# Thread: The edge of the universe

1. ## The edge of the universe

It's hard to believe that the universe is not infinite, i mean there is a place where the universe just stops and then there in nothing

no matter no space no time, but surly if the universe is expanding it must be expanding into something that wasnt there before

Ive read before that it's not the universe that's expanding just that the space within it is expanding if that makes sense

Hypothetical question : If i was standing on the very edge of the universe with a tennis ball in my hand and threw that ball over the edge what would happen to the ball ?

2. Originally Posted by luckyfrank
Hypothetical question : If i was standing on the very edge of the universe with a tennis ball in my hand and threw that ball over the edge what would happen to the ball ?
You assume there is an edge. Why?

Originally Posted by luckyfrank
[...] surly if the universe is expanding it must be expanding into something that wasnt there before
No.

From the Sean Carroll Cosmology FAQ (one of many fine resources listed in topic ** FAQs ** Resources On The Web):

What is the universe expanding into?
As far as we know, the universe isn't expanding "into" anything. When we say the universe is expanding, we have a very precise operational concept in mind: the amount of space in between distant galaxies is growing. (Individual galaxies are not growing, as they are bound together by gravity.) But the universe is all there is (again, as far as we know), so there's nothing outside into which it could be expanding. This is hard to visualize, since we are used to thinking of objects as being located somewhere in space; but the universe includes all of space.

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Originally Posted by 01101001
Originally Posted by luckyfrank
Hypothetical question : If i was standing on the very edge of the universe
with a tennis ball in my hand and threw that ball over the edge what would
happen to the ball ?
You assume there is an edge. Why?
Why not? It seems a reasonable and natural assumption. Is there any
other assumption one could make that is as reasonable or as natural?

-- Jeff, in Minneapolis

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Maybe Space/Universe is not all that "this" is made of. No way of knowing. We view space as nothing beyond that, for all we know there may be some type of other "space" outside of our universe. Or possibly space doesn't end within our universe? We just proceed it within our universe only.

All in the imagination until we know other wise.

5. Originally Posted by ccart3r
Maybe Space/Universe is not all that "this" is made of. No way of knowing. We view space as nothing beyond that, for all we know there may be some type of other "space" outside of our universe. Or possibly space doesn't end within our universe? We just proceed it within our universe only.

All in the imagination until we know other wise.
Well yes this may be true, but whats your definition of the "universe"?

The observable universe is one thing but the actual size, if one could even define size regarding the universe, is another. As 01101001 stated there may not be an edge in the classical way we might think of one. But then there maybe one but at present its just not observable. The universe could be a finite closed loop or perhaps infinite in space/time.

We just don't know enough to assume either one way or the other. All we can say is that from what we have observed so far the universe appears basically flat, around 14 billion years old and is expanding. Whats beyond that at present is an open book.

6. Originally Posted by Jeff Root
Why not? It seems a reasonable and natural assumption. Is there any
other assumption one could make that is as reasonable or as natural?

-- Jeff, in Minneapolis
What about the assumption that the thing we live in (the universe) is similar to the thing we live on (the surface of the Earth)?

If the Earth was flat, and had an edge such that if you walked far enough you could fall off, I would understand the assumption that the universe had such an edge.

7. Lf,
It's easy to ask the question.
But it's very easy inded to do a search of this website, say for "edge universe"
I found more than the 500 hits that the search can use.
By doing that you will find a great deal of information.
I suggest you do.

Meanwhile, the 'balloon' anology is imperfect, but may be helpful.
Imaginme a universe on the surface of a baloon.
It is a 2 dimensional universe, that expands as the balloon is inflated.
For a 2D inhabitant, it is getting bigger, but into nowhere.

Good luck!
John

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It's helpful to think of the universe's expansion as an expansion of space, instead of an expansion in space. So there isn't a need for room to expand into.

If i was standing on the very edge of the universe with a tennis ball in my hand and threw that ball over the edge what would happen to the ball ?
It would make a dull thud and bounce back.

10. or come round and hit you in the back of the head.

11. Or just cease to exist.

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Originally Posted by Jeff Root
Why not? It seems a reasonable and natural assumption. Is there any
other assumption one could make that is as reasonable or as natural?

-- Jeff, in Minneapolis
Physics is not restricted to what you think is reasonable and natural. As a matter of fact, it quite frequently acts unreasonably.

14. Originally Posted by speedfreek
What about the assumption that the thing we live in (the universe) is similar to the thing we live on (the surface of the Earth)?

If the Earth was flat, and had an edge such that if you walked far enough you could fall off, I would understand the assumption that the universe had such an edge.

Ok fair enough but lets say you were able to throw the ball faster than the universe is expanding ?

Another thing on my mind is when we look to the furthest reaches of the universe we are looking back in time to the start of the universe and everything is expanding from there then surly the origin of the big ban should be in the center of the universe right ? Do we know the location of the inital start of everything

15. Originally Posted by luckyfrank
Ok fair enough but lets say you were able to throw the ball faster than the universe is expanding ?

Another thing on my mind is when we look to the furthest reaches of the universe we are looking back in time to the start of the universe and everything is expanding from there then surly the origin of the big ban should be in the center of the universe right ? Do we know the location of the inital start of everything
The universe isn't expanding in a way that would allow the ball to surpass the 'expansion' as you suggest. It's apples and oranges.

And there is no 'center' or the universe objects are expanding away from. Search this forum for "expanding universe" and you'll find dozens of posts, including one started by myself a few weeks ago, that will answer your question...I've learned a lot since then...

16. Originally Posted by luckyfrank
Ok fair enough but lets say you were able to throw the ball faster than the universe is expanding ?
Since the universe is believed to be expanding at a rate of C or greater at the furthest detectable distances away from us, then based on relativity this would never be possible.

Originally Posted by luckyfrank
Another thing on my mind is when we look to the furthest reaches of the universe we are looking back in time to the start of the universe and everything is expanding from there then surly the origin of the big ban should be in the centre of the universe right ? Do we know the location of the inital start of everything
Yes, but based on the BB theory the centre itself is expanding, so the centre of the universe is everywhere.

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Originally Posted by luckyfrank
Ok fair enough but lets say you were able to throw the ball faster than the universe is expanding ?
That isn't possible. The expansion of the universe doesn't quite work that way. Recall the expanding balloon analogy. For anybody in this universe (the surface of the balloon), the local space around them does not seem to be expanding very rapidly, but only things far away seem to be doing so. So to overcome the expansion of space at the observer's location, you can just nudge the tennis ball to a few mm per decade and you'll have overcome the Universe's expansion at your local space. The expansion rate of the universe is ~500 km s-1 Mpc-1. That's 500 kilometres per second... per megaparsec. The expansion of the universe is 500 km s-1 across a million parsecs! Condering a million parsecs, 500 km s-1 is pretty much nothing. As such, if you were a Mpc away from an object, it would seem to receed from you at 500 km s-1. So let's say you throw something at 500 km s-1 so that it'll catch up with whatever object is a Mpc away. Well an object at 2 Mpc is moving away at 1,000 km s-1, so you'll never catch up with that one. So you can throw it harder if you want. Well an object three times as far (6 Mpc) is moving away at 3,000 km s-1. On and on... until you're aiming at something 600 Mpc away, which is moving away from you at 300,000 km s-1. You'll find yourself unfortunately incapable of throwing your tennis ball that fast.

Originally Posted by luckyfrank
Another thing on my mind is when we look to the furthest reaches of the universe we are looking back in time to the start of the universe and everything is expanding from there then surly the origin of the big ban should be in the center of the universe right ? Do we know the location of the inital start of everything
Again, recall the expanding balloon analogy. There's no point on the balloon's surface that you can identify as the start of the expansion. Any observer on the balloon would see the rest of the balloon expand away from his current location, regardless of where they are.

As such, no matter where you are in the Universe, you appear to be at the centre of expansion. In truth, the Universe lacks a centre much like the balloon lacks any true central point of expansion.

18. Originally Posted by Jeff Root
Why not? It seems a reasonable and natural assumption. Is there any
other assumption one could make that is as reasonable or as natural?

-- Jeff, in Minneapolis
The usual assumption (see for instance The large-scale structure of spacetime by Hawking and Ellis) is that space time is a 4-dimensional Lorentzian manifold without boundary (without an edge). That seems to be both reasonable and natural.

If you assume the structure of a manifold with boundary, then you have to contend with what is happening in the boundary. That boundary will be a 3-manifold without boundary, so locally 3-dimensional. Now, what is a 3-dimensinal spacetime? Is it locally timeless or is is locally 2-space with some notion of time ? Neither makes sense.

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Originally Posted by speedfreek
Originally Posted by Jeff Root
Originally Posted by 01101001
You assume there is an edge. Why?
Why not? It seems a reasonable and natural assumption. Is there any
other assumption one could make that is as reasonable or as natural?
What about the assumption that the thing we live in (the universe) is similar
to the thing we live on (the surface of the Earth)?
Why would anyone make *that* assumption? It appears to be completely
baseless. Not the least bit reasonable or natural!

Originally Posted by speedfreek
If the Earth was flat, and had an edge such that if you walked far
enough you could fall off, I would understand the assumption that the
What does the surface of the Earth (or anything else) have to do with
the extent of the Universe??? I see no connection. There is no apparent
reason to assume any connection or any similarity between the two.

The original poster's assumption is completely reasonable and natural.
There is nothing at all reasonable or natural about the assumption you
suggest.

-- Jeff, in Minneapolis

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Originally Posted by DrRocket
Is it locally timeless or is is locally 2-space with some notion of time ? Neither makes sense.
Why would a 2+1 spacetime not make sense?

Originally Posted by Jeff Root
What does the surface of the Earth (or anything else) have to do with the extent of the Universe??? I see no connection. There is no apparent reason to assume any connection or any similarity between the two.
The original poster's assumption is completely reasonable and natural. There is nothing at all reasonable or natural about the assumption you suggest.
I think the point he was trying to make was that asking what happens when you move off the edge of the Universe was similar to asking what happens if you move off the edge of Earth in that the two are based on incorrect assumptions.

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Originally Posted by korjik
Physics is not restricted to what you think is reasonable and natural.
Obviously.

Even if it weren't obvious, it would still be a completely reasonable and
natural assumption that physics isn't restricted to what I think is reasonable
and natural. I would not be surprised to learn that the original poster
assumes that physics is not restricted to what I think is reasonable and
natural. Nor would I be surprised to learn that he assumes that physics
is not restricted to what he thinks is reasonable and natural.

Whatever. The assumption that the Universe has an edge is completely
reasonable and natural. Is there any other assumption one could make
that is as reasonable or as natural? Is it your view that unreasonable
and unnatural assumptions are preferable to reasonable and natural
assumptions?

-- Jeff, in Minneapolis

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I think what the issue is here is that what is reasonable to one is unreasonable to another, perhaps due to a knowledge gap. Those more familiar with the BBT may find "every point appears to be the centre of the universe" to be reasonable, whereas someone unfamiliar with it wouldn't.

23. Originally Posted by Hungry4info
Why would a 2+1 spacetime not make sense?
Why would it ? If you travel to the edge do you simply flatten out ?

Huygens principle fails to work in even spatial dimensions, so does physics change at the boundary ? Remnember that this boundary, if it were to exist, would be a part of spacetime. It is not somehow fenced off.

The "cosmological principle" would fail dramatically at the edge.

Since the universe is, by definition, the whole enchilada, what is the importance of an edge ? It is not like there is something on the the other side, since there is no other side.

Originally Posted by Hungry4info
I think the point he was trying to make was that asking what happens when you move off the edge of the Universe was similar to asking what happens if you move off the edge of Earth in that the two are based on incorrect assumptions.
The huge difference is that the Earth is embedded in something else, the universe. The unverse is not embedded in anything else, since it is, by definition, all that is.

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Originally Posted by DrRocket
The unverse is not embedded in anything else, since it is, by definition,
all that is.
That's one definition.

Another definition would be the portion of the cosmos which contains
matter. It might be that most of the Universe is empty, and only a very
small part of it contains matter.

Another definition would be the part of the cosmos which is participating
in the Big Bang expansion. There could be many-- perhaps infinitely
many-- universes, some of which are expanding like the one we inhabit.

Originally Posted by DrRocket
The "cosmological principle" would fail dramatically at the edge.
Obviously. By definition.

But the cosmological principle is pretty weak. It isn't as strong as the
law of conservation of energy, and that law is only local, not global.
The cosmological principle is a description of what we can see. There
is no apparent reason that it should apply to everything beyond what
we can see.

-- Jeff, in Minneapolis

25. Originally Posted by Jeff Root
That's one definition.

Another definition would be the portion of the cosmos which contains
matter. It might be that most of the Universe is empty, and only a very
small part of it contains matter.

Another definition would be the part of the cosmos which is participating
in the Big Bang expansion. There could be many-- perhaps infinitely
many-- universes, some of which are expanding like the one we inhabit.
Neither of your two definitions apply to the a general relativistic model of cosmology.

You can, of course, use any ATM definition that pleases you. But such definitions are not amenable to treatment by means of mainstream theory. The only accepted theory that we have to deal with this question is general relativity, which has it limits, but which is supported by a large body of experiment and established theoretical models. Anything else is speculation.

26. Originally Posted by Hungry4info
On and on... until you're aiming at something 600 Mpc away, which is moving away from you at 300,000 km s-1. You'll find yourself unfortunately incapable of throwing your tennis ball that fast.
Hang on a sec. You're saying that an object 600 Mpc away is receding from us at the speed of light? 600 Mpc is nearly 2 billion lightyears, but we know of things that are (much) further away from us than that - e.g. the most distant galaxies known are about 13 billion lightyears away - and they aren't receding from us at faster than the speed of light (are they?).

27. Have we really come this far as to know the actual age of the Universe? If science says the Universe is 13.5- 14 billion years old.. does that mean its necessarily true? Sure, the most distant galaxy's that we know of are 13 billion light years away.. but I feel that the Universe has to be much larger in comparison to all that is known, in these terms the Universe feels small and compressed.

I feel the Universe can be a unlimited canvas to our comprehension, though I feel it is not infinite. When I think of it, I picture a bubble.. circular like Earth. Before Columbus discovered America, what do you think the people thought of the oceans .. what lay beyond them ? I'm more so inclined to believe in the multi universe theory, and sub dimensions than a infinite nothingness..

28. Originally Posted by Sententia
but I feel that the Universe has to be much larger in comparison to all that is known, in these terms the Universe feels small and compressed.

I feel the Universe can be a unlimited canvas to our comprehension, though I feel it is not infinite.
Science isn't about "feelings" or "belief" though.

29. Originally Posted by Sententia
Have we really come this far as to know the actual age of the Universe? If science says the Universe is 13.5- 14 billion years old.. does that mean its necessarily true? Sure, the most distant galaxy's that we know of are 13 billion light years away.. but I feel that the Universe has to be much larger in comparison to all that is known, in these terms the Universe feels small and compressed.
Your right, but I don't think you realize why.

http://en.wikipedia.org/wiki/Hubble_volume:

The distance c / H(sub)0 is known as the "Hubble length". It is equal to 13.8 billion light years in the standard cosmological model, similar to but somewhat larger than c times the age of the universe. This is because 1 / H(sub)0 gives the age of the universe by a backward extrapolation which assumes that the recession speed of each galaxy has been constant since the big bang. In fact, recession speeds initially decelerate due to gravity, and are now accelerating due to dark energy, so that 1 / H(sub)0 is only an approximation to the true age.
http://en.wikipedia.org/wiki/Observable_universe:

The age of the Universe is about 13.7 billion years, but due to the expansion of space we are now observing objects that are now considerably farther away than a static 13.7 billion light-years distance. The edge of the observable universe is now located about 46.5 billion light-years away [1].
So, while (the light from) the furthest galaxies we see are (is coming from) 10-13 billion light years away, that's where they were 10-13 billion years ago. The galaxies themselves are now closer to 46.5 billion LY away.

Originally Posted by EDG_
Science isn't about "feelings" or "belief" though.
Correct, but without them, where would science be?

30. Originally Posted by EDG_
Hang on a sec. You're saying that an object 600 Mpc away is receding from us at the speed of light? 600 Mpc is nearly 2 billion lightyears, but we know of things that are (much) further away from us than that - e.g. the most distant galaxies known are about 13 billion lightyears away - and they aren't receding from us at faster than the speed of light (are they?).
Well, yes they are (by a conventional definition of recession speed), and no they aren't; it all depends on what distance and time intervals one adopts since these intervals are not invariants (coordinates, coordinates, coordinates -- GR allows one to pick any self-consistent set that you want). Ned Wright's FAQs discusses several related questions; start here. It is also discussed in Lineweaver & Davis' Scientific American article, although out of simplicity they have adopted the simplest (and most common) explanation.

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