A Question about Universal Expansion

[M.W.T.B.F.]

Can someone please help me understand what exactly universal expansion is? I understand the red-shifted galaxy evidence but my problem lies in that an expansion implies that the universe has "borders" which are constantly growing outwards. Is this true? Does the universe have "borders" where you can theoretically be outside the known universe? Please help me understand this.

 

[borrofburi]

my problem lies in that an expansion implies that the universe has "borders" which are constantly growing outwards. Is this true?

Sounds about right.

Does the universe have "borders" where you can theoretically be outside the known universe?

It has borders but you can not be outside them; it's a bit complicated, and I am a bit of a novice. I do recall one proposal was that the universe acts as a sort of 4 dimensional sphere: i.e. if we were 2 dimensional beings living on the surface of a sphere we would find ourselves in a finite "universe" without borders. However I think current theoretically models indicate that space itself was the expansion of the big bang, and that it expands faster than light, so you can never even reach the boarders, but they are there... I might be a bit out dated though, or a bit... off.

 

[xman]

Imagine that the universe is an expanding balloon. Not the air inside mind you, just the rubber balloon. You can never reach the end, but it can still keep growing. It's not quite like that, but it's a useful image.

 

[ExeFBM]

The universe expanding is not the creation of new space from something that was previously nothingness, it's just everything that already exists getting further away from everything else.

If there were borders, then there would be a centre to the universe. All points in the universe used to be the centre at the singularity, but now no points are as we've expanded away from it. As was mentioned above with the balloon. If you imagine an infinitely stretchy balloon. Before it's blown up, all of it's mass is compacted at the centre, and then it expands away from the centre. All points on the balloon are getting further away from each other as the balloon inflates. It's something like that, but with more dimensions.

 

[mastercromo]

imagine a rubber ballon that is inflating. All that exists(all mass and energy) is on it's surface and the rubber surface itself is what you call space. Big Bang was a explosion of space, not a explosion of mather and energy. Space itsefl is not affected by general relativity so it can expand faster than light. This fact is the justification for having galaxys that are more then 15 billion light years away from ours.

PS: Any pertubation in the rubber surface(space) is also limited to the light speed.

 

[Master_Ghost_Knight]

No it doesn't imply that the universe has borders. Altough we are not quite sure of the configuration of space it could be as easily that if the unvirse stoped expading (and if we ignore the implications of it) it might be possible that you could sweep trough the universe and come back to the same place by coming trough the other side. In that sort of universe, there are no borders neither there is a center. The expansion is relatively simple, in one moment there is defined ammount of space in the universe and moments later there is more, sense this happens uniformly everywhere everything is pulled appart equally (sort of).

 

[AndromedasWake]

A few users have hit this one already, but try to imagine all 3 familiar spatial dimensions of the Universe projected onto a two dimensional sheet. We have to do this, because otherwise we'd have to fold a 3D volume around a 4th dimension, and our brains are not equipped to visualise this.

Consider that flat sheet, representing all of space. It has edges, meaning space has edges. However, if you wrapped that sheet into a sphere, such that the edges were all stitched up, you'd end up with a space with finite volume, but no edge. This is known as a finite but boundariless configuration, and we generally imagine the Universe to have a topology of this nature. However, there are other ways you can achieve this that better fit with observations. Imagine first wrapping the sheet around into a tube. You could cut open a toilet-roll tube and fold it out into a rectangular shape. You could also bend the tube around so both ends meet, and again you've eliminated any edges without needing an infinite surface area.

As for the expansion of space, that's a complicated and still rather mysterious issue. In classical cosmology, we consider the metric of space to be expanding. This means that if the metric expands by 3 times, what was once 1 metre is now really 3 metres. Matter interactions are able to overcome this expansion however, so galaxies do not expand or tear apart. But photons travelling through space as it expands will be "stretched out" according to their wavelength. In our example of a 3x metric increase, a photon with a wavelength of 1 metre now has a wavelength of 3 metres, making it a 'redder' photon. This gives us red-shift. We know red-shift is a doppler effect of relative motion, and not simply an abundance of old, red stars, because the actual absorption lines in the spectrum of the light are all shifted together. If we were simply looking at older stars, the absorption lines would not move in the spectrum, even though it appears more red overall.

Hope this helps some way. In the not-too-distant future, we'll have instruments that can measure the expansion of the Universe in real time - so-called 'Laser Combs'

 

[Pulsar]

I'll expand a bit further on this (no pun intended). First, we should make a distinction between the total universe and the observable universe. The total universe has no boundaries, even though it is expanding. The observable universe is defined as the part of the total universe that we can see, in other words, the maximum distance that any photon has theoretically been able to travel in 13.7 billion years before finally reaching us. As such, this area does have a border: it is a sphere, with the earth in the center, with a radius of 46.5 billion lightyears. Note that an observer in a different galaxy will see a different observable universe (because it would be centered on him). Naturally, as the universe gets older, the size of the observable part will increase. But curiously, because space itself expands, the recession velocity of the most distant galaxies will be so large that they actually start to escape from our view. In other words, in a few billions years, we will see more space, but less galaxies!

Anyway, back on topic: the total universe. Its shape (i.e. its topology) is unknown, and one of the biggest questions in cosmology. But it is possible to measure its curvature: it turns out that it is very close to zero, i.e. the universe is almost flat. By "flat", I mean that, for instance, the sum of the angles of a triangle are 180,°. This gives us several possibilities:

1. The total universe is closed, like a 3d-sphere (as mentioned by the other posters, it's curved like the surface of a sphere, but with 3 dimensions instead of 2). That means that the curvature should actually be positive, but is too small to measure (with current techniques). This is possible if the total universe is in fact much larger that the part we can observe, in the same way that the earth appears flat on small scales. Such a scenario can be explained by inflation theories, which propose that the early universe went through a phase of rapid expansion. If that is true, we are only seeing a tiny fraction of the total universe.

2. The total universe is closed, but multiply connected. Simply put, it has a shape "with holes in it". As AW mentioned above, the 3d-torus (a 2d-torus looks like a doughnut) is the simplest form (imagine glueing the opposite sides of a cube together). There are 10 of such possible shapes in total (e.g. the Poincaré dodecahedral space, see below). A 3d-torus is topologically flat.

3. The total universe is not closed. In other words, it is infinitely big. In that case, it could simply resemble the familiar Euclidian space. And yes, although it's infinite, it can still expand! Consider this thought-experiment: take an infinite number of balls, mark each of them with an integer number, and lay all the even-numbered balls on one line. Clearly, this line will be infinitely long. But it can still expand: between every two balls, you can squeeze an odd-numbered one! (ok, that was a weird sentence ). For completeness, the curvature could even be slightly negative, leading to a hyperbolic shape.

In any case, the total universe has no borders. And if it has a multiply connected shape, there is a fascinating possibility: the observable universe could be larger than the total universe! If that is true, the light of some objects has "gone round" the universe, and we could see multiple copies of it spread across the sky (the number of copies depends on the shape of the universe). And there are some indications that this could be the case, not from seeing mirror images of galaxies, but from patterns in the microwave background: the current best fit to the data is a Poincaré dodecahedral space (imagine glueing the opposite side of a dodecahedron together). However, this is by no means certain yet...

 

[M.W.T.B.F.]

Thanks a lot to all who responded

 

[scalyblue]

Note that if what we see is multiple copies of things, they will appear drastically different as we will be getting light from vastly different time periods in the histories of the respective objects

 

[sgrunterundt]

my problem lies in that an expansion implies that the universe has "borders" which are constantly growing outwards. Is this true?

Sounds about right.

No. As others have said that is quite wrong.

 

[borrofburi]

Sounds about right.

No. As others have said that is quite wrong.

Yah, thank you for going back to the first post in the thread before the academic phycisists came along, I really appreciate it and was totally unaware that my first semi-educated guess was significantly off. If you hadn't come along to show me just how wrong I was, I might have gone on thinking I was an infallible god who knew everything about physics. You sir, have done a great service to mankind today, I am not sure we will ever be able to repay this debt.

 

[scalyblue]

Yah, thank you for going back to the first post in the thread before the academic phycisists came along, I really appreciate it and was totally unaware that my first semi-educated guess was significantly off. If you hadn't come along to show me just how wrong I was, I might have gone on thinking I was an infallible god who knew everything about physics. You sir, have done a great service to mankind today, I am not sure we will ever be able to repay this debt.

 

[CupOfWater]

But if the space we live in is like the surface on a balloon, won't that mean that if we start off at a place, and just keep moving in a straight line, we will eventually end up where we started?

 

[scalyblue]

Separate balloon for every pair of points compared.

Yes, the universe is made of balloons.

 

[AndromedasWake]

But if the space we live in is like the surface on a balloon, won't that mean that if we start off at a place, and just keep moving in a straight line, we will eventually end up where we started?

Yes, but as we cannot exceed the recessional velocity of the edge of the observable universe (at any moment) we would never be able to make a full lap around the Universe. It's like an ant running round the balloon. If you keep blowing up the balloon, the ant always has further and further to run. If you increase the circumference of the balloon faster than the ant can move, he can never actually circumnavigate it. This is the position we think we're in.

By the way, relativity does not forbid us from carrying ourselves in a bubble of space at a speed faster than light (as seen outside the bubble). If we could do this, we could really travel around the Universe and we'd see very strange things indeed.

 

[aeroeng314]

By the way, relativity does not forbid us from carrying ourselves in a bubble of space at a speed faster than light (as seen outside the bubble). If we could do this, we could really travel around the Universe and we'd see very strange things indeed.

Are you sure? I was under the impression that one of the key assumptions in relativity is that information cannot exceed the speed of light. In fact, it comes up in the derivation of the Lorentz transformations. We even discussed how a mechanism of faster than light communication leads to a violation of causality since you can easily construct two reference frames where an event happens in the past of one but the future of the other and FTL communication would permit one reference frame communicating information about the event to the other before that event has happened in the other reference frame. Maybe my understanding of the situation is flawed, but I don't see the exception to this case.

 

[Gunboat Diplomat]

By the way, relativity does not forbid us from carrying ourselves in a bubble of space at a speed faster than light (as seen outside the bubble). If we could do this, we could really travel around the Universe and we'd see very strange things indeed.

Are you sure? I was under the impression that one of the key assumptions in relativity is that information cannot exceed the speed of light. In fact, it comes up in the derivation of the Lorentz transformations. We even discussed how a mechanism of faster than light communication leads to a violation of causality since you can easily construct two reference frames where an event happens in the past of one but the future of the other and FTL communication would permit one reference frame communicating information about the event to the other before that event has happened in the other reference frame. Maybe my understanding of the situation is flawed, but I don't see the exception to this case.

Amazingly enough, your understanding of the situation is not flawed, at least not where special relativity is concerned.

However, AndromedasWake is also correct. The reason that universal expansion can cause things to "fly apart" faster than the speed of light without violating causation through special relativity is because all of space is expanding so this cannot be used to propagate causation faster than light. If you have trouble imaging how this is true, think of two points separated by some amount of space. Because of universal expansion, the space between the two points is growing and so the points are moving away from each other. The farther apart they are, the more space there is between them to grow and thus the faster they will be moving away from each other. There is no limit to this so if they are sufficiently far apart then they can be moving away from each other faster than the speed of light. They also have no causal relationship; what happens at one point has nothing to do with the other. Suppose you were to try to make one? Suppose you wanted to send information from one point to the other? Whatever you choose as a means of communication would have to propagate from one point to the other. However, this propagation can only be hindered by universal expansion since any space you move through will grow as you move through it, sort of like walking up a downward escalator. Get it?

In the very same thread you linked, we've shown that there are many things that travel faster than light but those things don't propagate causation so it's okay. Universal expansion is the same way...

 

[aeroeng314]

However, AndromedasWake is also correct. The reason that universal expansion can cause things to "fly apart" faster than the speed of light without violating causation through special relativity is because all of space is expanding so this cannot be used to propagate causation faster than light.

Ahh, no, I understand that, I just misinterpreted what he said. I was under the impression that he was proposing some kind of "warp drive" kind of thing, not the idea that the space between two points can be expanding faster than light (because the metric expansion of space communicates no information, there's no violation). He explicitly mentioned it as a means of travel. So maybe I didn't misinterpret what he said. I don't know anymore, it's 3 AM and I've been grading papers for far too long.

 

[Gunboat Diplomat]

Ahh, no, I understand that, I just misinterpreted what he said. I was under the impression that he was proposing some kind of "warp drive" kind of thing, not the idea that the space between two points can be expanding faster than light (because the metric expansion of space communicates no information, there's no violation). He explicitly mentioned it as a means of travel. So maybe I didn't misinterpret what he said. I don't know anymore, it's 3 AM and I've been grading papers for far too long.

No, sorry, it was me who misinterpreted what he'd said. You're right, he was talking about some kind of "warp drive." I agree with you that that's impossible, unless by "weird things" AndromedasWake was including time travel...