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Question about the edge of the universe

  1. Jan 12, 2005 #1
    Question about the "edge of the universe"

    Okay, the farthter away you look, the farther back in time you are seeing, right? When you talk about seeing some object 11 billion light years away, you are seeing something as it existed a long time ago.

    So, since the universe is expanding, that means that 11 billion years ago, it was considerably smaller, right? Which means everything was closer together, right?

    So, now I'm tyring to understand this in terms of looking in two opposite directions and seeing two different objects 11 billion light years away in each direction. From my current perspective they seem to be 22 billion light years apart, but from the time/age issue they should be really really close to each other, right? Can someone straighten this out for me?

    And this just made me think of a related question. Can we see the same object by looking in two opposite directions (and without it meaning the universe is closed)? I was just thinking if we could see an object in one direction that was so old that it was from when the universe was really tiny, then anything equally far away in the opposite direction we see would have to overlap with that object if the universe was small enough back then.

    Which of course, make me want to ask, why can't we see the big bang itself at the edge of visible universe in all directions? Is this a limit on the power of our telescopes? Or is it because of the accelerating expansion of the universe so that those photons are just never going to reach us? Or is it from something else entirely?

    Thanks in advance for any help.
  2. jcsd
  3. Jan 13, 2005 #2


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    Everything was closer together, but, the universe was not smaller. To contend it was smaller requires something to compare it against, which is not available. The universe may very well have always been spatially infinite.
    Much bigger than 22 GY. See below.
    The universe appears to be too large to see the back of your head. See http://arxiv.org/abs/astro-ph/9801212.
    We can already see as far back as possible in the electromagnetic spectrum. They are called CMB photons. They were liberated about 400,000 years after the big bang.
  4. Jan 13, 2005 #3
    Since the universe is ever expanding, there is no edge. It keeps growing so how can you be sure you found the edge.
  5. Jan 13, 2005 #4


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    The last paper by N. Cornish: http://arxiv.org/abs/astro-ph/0310233 rules out topologies smaler than 24 Gpc in diameter, i.e. 40 GLy in radius.
  6. Jan 13, 2005 #5


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    Chronos - If the universe is expanding, i.e. relative to some standard metre ruler, then the past spatial separation of non-gravitationally bound objects would be smaller in the past than their present separations. So the universe could be said to have been smaller in the past. To make the statement precise one would have to state a specific region, such as the present observable universe, which was smaller in the past.

    gonzo's two objects that are at present 22 G.Lt.yr. apart would have been much closer together when the light from them was first emitted.

    The conclusion that we 'cannot see the backs of our own heads' is model dependent and is true for the standard Big Bang model, but there are so many questions today about acceleration, DM, DE, and possible gravitational anomalies such as MOND that it would be prudent to keep an open mind on this one!

  7. Jan 13, 2005 #6
    Thanks Garth, but I that's not the brunt of my question. It seems some posters, while hopefully being good intentioned, have nit picked my poor word choice instead of trying to understand the point of what I am saying.

    I am not really interesting in seeing the back of our heads. That's not what I meant.

    And getting hung up on the exact numbers I used wasn't the issue either. I will try and avoid specific numbers to avoid at least that problem.

    This is my main concern. If we look in one direction and see an object very very far away, and then look in another direction and see another object very very far away in that other direction. To us, it looks like these objects are incredibly far away from each other, much farther than either one is from us even.

    However, since what we are seeing is very very long ago, I thought that should mean these objects should actually be much closer together at the point we are looking at them, and so it is strange that we can see them in opposite directions, and I was hoping to understand this.

    The reason I mentioned seeing the same object in two directions is an extrapolation of this same idea. If we see these two objects by looking in opposite directions, but if they are far enough away, then it seem they should be close enough to see next to each other in the same direction (or in some arc, or whatever, I hope you get the idea of what I mean). You see? I am thinking that if they are close enough in the past to see in the same direction closer together, and that led me to wonder if you couldn't see the same object in two directions then if you looked far enough away, because the farthe away you look, the closer everything should be to each other and at some point you should be able to see these objects in the same small arc or direction.

    I guess it's a sort of mapping problem in a sense. as we look farther away, we seem to be looking at the surface of an ever larger sphere. But since we are looking back in time, we are looking at the contents of an ever smaller sphere (and don't get hung up on the fact that it really isn't a sphere and it's not expanding through anything and miss my point, I'm tyring to ask this without putting a million words on it and get the essence of my question across). So we are mapping a shriking space onto an expanding space in some sense, so unless objects take up more space, I'm confused on how that works out. Assuming I've managed to convey the essence of my confusion.

  8. Jan 13, 2005 #7


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    Actually I have already answered your question and it is related to 'seeing the back of your own head'! Think of the globe of the Earth. Your antipodes can be reached by going in any direction.

  9. Jan 13, 2005 #8
    I'm confused, I don't understand how this answers my question. Could you please elaborate? Thank you.
  10. Jan 13, 2005 #9


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    If great circles going through your position on the Earth's globe represent the null-geodesics of light rays coming to us from the depths of space, then, at the opposite side of the world, places close together, but on opposite sides of the antipodean point, may be 'seen' far apart, one great cirlce going 'eastwards' and the other 'westwards'. Its only a model, and the universe may not be space-like spherical in shape, but it is an illustration.

  11. Jan 13, 2005 #10
    I understand that analogy, but that is for something else and doesn't seem pertinent to my question.

    Maybe I'm not being clear. I'll try and say it again. As we look at objects farther away from us, we appear to be looking at the surface of ever larger spheres, with us at the center? Is that clear?

    However, as I understand it, since all of the stuff we are looking at on the surface of that sphere was much closer together since it also back in time, we should be seeing smaller and smaller regions of space instead of larger and larger ones.

    Back in time, things were closer together, but the farther out we look the farther apart they are. How is this resolved?
  12. Jan 13, 2005 #11
    try not to think too hard about it gonzo. It's pretty much as simple as a bag of marbles exploding. Would you ask the same mind bending questions about the marbles? no. A marble looks like a marble from all the other marbles. It's that simple.
    Marbles close together. Bang. Marbles far apart.

    We see distant objects the way they were millions of years ago. Yes, they were closer to other objects, but still not too close. To see the objects while they were still all stuck together in the original 'universe atom' we'd need to be alot further away from them. e.g. consider a very light particle, which has been traveling at very nearly the speed of light away from the universe since the begining. From this particle, looking in the oposite direction to which it is traveling, you could see the universe in the very compacted state it was in at a time just after the big bang.

    Yes, we do see galaxys to be closer to us and each other than they actually are.

    As for the looking in 2 directions and seeing the same thing, this is old crazy speculation from when the then new GR model first suggested the universe is closed in some respect, but in reality, it's never been observed.

    As for observing the big bang, yes, it's been observed, just the way you'd expect. A very redshifted faint old radiation, observed in all directions.
  13. Jan 13, 2005 #12


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    I will try to give an explanation but I am not sure whether it is correct. Consider the line element of the Robertson-Walker metric at scale factor ae (redshift ze) for only one angular variable:

    [tex]ds = a_e r d\theta[/tex]

    This can be written in terms of the current scale factor today a0:

    [tex]ds = \frac{a_0 r}{1+z_e} d\theta[/tex]

    Thus, the angular element for a given line element at redshift ze can be written:

    [tex]d\theta = \frac{1+z_e}{a_0 r} ds[/tex]

    Which is a function of r (a distance) and z (the redshift). When one approaches the particle horizon z goes to infinity very fast and r tends to a finite value (that of the radius of the observable universe). The angular element will increase.

    So in my humble opinion you are right in your assumption. However, the redshifts at which this phenomenon may be visible are surely behind our observational limits. Note that behind z > 1000 the universe gets opaque to electromagnetic radiation.
  14. Jan 13, 2005 #13


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    Yes I understand what you are saying.

    There are two effects to think of, and don't follow meemoe_uk's advice
    If we are not supposed to think hard why on Earth did God give us brains?!

    The first effect is caused by the curvature of space. If the universe is spatially spherical then a good 2D analogy is the surface of the Earth's globe. Consider the case where your 'space' is actually the surface of a wooden sphere. Start at the North Pole and paint the floor in ever increasing circles with yourself on the outside of a growing circle of wet paint centred on the North Pole. As you go on painting the area you cover will grow larger and so does the circumference on which you apply the paint. After a while you reach the equator, now something strange happens. The more you paint the smaller the circumference gets, and you eventually realise that, rather than being on the outside of a circle of wet paint, you find yourself on the inside of an ever decreasing area of unpainted wood surrounded by wet paint, until eventually you have painted yourself into one tiny spot at the South Pole.

    The second effect is caused by the fact that as we look out we look back to a smaller universe, so the expansion of the universe itself produces a lensing effect in which two 'rays' going off in different directions first diverge away from each other but eventually converge again as they traverse an ever younger and ever smaller universe.

    There may be regions of our universe that we can never see, for they are outside this small area in the early universe that we can see, the boundary between these two parts, the observable and the at-present-unobservable universe is our particle boundary.

    Last edited: Jan 13, 2005
  15. Jan 15, 2005 #14
    I think this may be the missing piece of the puzzle for me. Thanks.
  16. Jan 15, 2005 #15
    Actually, something just occurred to me. Doesn't this mean that far away objects are really much smaller than they appear?
  17. Jan 17, 2005 #16
    One other thing's been bugging me too. At first I assumed with the sphere analogy that there wasn't any real world equivalent to the equator, and that that was just a problem of the analogy. But I realized there actually HAS to be, if we assume the lensing effect.

    I know that for close objects my visual arc gets wider as I look out farther away, and then with this lensing issue, we assume that it gets narrower again very far away. That must mean there is a "turning" point out there somewhere, which becomes the equivlanet of the equator in the sphere analogy.

    So, how far out is this point, and what determines it?

  18. Jan 17, 2005 #17


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    If you apply 4 dimensional thinking, it all falls into place. Here and now observations are trapped in time bubbles when viewing distant objects. While what you are saying is true with respect to simultaneity, it is an illusion with respect to GR. Since time moves at a finite rate [speed of light], you see the past no matter what direction you look. And it is causally connected to the present. If the past is wrapped around itself, you would see 'circles in the sky'. But we don't. That suggests our universe is observationally infinite. But reality is not the same as observation. As many theorists have suggested, it may well be impossible to see the 'back of our heads' or mirror versions of this universe. I routinely discard such theories because they are impossible to affirm.
    Last edited: Jan 17, 2005
  19. Jan 17, 2005 #18


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    After thought, observe a distant supernova. Immediately initiate a search for another on the 'opposite' side of the universe. There should be a time delay [one will occur before the other because of the relative distance from our reference frame]. Supernova are so rare it should be a no-brainer. You would have at least a week to find its mirror image and collect your Nobel. I am almost serious about this. It would be a huge discovery, or non-discovery. Thanks for proposing a way to test this theory. Your idea is truly brilliant.
    Last edited: Jan 17, 2005
  20. Jan 17, 2005 #19
    That doesn't work because of the particle horizon, or whatever it is called, as I understand it.

    Adn I don't really understand how what you said answers my question.
  21. Sep 17, 2010 #20
    Re: Question about the "edge of the universe"

    -- James Ph. Kotsybar

    We can see fourteen billion light years out.
    For those still here a billion years from now,
    more light will have traveled to them, no doubt,
    the billion light years that space will allow.
    Distant descendants may not see much more,
    however, than what we can now observe.
    Despite larger radius to explore,
    their view won’t be a sight they can conserve,
    because space itself goes faster than light,
    as it expands relatively through time.
    This perspective's loss is ever the plight
    throughout our universe's known lifetime.
    We daily lose ability to see
    the things furthest back in our history.
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