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Could there be an edge to the Universe?

  1. Dec 7, 2007 #1
    The answer, I am told, is 'No'. But I do not understand why. Assume:

    (1) The Universe is spatially flat. It will one day stop expanding.

    (2) There is no weird dark energy.

    These are, I think, plausible assumptions.

    Then: What evidence is there to weigh against the conclusion that there is an edge to the Universe (and therefore a unique center)?

    It is certainly compatible with the observed expansion, no? To use a rubber sheet example: Imagine the Universe is a standard 2D sheet of computer paper, but made of rubber. Stick coins to it to represent galaxies. Then stretch it apart to represent the expansion--fast at first, but slowing asymptotically to zero. Then we have (1) all galaxies receding from each other, (2) a unique center, but not one that would be easily (or perhaps even possibly) evident to the galaxies' inhabitants.

    Thanks in advance.
  2. jcsd
  3. Dec 8, 2007 #2
    Says who? Links would be nice.

    Why does the universe need to stop expanding to have an edge?

    Again, I'm don't see what this has to do with the universe having an edge.

    Well, no one has ever seen an the edge of the universe for starters. I can't think of any evidence to contradict the notion that the universe has an edge, but why would you want to think it has an edge? Because objects humans experience in daily life have edges?

    I suppose it is compatible, if you think of it that simply. But I don't see how it is useful to think of the universe as having an edge. I mean, what would happen if I crossed this edge? Would I stop existing? Can you even reach it? If you can't reach it, then what does it matter, no one could ever study it.
  4. Dec 8, 2007 #3
    Something to do with space-time being approximately flat if visualised as a two-dimensional model but space being curved. Euclidean geometry applies on a cosmological scale - parallel lines never meet etc.

    Recent data seems to suggest the rate of expansion is increasing, and has been for about the last 6 billion years. http://science.hq.nasa.gov/universe/science/expanding.html


    Galaxies have been observed to rotate at a more or less constant rate. The laws of conservation (angular momentum etc) should apply and so stars farther from the centre should rotate slower than stars closer to the centre but it appears they don't. Rather, galaxies would appear to rotate as if they were solid discs, but there is not enough matter to permit that. The current accepted hypothesis is that dark matter which we can't see provides the mass to explain this phenomenon.

    Then, since gravity is always attractive and due to the mass of the universe, it should still be expected that the expansion of the universe ought to be slowing. But it isn't.

    If we regard the universe as having an edge, then it follows that there is an outside. What is outside? Nobody knows.

    The limitation is that we are used to operating in 3 dimensions where everything has boundaries. When we look to extremely distant galaxies etc, at distances in the order of 10 billion light-years, we're seeing photons from stars which began travelling towards us 10 billion years ago, so what we're seeing is the universe as it was 10 billion years ago - looking into the past.

    What if we're looking at it all wrong? If the result depends on the observer then perhaps how we're looking at it defines everything. The only constant in science is that understanding changes with discovery.

  5. Dec 8, 2007 #4


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    We are already at the temporal edge of the universe, so what is your point?
  6. Dec 8, 2007 #5

    As far as i am concerned there is no physical end to the universe, but you must remember that the universe is mostly "nothing" and only partly "matter." If we were to choose a point as the centre, then find the furthest out piece of matter, be it a star or galaxy or simply piece of rock, this would be the edge, or end of the universe, as beyond this point there would be nothing, just empty space. This would be the end of the universe. The universe is expanding, so this furthest out piece of matter would keep moving away from us, as would the end of the universe. However, we are unable to find the furthest piece of matter as our technology is limited, so we therefore are unable to find the edge of the universe
  7. Dec 8, 2007 #6
    Now, I'm not at all an advanced physicist, but I have always considered the idea of "an expanding universe", as the physical matter in the universe moving outwards (from a centre), not that the "edge of the universe" is being constantly pushed. My impression is that there is an unlimited amount of "nothingness" out there, and that you could fly in a spaceship beyond the "edge of the universe". You would not find anything though.

    If that is true, then it doesn't matter that the "universe" (physical sense) will stop expanding. The rest of the "nothingness" is still "there", and hasn't moved at all. Only the bits of matter are moving away from each other. Who said that the universe is only as far as the planets go?
  8. Dec 8, 2007 #7
    What do you mean by the Universe.? For me it means everything together. This implies that there can be no space outside the edge. If there is an edge, what is its nature. If something arrives at the edge, what will happen to it? I can think of two possibilities, it ceases to exist, or it bounces of the edge back into the universe.

    However, all this becomes complicated to reconsile with the present understanding of gravition as curvature of space-time. The only flat space-time is one without any masses.
  9. Dec 8, 2007 #8
    Pero, wouldn't your first paragraph imply that "the edge" is ceasing to exist all the time, so the universe is moving inwards?
    In any case, the fact that you consider "everything together" as the universe, does not imply that the universe has an edge. Rephrase please.
  10. Dec 8, 2007 #9
    Isn't the size of the universe commonly regarded to be defined by the volume of space (nothingness) it has, not by where the matter is in it?

    The concept of an 'edge' of space sounds like a paradoxical idea to me - if you have an 'edge' then there has to be something (more space) on the another side of it, so you can't have an 'edge' of space ...

    I would have thought that if you assume the universe is spatially flat then to avoid the paradox of an 'edge' you would also have to assume that the volume of the universe is infinite (and there would also be no centre). There would be nothing to stop it being a stretchable universe of course.

    If you assume the universe has a finite amount of space in it, the only way I can see of avoiding the paradoxical idea of an edge is to say that this finite volume of 3 dimensional space is curved around on it's self through some higher dimension to form some enclosed higher dimensional shape like a hyper-doughnut or hyper-sphere (and again there would be no centre on the 3 dimensional 'hyper-surface' (volume) of a hyper-sphere or hyper-doughnut).

    So in my view it causes a paradox to assume the universe is spatially flat and has a finite volume of space in it (because it would need a paradoxical 'edge') ...
  11. Dec 8, 2007 #10
    Tomtom my physics knowledge isnt brilliant either; I'm a 16 y.o. taking my GCSEs. I agree with you in that the universe is everything, and that there should not be an egde, as you can keep going way beyond any theoretical edge or boundary. But there is little need to go beyond the furthest point as it would simply be nothing. However, it is true to say that you would still exist and wouldn't explode or bounce back if you crossed this theoretical edge. The universe is expanding, yes, but there may possibly only be a theoretical boundary and not an actual or physical boundary, so it would not be affected by the expansion as it does not exist.
  12. Dec 8, 2007 #11


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    That isn't how the theory currently works. There is no 'nothingness' outside of the universe - it isn't expanding into anything. And there is no center, just like there is no central point on the surface of an expanding balloon.

    What you are describing is similar to what the OP is describing.
  13. Dec 8, 2007 #12
    I believe it is possible to have an endless universe and for it yet to have an edge. Think of the universe as a massive sphere, but its walls aren't true walls, but jump points. One point on the spherical wall can be connected directly to its twin jumpoint that is exactly opposite it on the other side of the universe, and so on for every other point on the wall. So if a spaceship (or comet, or light) would indeed reach the edge, they would never know because they would see more space in front of them which is really just the entrance to the other end of the universe. With that in mind, then light from stars, as we predict it to be billions of light years old, may very well be pretty young and very nearby, it may just be an illusion caused by going in and out of the jump points, traveling in a straight line, and yet going over the same space over and over, and eventually the light met with us. It could have "aged" because of some phenomenom of crossing from one edge of the time-space plane to the other. With a mirrored jump-point universe of this kind, we can have infinite linear mobility, just with repetition of areas traveled (a pseudo-multi-verse) and we can also have an expanding universe. This idea of a pseudo-multiverse may be able to explain why certain quantum theories, such as M theory, call for more than one universe (it may be just one universe that mirrors itself onto itself!) The universe may be expanding due to the theory that the jump-points are constantly trying to fall into each other (fall into the other side) and are thereby stretching the very fabric of the universe.

    See, we can have both theories without contradiction! I have not ran over this theory of mine yet anywhere else, I would like to call it the jump-point mirrored universe theory. According to this theory, the universe could be smaller and younger than we could ever imagine. It is just that our perception is a little off...its the mirrors, LOL.
  14. Dec 8, 2007 #13
  15. Dec 8, 2007 #14
    Thanks for the input, but these responses aren't quite addressing what I'm getting at.

    (1) I'm talking about a standard FRW model in which Omega=1. To answer Contrapositive's question, it's vital that the Universe be flat because if, say, the geometry were S^3, then there's obviously no edge. It's also vital that there's no dark energy because if dark energy exists, then even in E^3 the Universe will keep expanding.

    (2) To answer Sionnagh: My (1) and (2) are assumptions. I know some people claim that the Universe is open. And they might be right. But my (1) and (2) are plausible--note: not true--assumptions to make.

    And it is perfectly reasonably to talk about finite geometries. Think of an ant on a (idealized 2D piece) of computer paper, floating in air. The ant may one day reach the boundary of his world. He can no longer go in one direction.

    There seems to be a lot of quasi-religious sentiment against an edge, but where is the evidence?

    Thanks again.
    Last edited: Dec 8, 2007
  16. Dec 8, 2007 #15


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    I like the first part of your post. And agree with part of it.

    I like that you point out that if the universe is spatially S^3, then there is no edge (and of course it could be expanding as the data seems to indicate.)

    I think you may be mistaken about one or two details. I first studied my cosmology before there was any talk of "dark energy" and one assumed the cosm. const. Lambda was zero, just like you want to assume. What we learned was that the spatial flat case kept on expanding forever.

    But you say because if dark energy exists, then even in E^3 the Universe will keep expanding. It sounds like by E^3 you mean the flat (i.e. euclidean) case, which is fine, but you think it NEEDS dark energy in order to expand forever. Conventional wisdom is that it doesn't need. With your assumptions (1) and (2), flat and zero DE, it is already going to expand forever. But that is only a mistake in detail. I don't think it matters to your main message. Maybe it does so if it is important please explain how.

    Here I disagree strongly. You've got a basically reasonable approach but I think you have the shoe on the wrong foot. The key thing is Occam's razor. We don't need an edge to make model fit data, so an edge would be an unnecessary detail. Occam says don't complicate your model with unnecessary detail. So the burden of proof is on the people who want an edge in the picture. THEY have to supply evidence.

    To use myself as an example, I don't assume an edge. There is no evidence for one. The moment that data appears which favors some kind of boundary to space (I can't imagine what such data would be like but supposing) I will be instantly willing to entertain the notion. My preferences are in no way fanatical or "quasi-religious".

    So based on my own experience, I reject the idea that there is some kind of ideological or quasirelgious sentiment against spatial topology with boundary. AFAIK that simply is not true. I think what we do see is understandable skepticism of people saying "show me. show me evidence of boundary in the observational data". And in the absence of such evidence, they will not wish to add that unnecessary complication to the picture.
    Last edited: Dec 8, 2007
  17. Dec 8, 2007 #16


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    Only that we observe an sky that more of less looks the same in all directions. If we were near enough an edge to observe it then we would observe a gross anisotropy, the galaxies would go on forever in one direction and they would come to an abrupt end in the other.

    If the edge is further away than our particle horizon then we would not observe it, although the sky in that direction might show evidence of something 'different ' beyond the horizon, a change in temperature of the CMB for example.

    The "quasi-religious sentiment" is actually a fundamental principle used in theoretical cosmology, that of the homogeneity and isotropy of the universe on the largest scales. It might be sheer pragmatism, as a way of solving the Einstein GR field equation in the cosmological case, but it seems to work as far (in the spatial sense) as we know.

    BTW - a very warm welcome to all the 'PF newbies': Contrapositive, Sionnagh, kinshuk, adilghanty, Tomtom and nalA who have responded in this thread. :smile:

    Last edited: Dec 8, 2007
  18. Dec 8, 2007 #17
    I would say that empty space has no limitations, and if the universe is being defined
    as "all matter" then yes the edge of the universe would be the furthest piece of that matter from a given center.

    However if space travel was advanced enough to reach that furthest piece of matter, then the spacecraft venturing past that piece would always be "at" the edge of the universe only by definition. I cannot imagine any type of barrier that could exist as an edge, only as what we define as the edge of the universe. But the idea that many people i believe are confused on is if empty space has a limitation or edge, in which I would believe does not.

    On a somewhat related topic, I believe that there would be a good possiblility that the entire universe as we know it, being that it is now likely expanding and will one day stop, could be one of several "universes" out there in the same fashion of multiple galaxies. I dont believe in a billion more years of human civilization that we will ever find the last and final piece of matter that is out there, but we will continue to find more, and when we think we have found it all, we will find some more.

    I really like the final scene in men in black where our entire galaxy or universe is just a marble in someone elses game of marbles.
  19. Dec 8, 2007 #18
    Okey, I may be confused at this point. Assume Lambda=0. My understanding is: if Omega>1, then the Universe is S^3, and there is sufficient matter to overcome the expansion, and the Universe will eventually collapse. If Omega<1, then the Universe is H^3, and will expand forever.

    Now, what I'm interested in: If Omega=1, then there's just enough matter to halt the expansion of the Universe, but not enough such that it will collapse. The expansion will slow and slow, asymptotically to zero. Is this right?

    And in that case, even if there's not a hard-and-fast 'wall-like' edge (like in the case of ants marching on a 2D piece of paper), wouldn't there necessarily be a pseudo-edge where matter ends?

    Also, if the Universe is infinite and expanding everywhere, how could any finite amount of matter halt the expansion? If Omega=1, doesn't that mean that the amount of matter in the Universe is finite (and therefore, the Universe is, too)?

    Right. Of course isotropy we take as a presumption (perhaps a very reasonable one); but it's not as if there is empirical evidence in favor of it. But under the model I've been discussing, it wouldn't be surprising that we find ourselves on a planet where the Universe appears boundless: If there were an edge, only a very tiny minority of planets/star systems/galaxies would abut it.
    Last edited: Dec 8, 2007
  20. Dec 8, 2007 #19


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    That is basically RIGHT, but the previous words aren't. Expansion does not HALT. Expansion continues forever without bound, just slower and slower. So arbitrarily large expansion is attained if you wait long enough.
    In this case (Lambda = 0, and Omega = 1) if you pick some expansion target, no matter how large, like a thousand
    YOU WILL EVENTUALLY REACH THAT TARGET. That is an interval between two galaxies that is today a Gpc will eventually reach 1000 Gpc. Or pick another target, like a million. distances will eventually reach a million-fold.

    So saying "expansion will halt" gives the wrong idea. It doesnt halt, but it does get slower and slower, in this case.

    As I just said, in the case you are talking about, based on your assumptions, there is no such limit on expansion. But you are confusing expansion FACTORS with an imagined length or diameter. In the typical flat (Omega = 1) case the universe STARTS OUT INFINITE DIAMETER. It always was and always will be of infinite spatial extent.

    You have to be able to picture an infinite flat piece of paper and visualize it expanding 1000-fold. The overall size doesnt get any bigger because it was already infinite to start with. What gets bigger by a factor of 1000 is the distance between two points you pick.

    Anyway, in the case you are looking at there is no wall and there is no pseudo edge or any limit on expansion.

    Good thinking. It COULDNT. But in your flat case the amount of matter is not finite. Infinite volume of space and roughly evenly distributed matter, roughly constant density throughout, means infinite amount of matter.

    If you like finite volume and finite amount of matter, try thinking about the S^3 case.

    You ask "If Omega=1, doesn't that mean that the amount of matter in the Universe is finite ?" No it doesn't mean that. However Omega > 1 means finite spatial volume and amount of matter. It's a more intuitive case to study for that very reason IMO.
    First understand the Omega >1 case of spatial S^3 and then you can visualize the flat case, if you like, as a kind of limit taking larger and larger spheres.
    Last edited: Dec 8, 2007
  21. Dec 8, 2007 #20
    I known't we'r there be an edge or not, yet this conversation keenly outlines the concept of infinity.
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