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Where does the universe expend?

  1. Sep 18, 2007 #1
    as we all know and agree the universe is expending . what what is "expension" it is "to occupy a larger volume and space" but before that the universe occupy the new region by expension, what do we find in this region??
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  3. Sep 18, 2007 #2


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    The universe doesn't expand into anything - it is everything!
    Instead of picturing it as an explosion rushing out into empty space - picture it is a piece of paper with the plans on which is itself getting bigger.
  4. Sep 21, 2007 #3
    I believe the edge of this limit which you are talking about - namely the edge of the universe - is called the particle horizon.

    EDIT: And basically it would be the particle horizon from the point where the big bang took place - I believe.
  5. Sep 22, 2007 #4


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    to me it sounds like you are confused and suffer from one or more popular misconceptions so I hope you will pardon me if I suggest you read that Sci Am article that always gets recommended

    It is by Lineweaver and Davis. If you google "Lineweaver Misconceptions" you should get it. the title is Misconceptions about the Big Bang.

    In standard mainstream cosmology, at the present time:

    there is no one point in space where the BB took place

    the particle horizon is not an edge

    it is a continuously shifting limit---it is getting farther as we speak

    the universe is not expanding OUT INTO anything, there is no outside.
    Last edited: Sep 22, 2007
  6. Sep 23, 2007 #5
    how is a continuously shifting limit different from an continuously shifting edge?
  7. Sep 23, 2007 #6


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    I'll say it with more words and hopefully make it clearer.
    as far as is known, the universe has no edge, in the sense of a boundary where there is a physical change

    it is only that we see more and more of it as time goes on.


    sometimes people confuse this with distance-expansion but the two ideas are basically disconnected.

    there is a limit to how far we can see, because the light has had time to get here, but there is no EDGE there----the universe just continues out uniformly beyond what we can see at present. and as time goes on, more light comes in from further away and the part we can observe encompasses more galaxies.

    sometimes people picture "big bang cosmology" as a kind of EXPLOSION of crud from some central point flying out into empty space
    it is a misconception they get from the popular literature.
    Lineweaver and Davis SciAm article explains how that is wrong. Google "Lineweaver Misconceptions"

    the expansion is an expansion of DISTANCES in a boundaryless universe

    hope this helps

    Thanks Ganstaman, that is a good image (a 2D analogy to what happens in 3D) and you explained several things very clearly. I'll respond here, instead of making another post, since i can still edit this one.
    Last edited: Sep 23, 2007
  8. Sep 23, 2007 #7
    What if we could find out how fast the universe was expanding at all the spots. Couldn't we extrapolate inwardly and find out where and when the big bang was?
  9. Sep 23, 2007 #8
    This isn't a problem of what we can measure, as far as I understand. It's simply a matter of how things are. Everything is moving away from everything else. There is no center of expansion.

    The balloon example works best, IMO. Imagine blowing up a balloon. Now focus on the 2D surface of the balloon -- it is exanding (of course, it is exanding into the 3D space the air around it was occupying, so the analogy isn't perfect, but from a 2D perspective it's just expanding and not into anything). From this 2D surface, we can see that every point on the balloon is moving apart from all other points -- the speed they move away from each other is proportional only to their current distance from each other.

    Also note that on the 2D surface of the balloon, no point is the center of expansion. There is no center from which the 'big blow' started taking place. It happened everywhere on the surface at once when all points on the surface were close together.
  10. Oct 2, 2007 #9


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    I like the 2D balloon analogy too, it helps me to visualize the 3D somewhat.
    As far as new stuff appearing at the end of the universe as time goes by: I thought the opposite was true, that mass at the edges are moving away from us at greater that C (not that they are traveling at greater than C but the space is expanding between us and that matter at a combined rate that is greater than C). I thought less and less was visible as time progressed. Is that wrong?
  11. Oct 2, 2007 #10


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    As far as I am aware there isn't any new suff being created in the universe - other than small scale photons/pair production (e=mc^2 etc)

    That's correct. Less proportion of the total universe is visible, but the amount of volume observable is still increasing with time - at the speed of light.
  12. Oct 2, 2007 #11


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    But that ever-more visible volume contains less and less visible matter, correct? I mean to say that the volume expands at C but the matter is moving away at greater than C.
    Last edited: Oct 2, 2007
  13. Oct 2, 2007 #12


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    The horizon is only relative to our point of view--signals from events that happened at points inside the horizon would have had time to reach us since the Big Bang (though the closer you get to the horizon, the earlier the events would need to have happened for there to be enough time), while no signal from an event outside the horizon could have reached us. But that doesn't mean the horizon represents any sort of objective physical "edge of the universe", it's assumed that galaxies continue out past the horizon, we just can't see them.
  14. Oct 2, 2007 #13


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    Yes - I don't know which part contains the larger amount of matter, it would depend on the mass concentration in the early universe, I'm a bit out of date with inflatioanry cosmologies.
  15. Oct 8, 2007 #14
    This is actually a problem I've had with both the balloon analogy and the string theory explanation for the extra dimensions being "curled up":

    You can say that the 2D surface of the balloon isn't really expanding into anything, that all the points are moving out away from each other and there's no center to which to trace the expansion, but if the ballooniverse was actually 2D, would it be possible for it to expand? Isn't the third spacial dimension containing the balloon required for the expansion to take place at all? And if the 2D ballooniverse is curled up, from the 3D perspective there IS in fact a center of the universe, albeit one not perceivable by the 2D balloon inhabitants.

    Is there a satisfactory logical explanation of this? Or does the math work out so we fudge the details of the analogies?
  16. Oct 8, 2007 #15


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    Yes it is wrong. At the present time the number of galaxies (or amount of material) in our observable universe is increasing.

    (if the standard LambdaCDM model is correct then eventually, because of accelerating expansion, a time will come when the number of visible galaxies will start to decrease but that is very far in the future, the earth may no longer be inhabited etc etc. Dont worry about it :smile: )

    the other poster, MGB, seems to have said that what you assert is NOT wrong, but is correct. I don't understand his post. He seems to be saying that at the present time less and less is visble as time goes on. According to MGB the number of visible galaxies would be decreasing! We have a serious confusion here that should be cleared up.

    IMP perhaps you should learn what the current PARTICLE HORIZON is (if that idea is not already familiar to you). It is the absolute limit to how far we can see at present by any imaginable means, not just light but also neutrinos or gravity waves or whatever----more precisely it is the presentday distance of crud that we could be getting signals from at this moment.

    If I remember right the particle horizon is currently at some 47 billion LY. So just by the usual Hubble formula,
    OBJECTS at that distance would currently be receding at a speed of 47/13.8 = 3.4 c.
    But the horizon itself would necessarily be extending out faster than this, to include more of the universe---more objects in our field of view. I suppose the particle horizon is extending out at a speed of around 4.4 c.

    there is a catch---as distances expand, light from distant galaxies and stuff gets more and more redshifted. In the future this very old light will probably be getting too redshifted for us to detect. there will be a huge amount of early universe that we are getting signals from, but our "eyes" won't be sensitive enough to detect it, likely.
    Yes 47 is what Lineweaver says. I checked the figure of 47 billion LY.
    Anybody who wants to understand these things as clearly and easily as possible should look at the picture on page 6 of this paper:
    click on PDF and scroll to page 6, where there is "Figure 1"

    It shows the particle horizon and also the hubble radius (the distance where things are receding at speed c)
    and also it shows the "cosmological event horizon" which is currently at about 15 or 16 billion LY.
    This is the present distance of a galaxy from which, if TODAY they sent a signal, that signal would eventually reach us.

    Anything farther away than the cosmological event horizon, if today they flashed a light or sent some kind of signal it would never reach us even if we waited infinite long. that is, if the standard LambdaCDM model is true. there are other models where the signal WOULD eventually get here, but the LambdaCDM is what seems most successful in fitting all the data so it is the mainstream dominant model
    Last edited: Oct 8, 2007
  17. Oct 8, 2007 #16


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    * Isn't the third spatial dimension containing the balloon required*
    No. a third spatial dimension is not required. A two dimension sphere can be defined by specifying a set of points together with a distance function that gives the distance between any two points.
    Then it can be made to double in size simply by multiplying each distance by two.

    The property of being a two-sphere----a familiar 2D sphere---is an INTRINSIC property that a set of points can have without reference to anything else. Just those points and the distance function. (technically described with local coordinate patches but that's just a marginal comment, not the main message here)

    the distance function has to give what we know as the "great circle" distance or geodesic distance between the points.

    Geometry stopped being only about embedded things around 1850. What I just was saying is vintage 1850. Riemann discovered you could define a geometric space or figure INTRINSICALLY without referring to anything outside it. the distance function is called the "metric".

    in 1915 when it was time to describe spacetime, Einstein used Riemann-style intrinsic geometry.

    So the answer is no. a larger dimension space is not required. To do dynamic geometry you dont have to have space contained IN any larger context.

    *Is there a satisfactory logical explanation of this?*
    Certainly. the answer is Yes. Riemann style geometry is the basic way geometry has been done since 1850 or so. Sometimes called differential geometry---that's the school course textbook name.

    *Or does the math work out so we fudge the details of the analogies?* What do you mean FUDGE? the analogies carry no weight, and any details of the analogies that do not fit the picture can be ignored. there is nothing to fudge.

    You just define things logically and rigorously according to the standard methods of intrinsic geometry-----strangely enough Riemannian geometry seems almost as if it was MADE for expanding space Einstein cosmology----even though the methods of geometry were invented 60 some years earlier before Einstein needed them.

    Anyway, no fudge :biggrin:
    Last edited: Oct 8, 2007
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