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Question concerning the universe & the Hubble photos

  1. Jan 5, 2007 #1
    I have a decent question, I think, about the Hubble photos and the universe. When the hubble took the photos that show stars cluttered together, and that it is supposedly the first few hundred thousand years of the universes life, that got me thinking. If you pointed the hubble in any given direction and zoomed in like they did before, would the outcome always be the same? Will there always be these billions of stars cluttered together?

    And if so how is this explained in cosmology today? Does that somewhat verify that our universe was once a singularity? It's really hard for me to actually convey this question the way I like it, due to it's complexity. While I know that currently we have data that supports universal expansion, IF the hubble would show similar pictures in any given direction, how does that explain the big bang, and can it in any way verify or falsify universal expansion.

    I guess it just doesn't make sense to me if the pictures would always show these billions of stars, galaxies, etc clustered together how it can explain the big bang. I guess I think of it in a wrong sense, as there is some kind of "center" of the universe (which was the singularity) and now the universe is expanding. I guess my mind wants to think if you point it in on direction, you get less stars and galaxies, and more of a void, and the other direction being the beginnening with more stars and galaxies where the matter first started to form. I'm sorry if I confused any of you, but it's really hard for me to explain my question.

  2. jcsd
  3. Jan 5, 2007 #2
    Good question, but I think the answer is no: the big bang didn't occur at a particular place (which is what all the raisin bread analogies try to explain about hubble's expansion).

    Try this analogy. Stick a bunch of paper-stars to a deflated balloon and, as you slowly blow into the balloon, let a bunch of ants walk over the balloon measuring the changing distances between stars. Notice this expansion has no "center" in the two-dimensional space the ants live on, similarly our current theories predict the big-bang did not occur in any particular direction (in 3D space), and so far our astronomers continue to see roughly the same thing in every direcction.
    Last edited: Jan 5, 2007
  4. Jan 6, 2007 #3
    Ok riddle me this. I understand the rasin analogy for cosmic expansion and that space is expanding rather than objects moving apart. I can also accept the idea that the big bang had no centre but occured everywhere. However, and this may be wrong, i always thought the way that people came to the idea of a big bang was the extrapolation of the movement or expansion of space back to one, infinitely dense point, which contradicts the first thing.

    Where am i going wrong?
  5. Jan 6, 2007 #4

    Chris Hillman

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    Raisins in pudding analogy

    Hi, FunkyDwarf,

    Actually, the point is that raisins (galaxies) are moving apart, but are not themselves getting larger. To modify the analogy: put some raisins in some dough and bake. The raisins don't expand (lets ignore the fact that if anything they probably shrink due to dehydration), but the yeast puffs up the dough. Result: each pair of raisins is farther apart than before.

    I'd recommend that excellent popular book by Steven Weinberg mentioned here: http://www.math.ucr.edu/home/baez/RelWWW/reading.html#pop [Broken]
    Last edited by a moderator: May 2, 2017
  6. Jan 6, 2007 #5
    Here's my understanding, though I give it not as an answer but as a self evaluation. Maybe someone with greater understanding could correct my misunderstandings or misstatements.

    It may seem to be a contradiction but it is not. At the moment of the big bang the entire universe was indeed an incredibly dense singularity. The reason that it was incredibly dense was because all of the universe was contained within the point. When the big bang occured it simply expanded that point in every direction. Since then the universe has grown in size, but still contains the same components that were contained within the singularity. Because the big bang eminated from the singularity, everything contained in that singularity was part of it. When you collapse the universe back to that point, everything must still be contained within it. In order to assign the point specific location or coordinates you would have to step outside the universe to do it.
  7. Jan 6, 2007 #6

    Chris Hillman

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    Lensing and magnification of very ancient events

    No, because the reason we could image these very distant (hence very old) stars is that there happens to be a foreground galaxy which is acting as a gravitational lense. It turns out that one of the optical effects predicted by gtr (and spectacularly confirmed in these pictures) is a kind of optical magnification of the lensed image.

    Not sure I understand the question, but I suspect you asking about what would be called "fine-grained" versus "coarse-grained" in statistical physics. I.e. in something like an FRW model we idealize the universe by smearing out the galaxies to obtain a kind of averaged matter density. In the real universe, of course, we have stars separated by comparatively large and low density regions, and then at the levels of galaxies, the same thing, and at the level of clusters and superclusters, the same thing. Physicists would say that if we take progressively finer "graining", we see a more and more complicated density distribution at smaller and smaller scales.

    I was trying to help, but probably only confused you more, huh?

    Well, mainstream cosmology doesn't quite say that. It says that the universe was once much "smaller", hotter, and denser than it is now. If you run "time" backwards in imagination, in the idealized FRW models (and in various more elaborate models which are generalizations of the FRW models), which are exact perfect fluid solutions in gtr, you encounter a curvature singularity. But these models are highly idealized. In particular, they smear out the matter to a more or less uniform average density (at a given "time"). They should give a good qualitative picture back to the "decoupling time" and even earlier however, which means they should be adequate to explain the gross features of what we can see with optical telescopes. (Before the matter-light decoupling time, the universe was so dense that in a sense optical images could not propagate, for pretty much the same reason that we can't see very far into the Sun: photons can't travel freely inside the Sun because they run into something before they can escape from the surface of the Sun and propagate more or less freely. Roughly speaking.)

    Indeed. Well, I think the missing idea may be "averaging" or "scaling phenomena".
    Last edited: Jan 6, 2007
  8. Jan 6, 2007 #7

    Chris Hillman

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    I think I know what you mean, and if so, I guess I wouldn't disagree. The key here is probably to recognize that running "time" backwards to obtain a limiting locus makes sense, to a point (no pun intended), but starting from the Big Bang and trying to run "time" forwards is more difficult, unless you already have the FRW model in hand, as it were.
  9. Jan 10, 2007 #8
    thinking of the initial singularity as a point is not correct. Think of a very simple model, the real number line. This is 1D and infinite in extent. Lets consider the integers -inf,...,-2,-1,0,1,2,...,inf. Think of these as the universe today. Earlier this set would be -inf,...,-1,-.5,0,.5,1,...,inf. Now if you think about this the space between each member of the set has been cut in half, BUT the set still extends from negative inf to pos inf and contains the same number of members. The cuts can be made over and over again and the "space" between the members can be made arbitrarily small. The big bang corresponds to a huge density EVERYWHERE in the universe.
  10. Jan 11, 2007 #9


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    Infinite number of infinitesimals?

    Hi Allday, I like your simple model for the layman!

    Another 'layman' way to look at it is that our observable universe, extrapolated backwards for about 13.7 billion years, must have been the size of an infinitesimal 'speck' of near infinite energy density. However, there may have been an infinite number of these infinitesimal specks!

  11. Jan 11, 2007 #10
    And the laymen appreciate it as well.
    For further clarity:
    If we could step outside the universe and run the cosmic clock in reverse would the spatial dimensions that comprise our universe compactify and change shape as they became smaller, or would the dimensions collapse evenly and smoothly (as if it were moving away from the observer positioned outside the universe)?
    I have a sneaking suspicion that didn't make nearly as much sense as I was hoping, but I'll take what I can get. It might also be that I am the epitome of layperson.
  12. Jan 11, 2007 #11


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    The second point does not contradict the first point. Why should it? If everywhere you look, galaxies are homogeneously all around you and moving away from you, then extrapolating backwards in time, they and you were all at the same point at some time in the past.

    The problem is trying to figure out a geometric shape that can't really be pictured in your head. That's the purpose of the balloon and rasin muffin analogies. Just remember that they can expand or shrink infinitely - they can shrink to zero size.
  13. Jan 19, 2007 #12
    hi prosellis,

    IF you could step out of the universe like a flatlander in 3D world AND run time backwards ... you would have to show me how to do that ... you would hopefully see all the dimensions shrinking at the same rate. If that wasn't the case there would be a special direction in the universe and it would trash a lot of symmetry principles that never did anything to anyone
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