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B Can we identify the centre of the Universe?

  1. May 20, 2016 #76

    russ_watters

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    You agree that the 2d surface has no center on the surface, so therefore you must agree that the 3d universe has no center in its 3d space.
     
  2. May 22, 2016 #77
    What happens if in the near future we find to our dismay, there is a center to the universe?
     
  3. May 22, 2016 #78

    jbriggs444

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    The relevant feature of the universe is that it is homogeneous and isotropic. That is, it is the same everywhere and in every direction. In order for there to be a center, there must be a violation of homogeneity. If such a violation is observed and verified then we would need to come up with an explanation. That explanation might or might not involve a center.

    In the absence of any such evidence, this is pure speculation and not fit subject matter here.
     
  4. May 22, 2016 #79

    Drakkith

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    To bounce off of Jbriggs post, in the very unlikely event that we do identify a center we will simply modify our theories accordingly or develop new ones if our current ones can't be modified. That's how science works, after all.
     
  5. May 23, 2016 #80
    I always like the balloon analogy.
    The 2-D rubber represents our 3 dimensional space and as you blow the balloon up, the universe expands. That doesn't mean objects can't interact and collide with one another on the space that's expanding. It also highlights how it doesn't make sense (for the closed universe) to speak of a center, any more than asking someone standing on earth which lat and long represent the center of the earth. You CAN however, define a CENTER-OF-MASS frame, which is as close as physically makes sense to a center of the universe, this corresponds to the frame in which the microwave background is isotropic (more properly called co-moving coordinates).
     
  6. May 23, 2016 #81
    Glad it helps. I don’t like any of the analogies – Ballon, Fruit Bread, Donuts are all shown with 3D boundaries. Big Bang analogy is thought to be an explosion. That’s why there are all these questions about center of the universe and boundary.

    There is a lot of discussion about the observable universe, which I understand because that is all we can observe. But it seems to me that the original question relates to the total universe.

    I suppose that the questions about an imagined center of the universe arise from the assumption that the high original density had a source. A source implies causality, so if time started with the BB, there was no cause and therefore no source.

    Can somebody help me with the question: if we are saying that the original high density occurred everywhere, where does all this mass go when the density falls?

    Is it assumed that dark energy has mass? With the observed expansion of the universe, which we assume also to be the case in the non-observable universe, are we saying that there is an increasing emptiness (not containing dark energy or anything else) which is reducing mass per cubic light year?

    Or does dark energy maintain mass density? If not, where does the emptiness come from?
     
  7. May 23, 2016 #82

    Drakkith

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    Indeed. It is only natural to expect such a thing. It is, after all, extremely difficult for most people to wrap their head around a center-less expansion. We never encounter this in our everyday life.

    That assumes the big bang was a single event and that time started with it. All we know is that the universe gets denser the further back we look. Since we can only see so far, we have to extrapolate past backwards beyond a certain point in time, which is where the idea of a singularity comes from. If the universe continues to contract as you go back in time, then our extrapolation shows a singularity. Who knows what actually occurred...

    The mass of matter and dark matter simply spreads out along with those types of matter. The mass of the emitted light and radiation is, well, lost. Kind of. Sort of. (It's actually very complicated)

    It is not. It has the reverse effect that mass has.

    Dark energy is believed to remain at the same density while the density of normal and dark matter falls. This is what leads to the accelerating expansion.
     
  8. May 23, 2016 #83
    How can dark energy remain at the same density when the space between the (super) clusters is increasing? I can think of three reasons:

    • Dark energy is increasing in volume terms.
    • Dark energy is decreasing, which takes up more space because it is negative.
    • Emptiness is increasing.

      Which do you think is more likely?

      Has emptiness been fully discredited (by Dirac) or does the universe contain emptiness?

      I have a problem with expansion without available emptiness.
     
  9. May 23, 2016 #84

    Drakkith

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    One of the prevailing ideas is that dark energy is just the cosmological constant, a built-in property of spacetime that causes a change in the overall geometry (the origin of the accelerating expansion) when the density of matter falls below some level. If so, then dark energy has no choice but to "remain constant", as it isn't something that fills space at all.

    Unknown. I think that depends entirely upon what you consider "emptiness". If you count fields as "something", then the universe is never empty. But whether or not there is "true emptiness" really has no bearing on expansion. It's known to occur, regardless of the labels you and I place on things.
     
  10. May 25, 2016 #85
    Just forget about the raisin-bread, which has a boundary.

    The Universe is everything, without a boundary, and the main constituent parts are seen to be moving away from each other with increasing velocity.

    In the beginning they were compactly close together, without any space between, but still everything.

    You can’t have a center of everything.

    We have the phenomenon of a universe which was highly dense everywhere, without boundaries and which is now expanding exponentially, but not into anything, because it is everything. So we go from very high density to very low density with increasingly lots of empty space beween the constituent parts.

    It’s no use talking about raisin-bread. That’s ridiculous.

    What do you conclude. I am very interested in your opinion.
     
  11. May 25, 2016 #86

    phinds

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    No, it is not the least bit ridiculous, it is an analogy and analogies are by their very nature, flawed to one degree or another. The trick is to understand what part of the analogy is flawed and what part is helpful, but none of it is ridiculous. Similarly with the balloon analogy, as I explain in the link in my signature.
     
  12. May 25, 2016 #87
    So expansion only exists in empty space, between galaxies and clusters? Why would this be?
     
  13. May 25, 2016 #88

    phinds

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    Because at non-cosmological levels, expansion is SO incredibly tiny that it cannot even begin to overcome the forces of gravity, let alone forces like the strong force. This is discussed some in the link in my signature.
     
  14. May 25, 2016 #89

    mfb

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    Some posts that derailed the thread, and replies to those posts, have been deleted.
     
  15. May 25, 2016 #90
    There is no centre of the universe! According to the standard theories of cosmology, the universe started with a "Big Bang" about 14 thousand million years ago and has been expanding ever since. Yet there is no centre to the expansion; it is the same everywhere. The Big Bang should not be visualised as an ordinary explosion. The universe is not expanding out from a centre into space; rather, the whole universe is expanding and it is doing so equally at all places, as far as we can tell.

    In 1929 Edwin Hubble announced that he had measured the speed of galaxies at different distances from us, and had discovered that the farther they were, the faster they were receding. This might suggest that we are at the centre of the expanding universe, but in fact if the universe is expanding uniformly according to Hubble's law, then it will appear to do so from any vantage point.

    http://math.ucr.edu/home/baez/physics/Relativity/GR/centre.html
     
  16. May 27, 2016 #91
    I think a better question would be "If the universe stopped expanding and started shrinking instead, where in space would end up being in the middle of the final clump of matter before it went singularity on us?" Though that would probably still be 'everywhere', more or less, I suppose.
     
  17. May 27, 2016 #92

    jbriggs444

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    Yes. If the universe is finite then having it become smaller does not help localize the center. It's still everywhere. If the universe is infinite then scaling it down does not help localize the center. It's still infinite. In any case, there is no "final time prior to the singularity" any more than there is a smallest real number greater than zero.
     
  18. Jan 10, 2018 #93
    The solar sistem has a center of mass. Similar, a galaxy, a cluster of galaxies, etc, have a center of mass. What is the limit from where we cannot speak anymore about a center of a mass, in a finite Universe?
     
  19. Jan 10, 2018 #94

    russ_watters

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    Even in a finite universe there is no size limit applicable (it sounds like you're saying size large enough it looks infinite). It's a geometry thing: the surface of the earth has no center. The universe is like a 3D surface, curved in a higher dimension. It's hard to visualize, but just like flying around the world, you may be able to fly across the universe and end up where you started.
     
  20. Jan 10, 2018 #95
    So, can we say that, at a large enought scale, the vectors involved in calculating the center of mass are no longer liniar and cannot be added as usual anymore?
     
  21. Jan 10, 2018 #96

    mfb

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    Either that, or the universe is infinite and taking the limit (to cover an infinite volume) is not meaningful.
     
  22. Jan 10, 2018 #97
    I thought is a scientic fact that the universe is finite. Do you have a mathematical representation of a universe finit in time but infinite in volume?
     
  23. Jan 10, 2018 #98

    mfb

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    It could be finite or infinite, we don't know.
    The standard flat ΛCDM model is the easiest example.
     
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