Edge of the universe?

Factual Questions
1

Ok, I’ve been thinking about this for some time and there probably is some logical explanation out there that will make me want to slap myself. It may be a bit hard for me to phrase this question without a diagram of somesort… so just bear with me…

Anyway, let’s look at the Big Bang. The universe is born and it is expanding. But say we were able to travel into the farthest reaches of time and space, is it possible to travel so far back in time that we could reach or encounter the edge of the universe?

If the universe is expanding, what is it expanding into? If we reach the edge, will it be like a wall or something? So what’s on the other side? Any ideas? Perhaps dimensions like these won’t exist in such circumstances?

I’ll be happy to explain what I’m trying to get at here if anyone needs me to… :confused:

2

And how do you propose traveling back in time? If you have a working time machine, I’d suggest that you patent it and make yourself filthy, stinkin’ rich instead of posting to the SDMB :smiley:

Well, imagine that you’re a 2-dimensional creature walking on the surface of a balloon. You can travel forever in any direction without reaching the “edge”, yet your universe (the balloon) is still finite. Also, the balloon can expand without expanding into anything that you can perceive, or even understand.

Hopefully someone will come along and post some relevant links about the current scientific opinion of the shape and structure of the universe. But the above example shows how you can have a finite, yet unbounded universe by having it curve in a different dimension.

3

I should point out that the reason we see ‘back in time’ when we look to the ‘edge of the universe’ is simply that light from there is taking a goofy long time to get here because of the vast distances. We’re simply ‘seeing’ the state of things as they were, not as they are.

We have no idea what’s happening right now to the things we’re seeing right now. We only see them as they were. Symmetrically, anyone there can only see us as we were many many millenia ago.

To put it succinctly, the there at the edge of the universe, some distant galaxy, for example, could have, in its own ‘present’, exploded, vanished, or turned into banana loaf, and we won’t see it for billions of years.

Okay, so that’s simple enough. But travelling towards the things we see wouldn’t make us travel back in time. We can’t get to there at the time that we see there being. As we travel closer, we’ll also get closer to there’s present time. Eventually, we’d actually see the banana loaf. We couldn’t get to there, but at a time where there was no banana loaf.

You, as a time traveller, are sadly always travelling forward in the universe. You’re not always travelling at the same rate as the rest of the universe (darn that time dilation!) but you are never moving backward relative to any part of it.

Barring having some technology we don’t yet have. :slight_smile:

4

To second Joe on one point, when you look at the nighttime sky, you are said to be looking back in time, since the light from the distant objects you are looking at take a very long time to reach your eyes. However, if you actually travel to a distant object, you are in no way going back in time.

If this could be done, I believe I’d travel back to yesterday so that I can finish my tax return. :smiley:

5

Simple answers. The universe is not expanding into anything. Spacetime is all there is. The expansion of the universe creates new spacetime. Even so, there is no edge, no wall, no other side. You can’t get to the end of spacetime. You are always in the apparent center, no matter where you are. And spacetime can expand faster than the speed of light. You cannot travel even at the speed of the light, so there is no way to catch up. It expands in all directions around you. This is equally true even in the past when the universe was smaller.

Everybody always mentions Brian Greene’s The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory at this point and everybody is right. It is a wonderful book.

6

Even so, this month’s Scientific American’s cover story is about what’s beyond the edge of our universe (universe defined as the limits we could ever see, being that light has only travelled ~13 billion light years since the big bang. The universe is expanding by one light year each year in all directions).

So I guess there’s some disagreement among the astrophysicists.

The article went on to posit, with a huge, buried assumption in the middle of the article (assuming distribution of matter is uniform beyond our universe), and that with quantum physics limiting the possible number of time/space states in a universe, how far away, on average, an identical universe would be.

Don’t trash me; I’m just the messenger.

7

Just so people don’t get too confused: “Universe defined as the limits we could ever see” is a very different definition than “universe defined as everything within spacetime as we know it.” The latter is very much bigger than the former.

The distinction between these definitions is exactly the source of confusion in this thread.

8

Brilliant!

I’ll go and slap myself… :slight_smile:

9

//hijack?
re: William_Ashbless

suppose i were to travel away from Earth at the speed of light keeping a line of sight to her. looking back at earth, do i see it in stasis? or would i see nothing at all travelling at that speed?

10

After the obligatory reminder that you can’t go the speed of light under any circumstances, the answer is that you would see absolutely nothing traveling at the speed of light because nothing, including photons, could catch up to you.

11

But isn’t the speed of light always the speed of light in comparison to how fast you’re going, even if you’re going .999999999 times the speed of light? Or does light actually seem to “stop” if you were to theoretically go the actual speed of light?

12

OK… someone talk to me about the possibility of parallel universes then?.. :slight_smile:

13

What’s at the edge of the Universe?

I thought everybody here knew it was Milliways.
:smiley:

14

Shijinn:

Relativity does whacky things to the intuition. I’ll have to be careful lest I say something that’s incorrect. Very easy to do.

Special relativity dictates that if you’re in motion towards or away from Earth at nearly the speed of light (say, on a spacecraft), then, as seen from your brother on Earth, your passage of time is slower. You could detect this by sending out per-second light pulses to your brother, and he’d see your pulses arrive slow.

It’s a trippy consequence of special relativity, but it doesn’t matter if you’re heading away from, or towards Earth. (Tangential motion doesn’t apply).

The thing is, though, the situation is symmetric in special relativity. If you’re heading away from Earth, Earth is heading away from you. So as a consequence, you see your brother’s time frame slow relative to yours. He, and his light pulses back to you, will seem slow. So each of you perceives something different, that the other person is slowing down.

If you were to be going fast enough, you would indeed see Earth’s time slow to a crawl.

But there are some added wrinkles. In the presence of high acceleration or high gravity, the symmetry disappears. If you apply your rocket engines, say, to get up to that amazing speed, then your time frame slows down – but interestingly the rest of the universe speeds up (at least, that’s the way you see it). Furthermore, your brother on Earth, unlike the velocity-only case (non-inertial) above, actually agrees with you – you’re getting slower, not he. He, of course, doesn’t see anything unusual except that you’re slowing down.

To make matters more interesting (and I’m treading on thin ice here) it doesn’t matter if you’re accelerating towards/away, or tangentially, unlike the non-inertial case. This is a consequence of the equivalence principle stating that acceleration (say due to thrust) and gravity are indistinguishable.

Lastly, at these speeds you’d be seeing severe redshift (heading away) or blueshift (heading towards) as light was shifted in the spectrum due to doppler effects. This means as you looked back, you’d see Earth get redder and redder (as you got faster) until the light stretched off the visible spectrum. You could detect it with other sensors, and there might still be some very very faint visual cues based on higher frequency crap that we’re dumping out that get shifted down into the visual range. In any event, if you had sensors to turn infrared or lower signals into visual images, you’d still, and always, see the Earth, just getting slower and slower (unless you’re accelerating so much that the speed up of the universe from acceleration outweighs the slowdown of the universe because of velocity, of course… the two effects are cumulative!).

These are all just interesting properties for the sake of the question you posed that impact on what you might see.

Keep in mind that you can’t actually travel at the speed of light, so the exact question you posed is meaningless. In a sense, no light could catch up to you, but trying to speculate on it is like trying to speculate on plaid coloured hippos. You have to restrict the question to just shy of the speed of light (anything less will do just fine).

15

Damn, I was going to post something on this topic but kept putting it off. My thread would have been something like “Large scale topology of the universe”. I’ve wondered about this since I was a kid. I remember reading something by Sagan (I think) where he described the universe as a hypersphere. So you live in a 3-manifold warped in a fourth dimension such that it forms a 4-sphere. Recent discoveries related to the microwave background though (the W-MAP project) seem to indicate that the universe has a “flat” topology. This is disturbing. It implies that there may be an edge to spacetime.

The three possibilities seem to be that the universe is:

  1. finite and bounded (the W-MAP research seems to bear this out)

  2. finite and unbounded (a hyper-toroid, hyper-sphere or other unbounded shape)

  3. infinite and unbounded.

1 and 3 are a bit troubling. With 1 you wonder what happens at the edge. As one poster suggested though, if the edge recedes at >c there is no problem. With 3 you wonder how spacetime could have gone from a singularity at the big bang to an infinite size a fraction of a second later (note that I’m only suggesting that spacetime would do this, the matter and energy from the big bang would still be expanding sub-luminally).

16

Well, I have to add my comments on the ‘troubling’ nature of case 1, and its important enough because the universe does seem to stand a really good chance of being this way.

You’re making the claim that because something is bounded, there’s an edge.

The classic example is reducing the problem to 2 dimensions from a perception standpoint (which we can fathom better) by examining the proper outside surface of a balloon. So imagine you’re living on the surface, and there is no ‘height’ as you perceive it. Just length and width as you look around yourself.

The surface itself has no edge. You can ‘walk’ in any direction in two dimensions on this surface and not find one. In that sense, there is no edge. (We ignore the niggling detail of the ‘umbilical’).

Yet, the balloon can expand. As it inflates (for whatever reason), we end up with more surface area. This is trivial to show or even envision – someone blows up the balloon. So we get more ‘space’ on the surface.

But there’s still no edge!

Topology is sometimes as counterintuitive as relativity.

You might claim that this requires solving a two dimensional edge-less but bounded space by using a dimension you can’t ‘see’ – that is, the larger universe that must contain the true balloon.

Mathematicians and cosmologists play around with topologies all the time in dimensions sometimes much higher than our traditional 3. We might not be able to directly see higher dimensions, but they may exist to help the topology of the universe be the way it is.

The practical upshot of all this is that our universe can be ‘bounded’ but have no edge.

The universe doesn’t need anything to expand into.

In fact, some recent amusing results that are still highly tentative suggests that the universe isn’t even ‘spherical’. The universe could be toroidal. That is, it would be the 3 dimensional equivalent of the surface of a doughnut. No one would doubt that living on the surface of a doughnut, but not being able to look ‘up’ or ‘down’ but just ‘side to side’, that you cannot see an edge… any straight line of sight just keeps going.

This is actually almost easy to envision. Take a ‘cube’ that describes the universe, and number the faces. 1 on top, 2 on bottom, 3 on left, 4 on right, 5 on front, 6 on back.

If you could ‘stretch’ the cube in your hands, you could make faces 1 and 2 match up. If you assume that these faces are really just connected, then there really is no seam.

With some care and additional stretching, you could take faces 3 and 4 and wrap them together. If you assume these match up too, and are connected, then there really is no seam.

By ‘no seam’ I mean there’s no way to really detect where the edge/face is – in fact, there really is no seam, and you can look across this ‘transition’ like normal space.

Finally (and this takes some mental masturbation), you take faces 5 and 6, and match them up too. Problem is, if you’ve done everything prior to this, face 5 is inside of a spherical glob, and face 6 is outside. Yet, in a higher dimension you could connect these two faces without having to go through the intervening glob.

As it turns out you can connect up ‘edges’ of a universe in all sorts of interesting topologies to eliminate edges. In the case above, we now have a 3 dimensional space that is fully connected, and we can’t find any edges or seams, and yet, by ‘blowing up the inner tube’ so to speak or causing the doughnut to expand, we get more space.

There’s no necessity to have any edge of the universe, even one receeding away at C, or anything of the sort.

I mean, there might be…

…but there’s no requirement, yet, as far as anything we know.

17

By the way, (as far as I know) there is no observational evidence, for the universe, having large scale spherical geometry, though I do believe an experiment was conducted by 'drawing ’ a very large triangle (with galaxies as the points) and measuring the angles.

But the mass of the universe would certainly cause significant curvature.

18

[QUOTE]
*Originally posted by William_Ashbless *
**The classic example is reducing the problem to 2 dimensions from a perception standpoint (which we can fathom better) by examining the proper outside surface of a balloon. **

Yes, but hasn’t the jury pretty much come back in and told us the universe is flat? And wouldn’t this mean that the classic balloon model has popped?

Which leaves us stuck in a flat universe that does have an edge. True, we’ll never get there. But (and tell me where I’m making a bad assumption) the following also seems to be true to me:

  1. Some parts of the universe are closer than others to the edge, and there is a center.

  2. There is a point where you can go no further because space does not exist beyond that point. Okay, so there’s no way to ever catch up with that point, but it still does exist.

And if space-time expands > c, and matter and energy expand <= c, wouldn’t this mean that there’s an enormous buffer of empty space between all the stuff in the universe and the edge? Is space non-existent without stuff in it, or just “meaningless?”

Know I’ve got something wrong here. Would love to know more about what it is.

19

OK… I’m never going through that mental masturbation thing again :smiley: Ok I need some clarification here…

So if we had the ability to just keep travelling, we will end up in the same place as we started? :smack: Obviously not that simple, but is this the idea?

And taking the ‘cube’ model of the universe where we match all the faces of the cube together, doesn’t this essentially mean that there is no ‘direction’ of any sort that one could travel in?

20

Engywook:

The universe could be flat, or it could be ever-expanding, but that doesn’t destroy the balloon analogy at all. Space still could be fully connected in the balloon surface sense, and yet the expansion is occurring quickly enough that some parts of the balloon are moving away from us (along the surface) at faster than the speed of light.

Only in that sense is there an edge – but its an edge to observability, not an edge or skin of the universe.

This corresponds to your case 2, in a sense, but the thing is, its not like you hit a wall. It’s simply a place you can’t catch up to. From your perspective, no matter where you are, or how fast you’re going, you still can’t see any sort of edge.

Your case 1 makes no sense in a universe as we’re describing (and as we seem to be in). You keep thinking of the universe as having a distinct skin boundary from which you can measure.

The problem is, no such edge exists, and for everybody that ‘point of unobservability’, if it exists, is different. Me, in the Milky Way, can see some portion of the universe, but my brother Bob, in Andromeda, can see a slightly different portion of the universe. Neither of us is in the center. There is no universal skin.

To see this on the balloon, put yourself at one point, and measure out (along the curve of the balloon) some fixed distance. Put your brother Bob at another point, and measure out (along the curve of the balloon) some fixed distance.

There’s still no ‘boundary’ to the universe. A straight line still ends up going on forever without impacting any real wall. There may be a fixed distance that you can see because of the expansion, but that doesn’t change the topology of the universe.

None of this leaves us in a flat universe that has an edge. The connectedness may not be visible as curvature we can see in our paltry 3 dimensions.

Let’s put it another way.

As you blow up the balloon, various points on the balloon that you could see at some point in time seem to get further apart at another point in the future. (Dot the balloon with a marker).

The initial explosion of the universe is what keeps the universe expanding (possibly augmented by some sort of expansive force, speculated on by people), but for whatever reason, the balloon is being blown up.

The rate at which the balloon is being blown up is slowing due to gravity.

Each of the marked points on the balloon can be considered a galaxy, if you will, all emitting light.

Pick a point on the balloon where you are.

If the universe is expanding slowly enough, then light from the most distant possibly ‘galaxy’ on the balloon will be able to make it to you. This means that all points in the universe are visible.

If the universe is expanding just barely quickly enough, then the furthest possible galaxy on the balloon will be emitting light that, after an infinite amount of time, will reach you. This is sort of the cusp situation. This still means that all points in the universe are visible, except possibly for an infinitely thin region that is the furthest point away from you.

If the universe is expanding faster than that, then the furthest possible galaxy on the balloon will emit light that will never reach you. It cannot affect you in any way. Although you cannot see it, it may still exist, but you have no way of ever ever proving it. Ever. You can’t get to it, and it can’t get to you. This means that some parts of the balloon are not visible to you.

However, and here’s the important bit: None of this implies any sort of edge to the universe, except for the visible observable edge.

Okay, so you claim there’s still an edge.

Well, you can make the exact same claim at any point on the surface of the balloon.

So where’s the ‘center’ on the surface of the balloon?

There isn’t one.

Every point in the universe is the center of their own ‘observable universe’.

There’s no necessity to invent ‘skins’ or ‘edges’ or ‘buffers of empty meaningless space’, etc.

Now you might claim that anything that cannot affect us doesn’t exist, and I might not be able to debate that point. I’m merely pointing out that you don’t need to construct any sort of fixed edge or center or anything.