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Thread: Our known universe

  1. #1 Our known universe 
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    If we can only see 13.7 billion years back, how do we know one of two things? First, is that the end or does it go on forever? Second, could there be another or infinite other universes past that 13.7 billion years we can't see yet?


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    The Enchanter westwind's Avatar
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    I believe so. Better means of gathering receding light sources will reveal other Universes in an on going disarray pattern randomly distributed throughout on going Space. Why there is no mathematical order in the distribution of Universes is not obvious. Looking at our own Universe and the distribution of Galaxies, and within them the distribution of Solar Systems, there appears to be no Mathematical order regards size, age, how long our Universe will be in existance, wether we will alwau=ys retain our Universial Mass in redistributed form, or just keep on spreading out until we run out of Gravitational influence. westwind.


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    Anti-Crank AlexG's Avatar
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    The answer to your first question is, 'we don't know'.

    The answer to your second question is, 'almost certainly'.
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  5. #4  
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    These are the type of answers I like, so many people think that I don't know is a sin. There are more that one answer to every thing.
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by gene4452 View Post
    If we can only see 13.7 billion years back, how do we know one of two things? First, is that the end or does it go on forever?
    We don't know. The universe could be infinite or it could be "finite but unbounded" (think of it as curved back on itself). The universe appears to be very flat, indicating that if not infinite, it is very much larger than the universe we can see (the "observable universe"). I have seen estimates that the entire universe is at least 1023 times larger than the size of the observable universe.

    Second, could there be another or infinite other universes past that 13.7 billion years we can't see yet?
    Not that we will never be able to see beyond a certain time because before that point the universe was opaque to light.

    There are theories which include multiple universes in this sense, e.g. Eternal inflation - Wikipedia, the free encyclopedia
    ei incumbit probatio qui dicit, non qui negat
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    Quote Originally Posted by Strange View Post
    I have seen estimates that the entire universe is at least 1023 times larger than the size of the observable universe.
    That really is big!
    How do scientists arrive at such an estimate?
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  8. #7  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Halliday View Post
    Quote Originally Posted by Strange View Post
    I have seen estimates that the entire universe is at least 1023 times larger than the size of the observable universe.
    That really is big!
    How do scientists arrive at such an estimate?
    I'm not sure! This is outside my area of expertise. I think it is from measuring how "flat" the universe is (which, I think, means how close it approaches Euclidean geometry). But how they measure that .... (it may come frome CMB observation ... somehow ...)
    ei incumbit probatio qui dicit, non qui negat
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    Quote Originally Posted by Strange View Post

    I'm not sure! This is outside my area of expertise. I think it is from measuring how "flat" the universe is (which, I think, means how close it approaches Euclidean geometry). But how they measure that .... (it may come frome CMB observation ... somehow ...)
    I was puzzled when I first came across the adjective "flat" because I thought it was being used to describe three dimensional space but then understood it was linked to Euclidean geometry.
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  10. #9  
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    im not familiar with this stuff. im just curious. how could a three dimesional shape be curve back to meet again? i understand a sphere is an infinite plane, i just cant conceptualize that form.
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    Quagma SpeedFreek's Avatar
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    Quote Originally Posted by Halliday View Post
    Quote Originally Posted by Strange View Post
    I have seen estimates that the entire universe is at least 1023 times larger than the size of the observable universe.
    That really is big!
    How do scientists arrive at such an estimate?
    That estimate comes from inflationary theory, combined with our observations of the Cosmic Microwave Background, which put minimum constraints on how much the universe must have inflated early on in order to end up as we see it today, taking into consideration such things as the "flatness" of the universe, the even temperature across the region and the introduction of structure into a "smooth" universe! It turns out that, if inflationary theory is correct, the whole universe has to be at least that size for the CMB to look the way it does.

    If I remember correctly, it comes from measurements of slight temperature differences in the CMB, where the theory is that these little patches of difference represent quantum fluctuations in the early "smooth" universe that were magnified by inflation and were the seeds required for large scale structures (i.e. galaxies) to form later on.


    Quote Originally Posted by bigratlover View Post
    im not familiar with this stuff. im just curious. how could a three dimesional shape be curve back to meet again? i understand a sphere is an infinite plane, i just cant conceptualize that form.
    It is most common to compare this to the surface of the Earth, where you can travel in any direction and end up back where you started from. It is not about the sphere as a whole, it is about only the surface. If we consider the surface of the Earth to be two dimensional, it has no edge. You can move across it forever, and you will never leave it. In two dimensions, it is a plane, but it is not infinite. A finite surface can have no edge.

    In order to apply this concept to the universe, we have to step it up a dimension, which is pretty difficult to visualise. How can we imagine a finite 3 dimensional surface that has no edges? We could be to wrap it around a 4 dimensional sphere, but we cannot really visualise that. We might consider that in whichever direction we look within our 3 dimensional universe, what we think of as a straight line is actually a very small part of a giant super humongous circle that encompasses the whole universe! And if the universe had the "shape" of a 4 dimensional sphere, that would apply in whatever direction you choose!

    But for the universe to have the "shape" (or topology) or form the surface of that sphere, it would have to be really very huge indeed in order for our part of it to seem as "flat" as it is. It is simplest to assume the universe is flat, but unfortunately that only makes visualising a possible "shape" even more difficult! Strangely enough, a four dimensional donut is the alternative! Well that, or infinity.

    A good exercise for this is to start with a piece of paper with an equilateral triangle drawn on it. The angles inside the triangle add up to 180 degrees, which means the paper is flat. We cannot wrap that piece of paper onto a sphere without either tearing it or stretching it. If we stretch it so all the edges match up and none are left "flapping", we find that the angles inside the triangle do not add up to 180 degrees any more! That surface, therefore is not flat.

    But if we wrap that piece of paper into a cylinder instead, so 1 edge matches up with the other and we leave the ends open, we find that the angles inside the triangle still add up to 180 degrees, as we did not have to stretch the paper. So the surface of a cylinder is also flat.

    Now, if we bend the ends of that cylinder round to meet each other, forming a donut or torus, we find we have to stretch the paper so the surface is not flat. But interestingly, the situation is not the same when you step all this up a dimension! It turns out that a 3 dimensional surface with the topology of a torus is also flat! (You'll have to trust me on this, as I have no way to describe it, except to say that if a 2D surface remains flat when bent one way like a cylinder, then a 3D surface remains flat when bent two ways like a torus!)

    So, if our universe is flat it is either infinite, or if finite it might be a four dimensional donut!
    Last edited by SpeedFreek; June 8th, 2012 at 05:30 PM.
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  12. #11  
    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by Strange View Post
    The universe could be infinite or it could be "finite but unbounded" (think of it as curved back on itself).
    It could also be infinite, yet bounded...here's where the fun really starts !
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