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Space Density

  1. Mar 20, 2006 #1
    Each time we look deeper into space we find more stars and galaxies.

    Perhaps the universe is much, much, larger than we believe.

    If so, maybe the universe is more dense towards the center and light travels at a different speed?
     
  2. jcsd
  3. Mar 20, 2006 #2
    Which centre was that then? Since when did the Universe acquire a centre?
     
  4. Mar 20, 2006 #3

    russ_watters

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    Large-scale surveys (such as Hubble Deep Field) show clusters and superclusters, but they are consistent with the idea that there is no center. Things like the microwave background imply that the density is relatively uniform.
     
  5. Mar 20, 2006 #4

    Phobos

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    Virtually everyone agrees that there is more universe out there than we can see. And many scientists believe the universe to be infinite.

    As noted above, the universe has no center (or edge) in 3D space. The Big Bang was a rapid expansion of all space, not an explosion from some center into emptiness.

    Light speed is constant (see Einstein's Relativity)
     
  6. Mar 20, 2006 #5
    Light speed is not constant through something more dense.
     
  7. Mar 20, 2006 #6
    Since when is this an implication that the universe has no center? Uniform density doesn't imply anything one way or the other. Perhaps you can explain your logic.
     
  8. Mar 21, 2006 #7

    Chronos

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    SDSS, and other studies, show no statistically significant overdensities at large scales. In other words, matter is uniformly distributed in every direction no matter where you are located in the universe. No professional astrophysicist, to my knowledge, seriously entertains the idea of a 'center' to the universe. The very concept of a center requires edges to make it meaningful. While the universe is believed to have a temporal edge [i.e., the age of the universe], it is not absolute and differs depending upon your location in the universe. It is therefore not physically meaningful. The CMB reference merely points out there is no preferred direction in CMB, which would be expected in a universe with fixed spatial edges.
     
  9. Mar 21, 2006 #8
    If I remember correctly, the average density of space is something on the order of one atom per cubic centimeter.
     
  10. Mar 21, 2006 #9
    My astronomy teacher explained it like, everything is equally close or far no matter where you are in space to the big bang..
     
  11. Mar 21, 2006 #10
    have a look at this.

    common starting position to it's current position
    doesn't this mean that the universe has a center?
     
  12. Mar 21, 2006 #11

    russ_watters

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    In order to have a center, it must also have an edge. Since no edge is visible: no center.

    The consistency of expansion is also evidence of a lack of a center.
     
  13. Mar 21, 2006 #12

    russ_watters

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    No, it doesn't. Imagine an expanding balloon - the surface of the balloon has no edge or center. If the balloon could shrink, it would end up as a single point, but the surface would never have a center.
     
  14. Mar 21, 2006 #13
    I would suggest that the problem has more to do with our representation of the big bang that with any actual observation of the universe. If one looks at the usual inflation diagram, it certainly does look as if there is a center, as well as edges. Even the most recent reports about the first trillionth of a second are shown as if there is a center. These representations are misleading.

    Of course people who have studied futher realize that the diagrams are spacetime diagrams, not merely space diagrams, but that is not clear to the average reader. And they are flawed, IMO, even as spacetime diagrams, since they never seem to account for the compression of time at the so-called first instants. One would think that the measure of time is the same from beginning to end, but that is not so.

    R
     
  15. Mar 21, 2006 #14
    Seems like flawed logic to me. This is not to say the opposite is true, but that there is no preference.

    How so? I could just as easily explain a model with a center, with consistency of expansion. Again I see no preference here.
     
  16. Mar 21, 2006 #15
    russ watter's logic seems ok to me. How will you define the center if there is no edge?

    I would like to see the explanation of a model with a center and consistancy of expansion. I don't get any physical image from this choice of words.

    R
     
  17. Mar 21, 2006 #16
    Well if I take two guys and put em in the center a room with no light, where one guy says there are no walls, and the other guy says there are, wherein these two dudes can't take more than one step in a direction, and that step leaves them shy of finding out if there are any walls to be found. Do you accept the guys statement that there are no walls, or the one that says that there is?

    Clearly the guy saying there are no walls would be making an educated guess, but in this particular scenerio he would be dead wrong, while the guy saying there is a wall has nothing to go on but pot luck.
    In the case of the universe having no center ........The educated guess is that there is no center, but by no means do we have a slam dunk.

    Imagine a thousand black dots on a white screen monitor equally spaced, wherein all dots move away from the center dot on the screen, and imagine that the further away each of these dots are from the center dot ... the faster they move from the center dot. In this scenerio no matter which dot you choose ...... all dots will be seen to be moving away from the dot you choose, and with a consistancy of expansion overall.
     
  18. Mar 21, 2006 #17

    russ_watters

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    Please explain: how can there be a center if there is no edge? I suppose there could be an edge that we can't see, but in science the default assumption is always that if you don't have any evidence to say that something exists, you must assume that it doesn't. Besides, there is also the next point...
    I probably could have worded that a little more descriptively, but what I mean is that if there were a center, the motion of galaxies could be plotted in 3d and their directions extrapolated back to a central point. But in fact, if you tried this, every galaxy would appear to be the central point.

    Ie, if you look at galaxies in the direction of the center, they'd be denser and moving slower. If you look at galaxies in the opposite direction of the center, they'd be less dense and moving faster. Like an explosion.
     
  19. Mar 21, 2006 #18

    russ_watters

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    For the purpose of science, both are correct in that the hypotheses are supported by their observations. You cannot assume the existence of something for which there is no evidence.
    Well, with all the other evidence, yeah, it's pretty much a slam-dunk.
    No, that isn't correct. For a dot a little to the right of center, looking to the left it will find dots closer together and moving slower than when it looks to the right: just like in an explosion. (edit: not sure about the velocity distribution, but I am sure of what it does for density....I'll work on that, though)
     
    Last edited: Mar 21, 2006
  20. Mar 21, 2006 #19
    "in the center a room with no light"

    My question to you was, how can you know there is a center if there is no edge? In fact you could generalize the argument, as you have here, and assume that there could be edges but you cannot detect them, but the question is not really about the edges, it is about how you know there is a center.

    Your placing two guys in the center of a dark room starts out saying there already is a center, so your statement is like saying "I know there is a center because two guys are standing there." Of course it is likely that in an otherwise empty room, one of the two guys is you, and the other is me. Now we still have an argument. You say there is a center because you are standing on it. But I argue that I could just as well be the center, and in fact, moreso, that everywhere around us is equally likely to be center. Then since there is no place that is more likely to be the center than any other place, your statement is "I am the center of the universe", and my argument (well, I won't speak for russ waters, but maybe he would agree with me here) is that since I have no evidence of a center in any particular place I cannot say that any particular place is the center, not even the place where I myself stand. So your argument is that you are the center of the universe, and my argument is simply that you are not the center of the unverse. Neither of these arguments can be proven, but your claim to be some special being among all the other beings, being in the center of the universe, will require some substantial evidence if you are not to be, universally by everyone except yourself, thought to be mistaken.

    Moreover, if you persist in your argument, you will come into contact eventually with someone else who also claims to be the center of the universe. Is it worth fighting over? Maybe both of you will think so and get in a fist fight. I wouldn't like to see that, but it is an amusing thought.

    On the other hand, if russ waters and I happen to meet and discuss this or some other issue, the chance of our getting in a fist fight is rather small. I hope so any way. Both of us can easily agree that there is no detectable center.

    Now you really must think about the density question carefully. No matter which direction we look in space, and no matter how far away to the limit of our ability to discriminate, the universe looks the same. That is a startling fact, and you are right to think that it seems very unlikely. But all the evidence says it is the same, no matter where you go, or how long ago, or, for all we know, how far you get to go into the future.

    Richard
     
  21. Mar 21, 2006 #20

    russ_watters

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    Ok, I was thinking about this more and yeah, I was right about the velocity distribution too. The reason is the overall gravitational field of the universe has a similar effect in the accepted model and in your explosion model: the expansion slows down with time. But in the accepted model, since the gravitational field is uniform, the slowdown is uniform. In the "explosion model", since the gravitational field has a center, objects closer to the center will be slowed more than objects further away and thus you will see different velocities for the same distance if you look toward and away from the center.
     
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