What Is the Universe Expanding Into?

[hmetkaya]

Dear all,

I just read a great question on TED.com, however I think people over here could answer this question perhaps better.

"If the universe is expanding - what is it expanding into?

what is THAT space called
and why isn't it part of the universe now?"

 

[hmetkaya]

For me the next analogy comes in handy:

If a table could've been given the possibility of speech, upon asking what it's creator is made of it would answer with terms it is familiar with. Wood, oak, brown/black, painted, 4-footed, etc.

When trying to explain something which consists of flesh and blood, it would fail to understand because of the lack of familiarity with these kind of substances.

This analogy helps me in trying to (or rather lack) understand how the universe can expand within the maximum space assigned for it already.

 

[Chronos]

There is no simple answer to this question. The universe is expanding, but, nothing is actually moving in any meaningful way. The empty space between the galaxies increases while the galaxies sit around doing nothing. If it helps, you may simply assume the universe is infinite, in which case the question becomes meaningless - even though we don't actually know if it is spatially infinite or just really big.

 

[phinds]

Dear all,

I just read a great question on TED.com, however I think people over here could answer this question perhaps better.

"If the universe is expanding - what is it expanding into?

what is THAT space called
and why isn't it part of the universe now?"

The problem with positing an edge to the universe (which would be needed if it were actually expanding INTO something) is that there is no known way to explain the utter weirdness that would happen to all our physical models at the edge. It just doesn't make sense. Try this:

www.phinds.com/balloonanalogy

 

[Mark M]

You're misunderstanding what it means for the universe to be expanding. Before I explain, I want to very strongly emphasize a few points.

1. The universe has no 'edge'. This is because we model the universe as homogeneous and isotropic. That is, there are no special places. Also, the universe at large is described by general relativity, which describes spacetime as a smooth manifold, which excludes the possibility of an edge.

So, one possibility is that the universe is infinite, it just extends forever. However, it may also be finite. If this is the case, then it's either simply or non-simply connected. That means it would be like the three-dimensional analogue of the surface of the Earth - it's definitely finite, but it has no edge. So, in a finite universe, going far enough will bring you back to your original position.

So, you can see from that why your question is flawed - there is no such thing as 'outside' of the universe because it has no edge.

2. The universe has no center, and the big bang was *not* an explosion. This will make more sense in a little bit, but it's key to remember that the universe has no center - it expands from every point in intergalactic space. The big bang was the point in time when the entire universe was filled with a hot, dense, plasma and radiation. As space underwent expansion, the radiation redshifted and cooled, and the plasma cooled into hydrogen gas. This then accumulated into clumps, which became galaxies. The space in between these galaxies continues to expand.

So, what does it mean that the universe is expanding? It means that the distance between bound objects (i.e. galaxies) is increasing. The galaxies themselves aren't moving (well, they have a very movement that has nothing to do with expansion, but this is besides the point), they just appear to be because the distance between them is getting larger. So, you can see that 'expanding' in this context is a lot different than how you think of it - the distance in between all galaxies getting larger.

Now it makes sense why the universe doesn't have a center - since the space in between everything is growing, every galaxy sees every other galaxy moving away. So, everyone appears to be at the center, which is why there isn't an center (depending on whether you're a half glass full or half glass empty type of person, you may say the universe has no center or that every point is the center, both mean the same exact thing).

I highly recommend reading this article, it should clear up a few misconceptions:

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

 

[hmetkaya]

Interesting replies, thank you all.

The balloon-analogy helped deeply in getting a grasp of the thought how everything expands.

Another thought has kept me awake for quite some time.
Every planet has its own orbit, whereby every bundle of planets also have their own orbit (looking at the Milky Way). There has been, in my knowledge, no collision up to date. When we would make a model of these planets' orbit, it would just be a matter of time for them to collide.

In the balloon-analogy this was stated:

(1) The universe is expanding OUTSIDE of systems that are gravitationally bound, or bound by other local forces (e.g. strong and weak forces) That is, things the size of a local cluster of galaxies and smaller (like, the Milky Way, Earth, you, me, atoms, and so forth), do NOT expand.

This of course explains that expansion takes place without the orbits of smaller systems being disrupted. However I do not think that solves the issue of the possibility of collision.

Any thoughts about this?

 

[DeepSpace9]

This question is asked once a week, to all the senior members, I applaud you for answering it in detail every single time.

 

[Khashishi]

The expansion of the universe can be thought of as having the distance between every object increased on average at large scales. All the galaxies move farther apart from each other, so the universe becomes more sparse. It's like zooming into a sheet of grid paper, except that the grid paper stretches on to infinity so there is no edge.

 

[phinds]

Interesting replies, thank you all.

The balloon-analogy helped deeply in getting a grasp of the thought how everything expands.

Another thought has kept me awake for quite some time.
Every planet has its own orbit, whereby every bundle of planets also have their own orbit (looking at the Milky Way). There has been, in my knowledge, no collision up to date. When we would make a model of these planets' orbit, it would just be a matter of time for them to collide.

In the balloon-analogy this was stated:



This of course explains that expansion takes place without the orbits of smaller systems being disrupted. However I do not think that solves the issue of the possibility of collision.

Any thoughts about this?

There are entire GALAXIES that collide, so I don't know where you get the idea that there are no collisions. I seem to remember reading an estimate, however, that if two galaxies of a billion stars each collide, as happens sometimes, the chances of two solar systems within them colliding is near zero and it's likely that if there are any such collisions, it's only a couple.

 

[QuantumHop]

"If the universe is expanding - what is it expanding into?

what is THAT space called
and why isn't it part of the universe now?"

Its to do with the properties of infinity.

Infinity +100 = infinity

You can't make it any bigger by adding to it, its already infinite.

So if the universe is infinite in size and its expanding its still the same size (infinite), basically its just expanding into itself.

 

[Mark M]

Its to do with the properties of infinity.

Infinity +100 = infinity

You can't make it any bigger by adding to it, its already infinite.

So if the universe is infinite in size and its expanding its still the same size (infinite), basically its just expanding into itself.

No, this isn't right. Even if the universe is finite, it isn't expanding into anything. See my above post.

 

[QuantumHop]

No, this isn't right. Even if the universe is finite, it isn't expanding into anything. See my above post.

I didn't say its expanding into anything other than itself, its falling outward.

Its dimensionless, it has no definite size to begin with and therefore its perfectly happy to expand and not get any bigger. I think looking at the properties of an infinite size number and realising you can't make it bigger by adding to it is a good analogy for expanding space. It gets rid of the idea that its expanding into something else.

No matter how much it expands its still the same size.

 

[QuantumHop]

The important property of expanding space is that by volume matter is getting scarcer.

 

[Drakkith]

I didn't say its expanding into anything other than itself, its falling outward.

Its dimensionless, it has no definite size to begin with and therefore its perfectly happy to expand and not get any bigger. I think looking at the properties of an infinite size number and realising you can't make it bigger by adding to it is a good analogy for expanding space. It gets rid of the idea that its expanding into something else.

No matter how much it expands its still the same size.

I think you have a misunderstanding of what we mean by saying that the universe is infinite but expanding. The distance between ALL objects not gravitationally bound to each other gets larger over time. That is it. There is nothing that says that space is moving anywhere, or expanding into anything else, or anything of the sort. The universe isn't expanding into itself or anything else. Things are simply moving away from other things without getting closer to anything.

 

[QuantumHop]

I think you have a misunderstanding of what we mean by saying that the universe is infinite but expanding. The distance between ALL objects not gravitationally bound to each other gets larger over time. That is it. There is nothing that says that space is moving anywhere, or expanding into anything else, or anything of the sort. The universe isn't expanding into itself or anything else. Things are simply moving away from other things without getting closer to anything.

I think you have a misunderstanding of what we mean by saying that the universe is infinite but expanding.

I'm pretty sure I understand it.

ALL objects not gravitationally bound to each other gets larger over time.

If you mean the distance then yes I know.

There is nothing that says that space is moving anywhere, or expanding into anything else, or anything of the sort.

I never said it did, saying that it expands into itself isn't the same as saying it expands into "something else"

 

[QuantumHop]

If I have an infinitely long elastic band and I stretch it some more does it get longer? Of course not.
So if space is infinite in size and it expands does it get bigger? obviously not.
Point is infinities are not additive and I think it makes perfect sense to say that space simply expands into itself.

 

[Drakkith]

If I have an infinitely long elastic band and I stretch it some more does it get longer? Of course not.
So if space is infinite in size and it expands does it get bigger? obviously not.
Point is infinities are not additive and I think it makes perfect sense to say that space simply expands into itself.

I'd say that it most certainly gets longer. It's original length can be finite or infinite, but if you stretch it then it is longer. And again, space isn't expanding. Matter and energy within spacetime is getting further apart. To my knowledge spacetime is that which everything occupies, not something physical in itself.

 

[QuantumHop]

If I had an infinitely long elastic band and I stretched it some more by say 10 feet then by your reckoning its now infinitely long + 10 feet, that's just silly try measure it :) , its still infinitely long.

Half of infinity is infinity, you cannot divide infinity into a measurable unit, nor can you add to it to make it bigger.

If an infinite volume of space expands it doesn't get bigger, its still infinite.

If you stretch something that has a finite length then it gets longer but if its infinitely long its length stays the same.

 

[QuantumHop]

You cannot think of an infinite distance as "measurable" its simply infinite even if it expands or contracts.

 

[Drakkith]

Take an infinitely long rubber band with marks every 1 inch and stretch it. the marks are all now further apart. The rubber band has stretched but it has not stretched or expanded into itself. I would certainly call that longer, but as I am not educated in mathematical infinity I won't try to argue it, I'll save that for one of the other guys here.

 

[QuantumHop]

he he I see where your coming from and the idea for marking the rubber band is interesting but it won't change the overall length, that remains infinite.

Think about this, all of space is infinite in all directions and in front of you is an infinitely long rubber band. What is the volume of the rubber band? If you think it through it has an infinite volume and yet its not consuming all of space.

Your thinking about it as if it were a number.

 

[phinds]

... and I think it makes perfect sense to say that space simply expands into itself.

Yes, you have the right to believe whatever you like, but you do NOT have the right to dictate terminology, and the terminology you are using is non-standard and confusing. We do NOT say space is "expanding into itself". "Itself" is something so that's just another way of saying it expands into something, which is wrong.

 

[Mark M]

I'm pretty sure I understand it.

Really?

There is absolutely no evidence that the universe is infinite. It's irrelevant. The metric of space is different at a later time, not that anything has 'grown'. So, your rubber band analogies are all false, since the rubber bands are literally being expanded, whereas the expansion of the universe is metric.

For example, the de Sitter solution to the Einstein field equations is a perfectly legitimate cosmological solution, yet it is finite. The expansion of the universe is a local phenomenon, it has no relation to the overall size.

 

[Mark M]

Take an infinitely long rubber band with marks every 1 inch and stretch it. the marks are all now further apart. The rubber band has stretched but it has not stretched or expanded into itself. I would certainly call that longer, but as I am not educated in mathematical infinity I won't try to argue it, I'll save that for one of the other guys here.

Well, this isn't the same as the expansion of the universe. In this example, you are expanding the rubber band - you're pulling the atoms that make it up apart. This, like the balloon analogy, gets the general idea across (that the expansion occurs everywhere) but it isn't precisely correct. During metric expansion, the distance in between galaxies is larger, but nothing has really expanded or grown.

Analogies are good, but it's always good to keep in mind that the universe isn't a balloon or a rubber band. For a concrete explanation, take a look at this equation: $$d(t) = a(t)d_{0}$$ Where $d(t)$ is the distance to some other object, say, a galaxy. Now, $d_{0}$ is the distance at some earlier time. Finally, $a(t)$ is called the scale factor, and it encodes, as you can see, how much larger the distance in between you and the other galaxy is now, compared to before. In time, the scale factor gets larger and larger. So, that means that the distance in between any two objects is becoming larger.

Notice how this doesn't need to make an mention to anything 'expanding'. Just the fact that there is now more distance in between all bound objects.

 

[Axefly]

So then is it like the universe is infinite and everything inside it, is just moving further apart from each other, that is, if the universe is really infinite.

 

[phinds]

So then is it like the universe is infinite and everything inside it, is just moving further apart from each other, that is, if the universe is really infinite.

Whether the univeser is finite or infinite is unknown, but in EITHER case, the expansion is metric expansion (look it up and yes, it mean things are just moveing farther apart)

 

[sssamir]

therefore big bang theory stands disproved!

 

[sssamir]

infinity cannot begin from a finite point!

 

[sssamir]

to imagine an entity as vast as the present observable universe to have emanated from a
single point (a.k.a. big bang theory) before which nothing existed;not even empty space
since space is something is a practical and scientific impossibility.if nothing existed before
the big bang what did the exploding matter expand into?--------!

 

[phinds]

therefore big bang theory stands disproved!

No, it has not. The big bang theory, contrary to your misperception, most emphatically does NOT say that everything started at a point. In fact, it does not say ANYTHING about how things started, it just describes what happened starting at one Plank time AFTER whatever the singularity was. "Singularity" in this case does NOT mean "point" it means " the place where our models break down".

Your attitude that you have disproved something that is accepted by all of physics, is just ... well, forum etiquete prevents me from saying what I think of it but if you plan on staying around, you'd best lose the foolish arrogance.

 

[Mark M]

to imagine an entity as vast as the present observable universe to have emanated from a
single point (a.k.a. big bang theory) before which nothing existed;not even empty space
since space is something is a practical and scientific impossibility.if nothing existed before
the big bang what did the exploding matter expand into?--------!

This is totally incorrect. The big bang model doesn't hold that anything 'exploded', just that the early universe was very hot, and very dense. Nor does is attempt to assert the size of the universe, which as phinds said, may be finite or infinite.

See here:

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

 

[audioloop]

So then is it like the universe is infinite and everything inside it, is just moving further apart from each other, that is, if the universe is really infinite.

read this thread

https://www.physicsforums.com/showthread.php?t=632836

 

[Brainiac2]

I didn't say its expanding into anything other than itself, its falling outward.

Its dimensionless, it has no definite size to begin with and therefore its perfectly happy to expand and not get any bigger. I think looking at the properties of an infinite size number and realising you can't make it bigger by adding to it is a good analogy for expanding space. It gets rid of the idea that its expanding into something else.

No matter how much it expands its still the same size.

That's mind boggling!

 

[marcus]

Mark had an interesting point here that I want to bring up again. Most people participating here do I think realize that in standard cosmology the expansion of distances between stationary observers does NOT imply that there is any "surrounding space" that space is "expanding into".
Mark summarized that early on, but also made the additional point that this has nothing to do with the (unnecessary) assumption that space is infinite volume. It is a common misconception that the boundarylessness of space comes from its being infinite. The math model used in cosmo can easily be finite volume and still not be embedded in some surrounding space. I'll quote Mark's summary as a reminder and then a subsequent post regarding this.

... I want to very strongly emphasize a few points.

1. The universe has no 'edge'. This is because we model the universe as homogeneous and isotropic. That is, there are no special places. Also, the universe at large is described by general relativity, which describes spacetime as a smooth manifold, which excludes the possibility of an edge.

So, one possibility is that the universe is infinite, it just extends forever. However, it may also be finite. If this is the case, then it's either simply or non-simply connected. That means it would be like the three-dimensional analogue of the surface of the Earth - it's definitely finite, but it has no edge. So, in a finite universe, going far enough will bring you back to your original position.

So, you can see from that why your question is flawed - there is no such thing as 'outside' of the universe because it has no edge.

2. The universe has no center, and the big bang was *not* an explosion. This will make more sense in a little bit, but it's key to remember that the universe has no center - it expands from every point in intergalactic space. The big bang was the point in time when the entire universe was filled with a hot, dense, plasma and radiation. As space underwent expansion, the radiation redshifted and cooled, and the plasma cooled into hydrogen gas. This then accumulated into clumps, which became galaxies. The space in between these galaxies continues to expand.

So, what does it mean that the universe is expanding? It means that the distance between bound objects (i.e. galaxies) is increasing. The galaxies themselves aren't moving (well, they have a very movement that has nothing to do with expansion, but this is besides the point), they just appear to be because the distance between them is getting larger. So, you can see that 'expanding' in this context is a lot different than how you think of it - the distance in between all galaxies getting larger.

...
I highly recommend reading this article, it should clear up a few misconceptions:

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

Its to do with the properties of infinity.

Infinity +100 = infinity

You can't make it any bigger by adding to it, its already infinite.

So if the universe is infinite in size and its expanding its still the same size (infinite), basically its just expanding into itself.

No, this isn't right. Even if the universe is finite, it isn't expanding into anything. See my above post.

If I understand what he's saying here, Mark is making an important point. It is still an open question whether U is spatially finite or spatially infinite. We don't have to assume infinite in order to avoid the idea of a U with boundary that is "expanding into" someplace that is not the U.
Boundary and "space to expand into" are not logically necessary to what cosmologists mean by expansion.

So if anyone finds this puzzling, here is a good opportunity to get a more accurate idea of what they mean by Hubble law---the pattern of expansion of internal distances--and the related idea that geometry (experienced from within) can be dynamic and not a fixed rigid framework in which things happen. Geometry interacts with the rest of nature.

 

[Lino]

... Just the fact that there is now more distance in between all bound objects.

Mark, I thought that it would be more distance between "unbound" objects. In what sense are the objects bound?

Regards,

Noel.

 

[marcus]

Mark, I thought that it would be more distance between "unbound" objects. In what sense are the objects bound?

Regards,

Noel.

Heh, I think I see what you are driving at, Lino---but what I see is more a verbal ambiguity than a disagreement. A distance between bound objects (in Mark's words) could mean the distance between two objects, each of which is bound, but the two objects are not bound gravitationally to each other.

So the distance could be the distance between two rocks, or two galaxies, or two gravitationally bound clusters of galaxies. Each of them separately is a "bound object" because held together by its own internal forces.

But it's assumed (as I read Mark's post) that the two bound objects do not form a larger bound system. So it is an "unbound" pair of bound objects.

Ordinary language is frustrating, so many chances for ambiguity.

 

[skydivephil]

I thought the best way to define the expansion of the unvierse is the distance between galaxies is growing. It doesn't mean the unvierse is getting bigger (it might have been infinite at the start of the expansion). Also it doesn't mean the distance between objects smaller than galaxies is expanding as these are gravitationally bound. However I have just realized a difficulty here that I am not sure how to square. The propblem is if we define the expansion this way, what do we mean by the expansion before galaxies formed? Woudln't that imply the distance between unbound obejcts is growing and hence invalidate the earlier description or deny that the unvierse was expanding before galaxies formed. Both seem wrong to me although obviously the latter seems a little more absurd. not sure how to make my deifnition correct though.

 

[Lino]

Thanks Marcus (& Mark). Much appreciated.

Regards,

Noel.