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Earth at the center of the Universe

  1. Jan 19, 2012 #1
    So i read the answer of Marcus in the thread "Where is the Center of the Universe" where there was a brief mention of the "Un-Copernican" view of the Cosmos. Now i do not think that we are at the center of the Universe but this got me thinking.

    "IF" the homogeneity of the Cosmos really was only the consequence of us being in the center of the Universe, How far away from our planet would we have to observe the universe to start seeing the heterogeneity of the Cosmos?

    10,100,1000 billion miles?
  2. jcsd
  3. Jan 19, 2012 #2


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    I'd be happy to hears how some other people would reply to that. I think it would depend on how ABRUPT you imagine the inhomogeneity to be.

    We see a fairly homogenous U out to a presentday radius of about 45 Gly.

    At that distance (45 billion ly) we see the emission of the ancient light from nearly uniform density glowing hot hydrogen gas. The unevenness in density of the gas is about 1/1000 of one percent.

    Suppose we lived in the center of a 46 Gly radius bubble and outside that there was a much denser layer. This idea has been explored by cosmologist David W. as a way to explain the very gradual acceleration of expansion. Could it be the "pull" of a denser surroundings that we cannot see?
    Then the U that we can see would be, in effect, a comparatively low density "void".

    Well depending on how abrupt the transition was we would probably not be able to tell. People in a galaxy 1 Gly to the east of us would be closer to the denser shell and would be able to see it as an anomalous spot in their CMB sky.

    Any spot that is not due to our own motion doppler and which is substantially hotter/colder than the normal temperature fluctuations which are 1/1000 of one percent, would be a giveaway that there is some inhomogeneity.

    But the conditions imagined in order to "fool us" are finetuned and far-fetched. It is like fairies having put fake dinosaur fossils in the rocks so as to make us believe dinosaurs.

    We are contriving an inhomogeneity and then contriving to put Earth right in the center of a void so that we do not see it. :biggrin:
    Last edited: Jan 20, 2012
  4. Jan 19, 2012 #3
    It's a great imagination firing question, almost like an inversion of the Krikket from Life, the Universe and Everything. If we travel a billion light years left and suddenly see a vast void, will we be sent running back to our rooms to hide beneath the sheets?

    I find it interesting that pursuing the posters question seems to server as a better way to understand the accepted model, than trying to explain the model. "Well, nothing else works, so it must be this." Sherlock would be proud.

    I'm only a weekend scientist. Like a weekend warrior who goes paint-balling and pretends to be on a real battlefield, but never has to deal with bullets, I like to theorise on lazy sunday afternoons, pretend I know what I'm doing and never have to deal with maths! Sheldon Cooper might suggest this makes me an experimental physicist. Anyway, as the above poster says, the homogeny in the CMB is such that you'd prolly decide there isn't an edge before ever finding it.

    Apparently, I decided my contribution to this discussion would be rambling and backing up someone who knows way more than I do. Enjoy.
    Last edited: Jan 19, 2012
  5. Jan 20, 2012 #4


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    I think you're mixing up terms. Homogeneity means the same everywhere. So it is directly contrary to the notion of us being the center.

    You probably mean isotropy, which means the same in every direction. If the universe is isotropic but not homogeneous as viewed by us, then this means moving some distance away it would cease to look isotropic. Exactly how far we'd have to move, then, depends upon how things change as we move away from the Earth. But at the very least we'd have to move far beyond our own galaxy, which is around 100,000 light years across (As in close to a billion billion miles). We'd be talking millions of light years at least, perhaps hundreds of millions of light years, in order to see any difference.
  6. Jan 20, 2012 #5
    So isotropy is an apparent homogeny? If I were in the middle of the ocean and knew nothing of the land, I might think the world to be a homogeny of water - and still not a drop to drink - when it's really just isotropic?
  7. Jan 20, 2012 #6


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    No. Homogeneity is the same everywhere. Isotropy is the same in every direction. The two don't have to be the same thing at all. You can have a system that is homogeneous (the same everywhere) but not isotropic. A good example of this is a regular crystal. Within the crystal, the atoms line up in an ordered fashion that makes certain directions very different from other directions. But that ordered structure repeats, so that it is the same throughout the crystal.

    Similarly with isotropy, you can have isotropy but no homogeneity as long as things vary along the line of sight. So if you were sitting in the center of a big hollow sphere, things would sure look isotropic, but they definitely wouldn't be homogeneous.

    It would be better to state that that system is locally homogeneous. But it may not be globally homogeneous.
  8. Jan 21, 2012 #7


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    Just to keep this in perspective, 1000 billion miles isn't even off the solar system's front stoop. You'd have to get four times farther than this just to reach our nearest neighboring star, A. Centauri, let alone crossing town to the next galaxy.
  9. Jan 23, 2012 #8
    If the universe is infinite in space (i.e. Euclidian in all directions), then maybe > 45 Gly. If space is compact and behaves like a 3D analogue of a 2D balloon surface, then it is unlikely that Earth or any other point in space could be at the center; the Universe would look homogeneous and isotropic. I’m still struggling with this concept, but if space was infinite, then there could be a point where the Universe was not homogeneous and isotropic.
  10. Feb 4, 2012 #9
    This brings up a good point. Why do we believe the universe is homogeneous? It is homogeneous as far as we can see but the leap from "homogeneous as far as we can see" to "the universe is homogeneous" seems unjustified to me.
  11. Feb 4, 2012 #10


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    Mediocrity Principle.

    In short, it makes more sense to assume we are not in a specially-privileged part of the universe than that we are in a specially-privileged part of the universe. If we are in a specially-privileged part of the universe then we have to have theories to explain what the differences are and why they are different.
  12. Feb 4, 2012 #11


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    I don't think it makes sense to assume that the universe is globally homogeneous. It is obviously locally homogeneous, of course, but global homogeneity is unlikely. So is any sort of global spatial symmetry about which you could possibly define a center.
  13. Feb 4, 2012 #12
    I think the safest answer is "unknowable". In anthropic universe model's the larger universe beyond our observable one is considered to vary considerably. Aren't you a supporter of this?
    The possibilities are by no means black or white, however. a billion lightyears beyond the observable univere to our left may be a void, but we might need to go 4 billion years beyond the observable universe to the right before discovering any inhomogenity. Detractors of inhomogenity often seem to go with this idea that suggesting inhomogenity automatically put us at the center of the local homogeny.
  14. Feb 4, 2012 #13


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    Except we don't know that. Direct observation is not, after all, the only way to glean this information. We could also do it by understanding better the origins of our own region of the universe, for example.

    I think this is the most likely, yes. So this would be a universe that isn't globally homogeneous. But global inhomogeneity is very likely for a different reason: homogeneity over large scales is a low-entropy configuration. That is, there are far more ways for the universe to be, on large scales, very inhomogeneous than homogeneous. To explain the homogeneity of our own universe we typically resort to some sort of physical process, such as inflation, that produces the large, homogeneous universe (note: inflation doesn't actually explain this point, as it does require homogeneity to start, albeit on a much smaller scale). And these physical processes tend to only produce local homogeneity, not global homogeneity.

    I don't think anybody seriously believes that the inhomogeneity starts immediately beyond our cosmological horizon. Sensible models predict that the typical volume of the universe that is homogeneous is much, much larger than our observable universe. But not infinite: beyond some distance, the likelihood of significant changes becomes greater and greater.
  15. Feb 6, 2012 #14
    If we were afloat in the Pacific we may believe that the surface of the Earth was homogeneously water. We would be wrong, but we would still not be in a special place on the earths surface.

    Suppose our "fundamental" constants varied randomly across the universe on Tly scales or larger. That would be a convenient solution to the fine tuning problem, though I have no idea how we'd ever test it.
  16. Feb 6, 2012 #15


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    Exactly. We have no way of knowing. We can only say that to the best of our knowledge the Earth's surface is homogeneously made of water.
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