to nearly 14 billion years old this doesn't mean that the observable universe is only 28 billion light years in diameter.  Due to the expansion of the universe, what we see as being ~14 billion years old is now actually about 46.5 billion light years away, making the current diameter of the observable universe about 93 billion light years.  The observable universe would only be ~28 billion light years in diameter (being ~14 billion years old) if it were not expanding and everything is still where it was when it emitted the light 14 billion years ago.

As to the boundary of the universe, well in one way it depends on whether you think that the future already exists and is predestined, or whether we are constantly moving into 'new' time.  This can't be proven, either way, but on the assumption that we are moving into 'new' time (because that's how it appears) then we exist right on the temporal boundary of the universe - right on the very edge.  Also note that in this respect, the universe is clearly finite i.e. finite in age.

Regarding the spatial boundary of the universe and as to whether something might disappear out of one side and reappear on the other, it depends upon whether the universe is open or closed, as graham.d says.

This diagram (from NASA via wikipedia) shows it quite well, although you need to get your head around the fact that what is being shown here is a two dimensional representation of three dimensional space i.e. a  3D volume is being represented by a 2D surface so that we can see the curvature:



The key to that diagram is:
Quote
Diagrams of three possible geometries of the universe: closed, open and flat from top to bottom, corresponding to a density parameter Ω0 which is greater than, less than or equal to 1. The closed universe is of finite size and, due to its curvature, traveling far enough in one direction will lead back to one's starting point. The open and flat universes are infinite and traveling in a constant direction will never lead to the same point.

Permission of NASA Official: Gary Hinshaw
Note that copyright should be assumed.

Another good diagram is this one (courtesy of the NASA/WMAP Science Team):



which shows the universe as an open 'cone', where the profile of the cone shows the rate of expansion over time.  While the 'end' of the cone continues getting bigger the universe appears to be heading for an 'open' future, but if the 'end' of the cone were to start contracting and eventually close up then the universe would be closed.

Open universes might be currently finite in extent, limited by the currently finite age of the universe, but if the extent of time is infinite and has no end then the universe too is essentially infinite in extent, even if it might not yet have extended to infinity.  However, even if the universe is not currently infinite this doesn't mean that there is necessarily something 'outside' it, for just as we seem to move into 'new' time and not a pre-existing future, the universe could be expanding into 'new' space that doesn't exist until the universe expands into it and fills it.

Should a hypothetical space probe reach the edge of a currently finite open universe (it couldn't because the universe has a 14 billion year head start) it would not disappear and then reappear on the opposite edge, but instead would just cause the universe to extend (because the probe is part of the universe, don't forget) to to still include the probe within it.
« Last Edit: 27/03/2010 18:42:33 by LeeE »
 

Offline Murchie85

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« Reply #4 on: 27/03/2010 21:31:57 »
Great info guys, and thanks for all the effort Lee,seriously that's a good whack of stuff I had read in places but putting it together helps me gain a new perspective. Although that was something I had a problem with, I was taught the universe was 14 billion lighyears in diameter but that would assume that we live in the middle and see 7 each way or 10 and 4 etc. Is there a current accepted value for the diameter of the known visible universe?
 

Offline yor_on

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« Reply #5 on: 27/03/2010 22:51:00 »
The Shape and Topology of the Universe 
===

And after reading that one you will naturally want to take a look at THE PLANCK MISSION. too see if they have in any way confirmed a topology. Their 'homepage' are at project=PLANCK 
« Last Edit: 27/03/2010 23:48:40 by yor_on »
 

Offline yor_on

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« Reply #6 on: 28/03/2010 00:17:47 »
That one confuses me LeeE? "but if the extent of time is infinite and has no end then the universe too is essentially infinite in extent, even if it might not yet have extended to infinity."

Topologically you can have different 'shapes' to a universe it seems to me without therefore limiting 'time'? Or am I misunderstanding it?
 

Offline LeeE

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« Reply #7 on: 28/03/2010 13:42:08 »
That one confuses me LeeE? "but if the extent of time is infinite and has no end then the universe too is essentially infinite in extent, even if it might not yet have extended to infinity."

Topologically you can have different 'shapes' to a universe it seems to me without therefore limiting 'time'? Or am I misunderstanding it?

Perhaps the key word there is yet.  The amount of time that has passed appears to be finite, but time itself appears to be infinite: it just hasn't all happened yet (and if it is infinite then it can't ever all happen, so that at any point in an infinite range of time, the amount of time that has passed will still be finite).  At the same 'time' it is impossible to run ahead of time, so no matter how far you travel in time you'll still be within it.

In the same way, if the universe is spatially open it might not yet be currently infinite in size but if there is no limit upon its size it'll impossible to get outside it, for anything approaching the boundary, including EMR and virtual particles, will extend that boundary.  Even without the expansion of the universe within its boundaries, if the universe is currently finite in size it would appear to be extending, at its boundary, at the speed of light, hence the point about not being able to catch up with the boundary because of that 14 billion year head-start it had.

Note the bolded provisos.
 

Offline yor_on

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« Reply #8 on: 28/03/2010 17:04:04 »
I first thought you might consider time in relation to the universes relative 'size' :) Even though we call time a dimension, and we need it to describe any event happening inside SpaceTime time, it still reminds me of those smaller and larger infinities mathematicians labor with. And in that motto I would call time as infinite as the universe might be, and having no relation to its possible size, just as a 'smaller infinity's' also are infinity's, meaning that there aren't any real limit to them. But that wasn't your beef here, right :)

You were considering how 'times arrow' might express itself in a unbounded open universe.

I think matter takes times arrow with it wherever it 'goes'. So thinking of your example of an unbounded universe then it either exist, and so will have an arrow of time, or it isn't 'there' for us, and then the arrow won't be there either. I don't expect any 'space-boundary' to split SpaceTime's 'lawful areas' from the 'wild magic areas' outside our arrow of time :) that as we're creatures of times arrow and so will bring it with us, wherever we can exist. Doesn't mean that there isn't a 'boundary' of sorts and that SpaceTime can't 'grow' though, just that we won't be able to define what those 'areas' really are like, as a guess that is :)

Times arrow is to me the property making us able to discuss, but without that arrow? So to me it's a must for any SpaceTime making sense to me. And if that is correct then it's impossible to say where it 'ends'. Then it will end when a SpaceTime ends, and even with a gluon soup I would expect 'time' to exist although what we see as the arrow might be gone, possibly? It's like 'space', not there at all, but very necessary for our universe to 'work'
===

Did this make sense?
« Last Edit: 28/03/2010 18:20:15 by yor_on »
 

Offline Murchie85

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« Reply #9 on: 29/03/2010 00:28:42 »
Its not hard to see how these debates are considered some of the most challenging and hardest conceptual pieces in human thought. On your point YOR, another thing people sometimes take for granted is the universe may have existed prior to the big bang in constant exploding,imploding cycle so in that sense talking about infinities of time may also suggest that the past could also be infinite which would be another spanner in the works.

 It is easy to see when discussing this just how little we actually know about the whole picture and how much more work is needed. I think one major obstical is determining if the universe is open or closed, if for arguments sake the universe was closed and quantum/string theory posits alternate universes then the picture may still be as big as an open universe.

 

Offline yor_on

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« Reply #10 on: 29/03/2010 03:52:57 »
well yes, I can't help loving physics. It's the best RPG there is.
 

Offline LeeE

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« Reply #11 on: 29/03/2010 08:54:51 »
Did this make sense?

I'm sorry Yor_on, but I must confess that I didn't quite follow most of that.

It seems to me that you're trying to understand something using concepts that won't really allow the understanding you want.  For example, in adhering to the idea of 'Time's arrow' you still seem to be thinking of time as something that flows past us, rather than a dimension through which we move.  Similarly, I'm afraid that thinking in terms of 'lawful areas' and 'magic areas outside our arrow of time' seems more like resorting to mysticism rather than reason and is only going to lead to more confusion rather than clarity and understanding.

Sure, getting your head around some of the concepts involved is, as Murchie85 says, amongst some of the most challenging of mental tasks but if you find that you can't explain something using ideas that you're happy with there's no option but to abandon those ideas and put the effort in to understanding the ones that are harder.

On the plus side though, you don't need to dive straight in at four-dimensional space-time.  For example, try thinking about a one-dimensional space-time and try to work out the possibilities that would offer ;)
 

Offline yor_on

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« Reply #12 on: 29/03/2010 19:46:40 »
Yes LeeE, I agree.
Don't really know what time is :)

We will all, sooner or later 'know' one way or another though.
And then, he**, then I won't be able to tell, will I ::))
 

Offline Ron Hughes

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« Reply #13 on: 29/03/2010 20:19:16 »
Has the thought ever occurred to anyone that the answer to what lies outside our Universe may be found in what caused the BB in the first place?
 

Offline yor_on

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« Reply #14 on: 29/03/2010 21:06:59 »
Well, that one is in the realm of the Gods it seems to me?
There is no way you can prove your ideas, alive :)
No matter your math.
==

You might be able to construct a cosmos of course, having sufficient knowledge?
That might be seen as a proof, but even so it wouldn't state that you knew how this one came to be, would it :)

A lovely idea otherwise.

"Charles stop playing with the cosmos."

"Yes mum."

:)
« Last Edit: 29/03/2010 21:10:24 by yor_on »
 

Offline Ron Hughes

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« Reply #15 on: 29/03/2010 21:46:15 »
I would think whatever was the cause would have some effect on our Universe we would not understand unless we were to correctly guess that cause. Maybe something like the density of galaxies lining up along imaginary lines through the Universe, or an unexplained accelerated expansion of the Universe.
 

Offline Ron Hughes

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Offline LeeE

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« Reply #17 on: 30/03/2010 09:09:03 »
Has the thought ever occurred to anyone that the answer to what lies outside our Universe may be found in what caused the BB in the first place?

Yup, but all we can do is speculate about it.

The simplest solution is that our four-dimensional space-time universe is within a higher order dimensional universe i.e. a five-dimensional, or greater, universe.  Just as there is 'space' for an infinite number of points along a line of finite length, and there is room for a line of infinite length within a finite area (and even an infinite number if infinitely long lines), and so on, the idea of a potentially infinitely sized four-dimensional universe within a finite structure inside a higher order dimensional universe is quite straightforward and may even be provable one day.
 

Offline Murchie85

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« Reply #18 on: 30/03/2010 09:43:54 »
LeeE I think you have hit the nail on the head, speculating is the first step into building a theory or provable idea. It takes time and evidence, but before we go looking for proof as scientists we ourselves have to decide on what it is we are searching for. For example a way or possible way of proving the concept of higher dimensions is to invest more time and money in the study of so called virtual particles (which could be the result of higher energy particles dropping from their higher universe into ours). Studies into dark energy and dark matter may also help us fit pieces of the jigsaw puzzle together.
 

Offline Farsight

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« Reply #19 on: 30/03/2010 13:37:33 »
What I would like to know is what defines the external boundary of space and the universe? For example if the universe is 14 billion years in age, what lies beyond the void? Does it have potential for matter to exist there and thus just the same like the space here? Or is it a physical/non physical thing that prevents light from going further? I have read journals positing that there is no edge and light would end up at the other side like a spaceship going out one side of the screen in a computer game, although I believe that might have been in connection with string theory. So what is the current astronomical/physics view on this?
This is an interesting one. There's no current view covering all aspects of this, but the universe is currently thought to be "flat". See http://en.wikipedia.org/wiki/Shape_of_the_Universe where it mentions WMAP, and this WMAP page http://map.gsfc.nasa.gov/universe/uni_shape.html. That means the universe isn't a hypersphere, if you go thataway you don't end up coming back thisaway from the other direction.

Like Lee said, the universe is expanding faster than you can travel, and you can't actually catch up with the "boundary".  But let's suppose that it isn't, and you can. The thing to remember is that space is expanding. If you're at this "boundary", there is no space beyond it. There is no void beyond the boundary of space, you've run out of space.

But let's say you decide to boldly go forward anyhow. There's at least a couple of options for what might happen. One is that you notice nothing. This is where the boundary is kind of an infinitely flexible thing. You go forward, and you push space outwards, leaving it trailing out behind you, like Lee said. Another option is you can't move forward at all, because there's just nowhere to go. The boundary is then like a brick wall. A middle option is that you can push forward, but it gets harder and harder to make progress because the boundary is acting like elastic. I quite like this last one myself, because it's something like "the bag model" of quark confinement. There's no hard and fast answers here, at least none that I know about, but it is fun to think about on a dull day! 
« Last Edit: 30/03/2010 13:39:08 by Farsight »
 

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