Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Can we identify the centre of the Universe?

  1. May 1, 2016 #1
    Hi



    I have two novice question

    If the universe is expanding and it is all moving apart, how is it that galaxies are colliding?



    If our galaxy and all others are moving away from some singularity central point. And as we see other galaxies moving away from us at a speed relative to there distance from us.



    Shouldn't we be able to find the general direction of that singularity central point of the universe by identifying galaxies that seem to be heading in the same direction as our own galaxy but more slowly instead of fast according to there distance from our home galaxy because they are closer to that point?

    jefsART
     
  2. jcsd
  3. May 1, 2016 #2

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    The putative 'center' of the universe is a common misperception. It is widely accepted that the universe has no center, or if you prefer, the center is wherever you happen to be in the universe. No matter where or when you happen to be in the universe, you are always exactly one Hubble radius from the edge of your observable universe in every direction. The simplest conclusion is the universe has no center. Part of the blame for this confusion has to do with popular science descriptions of the big bang, where the universe originated from a tiny primordial singularity. The term singularity, however, does not imply anything tiny nor confine it to any particulaqr location. It merely means our mathematical models fail to yield sensible solutions when pushed to extreme limits - like t=0.
     
  4. May 1, 2016 #3

    mathman

    User Avatar
    Science Advisor

    In addition to expansion (significant only in large scale activity), galaxies have their own proper motion, so they can collide.

    The universe has no center.
     
  5. May 1, 2016 #4

    phinds

    User Avatar
    Gold Member

    I recommend the link in my signature. It's an easy read and addresses your question.
     
  6. May 1, 2016 #5
    Thanks for the replies. I shall look up the link. So this means (the universe has no centre) that if we rewind the video it would not contract to a singularity?
     
  7. May 1, 2016 #6

    phinds

    User Avatar
    Gold Member

    No, it WOULD contract to a singularity. What has already been explained to you is that "singularity" does not mean "point", it just means a place where the math model breaks down and we don't know what is going on.
     
  8. May 1, 2016 #7
    True but they are moving away from each other at tremendous speeds. Moving away and colliding at the same time creates a problem for me. The mode has problems. No?
     
  9. May 1, 2016 #8

    phinds

    User Avatar
    Gold Member

    No. There are two different modes involved. Read the link I provided.
     
  10. May 1, 2016 #9
    It hasn't been explain its been stated. In the sub atomic things dont make sense, ok. But on the large scale if it dont make sense then you must resolve the contradiction. Dont take offence but your words "we don't know what is going on" seem to apply.
     
  11. May 1, 2016 #10

    phinds

    User Avatar
    Gold Member

    What do you think the sub atomic scale has to do with the expansion of the universe?

    There IS no contradiction.

    Only because you haven't studied the stuff yet.

    Gravitationally bound systems such as galactic clusters, solar systems, you, atoms, etc do not expand. Galactic clusters and larger objects recede from each other.
     
  12. May 1, 2016 #11
  13. May 1, 2016 #12
    Imagine beings like ants or bacteria walking on a sphere and having only knowledge of the sphere. Then some ant asks where the center of their universe is. You can't really show them since while there is a center, it is not part of their universe. In our case it's even harder since there is nothing outside our universe, so there is no center at all.
     
  14. May 1, 2016 #13

    Drakkith

    User Avatar

    Staff: Mentor

    Singularity, in this context, does not mean a single location in space. It means that the density everywhere in the universe goes to infinity as you look further back in time.
     
  15. May 2, 2016 #14

    phinds

    User Avatar
    Gold Member

    Would it not be better to say "... approaches infinity" ? We don't know what the singularity is but saying " ... goes to infinity" seems to imply that it is infinitely dense and we don't know that.
     
  16. May 2, 2016 #15

    Drakkith

    User Avatar

    Staff: Mentor

    Not sure.
     
  17. May 2, 2016 #16

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    It may help to think in terms of the observable universe, which has always been the only portion of the universe that is observationally accessible to us. The size of the observable universe has, and will always be limited by the age of the universe due to the finite speed of light and basically works out to cT, where T = the age of the universe and c = the speed of light. As is evident the size of the observable universe approaches zero as T approaches zero. Which, of course, wreaks havoc when you try to make any mathematical sense out of the size of the universe at T=0. Cantors work on set theory is still regarded as a good place to start unraveling this mystery.
     
  18. May 2, 2016 #17
    "Only because you haven't studied the stuff yet."
    True, but I know a contradiction when I see one.
    "Galactic clusters and larger objects recede from each other."
    If they recede from "each" other then they can collide. Study that!



    Hi Gold Member. Thanks for giving an explanation.

    I will rephrase my question.

    Seeing as although we are limited by the “observable universe, which has always been the only portion of the universe that is observationally accessible to us. “

    Yet we can draw all sorts of conclusion such as we are expanding from a singularity (can we still use that term?).

    question:
    “Shouldn't we be able to find the general direction of that singularity 'central point?' of “observable universe" by identifying galaxies in the “observable universe” that seem to be heading in the same direction as our own galaxy but more slowly instead of fast according to there distance from our home galaxy because they are closer to that point of creation (although not observable to us) from which they are expanding (receding)?

    Is there a difference between recede and expand? Is expand out now? If everything is moving away from everything else isn't that an expansion?

    We draw a lot of conclusions even though we have this limitation of the “observable universe“.

    Isn't it very relevant to our present model that we see more distant galleys 'receding' faster than closer galaxies?


    "Galactic clusters and larger objects recede from each other." And bump into each other.

    That may be a local thing but still it may be a hint of something more.

    Are the more distant galaxies at the edge of “the observable universe” bumping into each other???

    It seems to me we have much more to understand. When ever we have some understanding wrapped up nice and neat some trouble maker like Einstein comes along and changes every thing. He ended the billiard ball concept of the universe which was a happy explanation of things at the time. Just asking.
     
  19. May 2, 2016 #18

    davenn

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    You didn't heed what you were told above, aye

    you were told that the singularity DOESNT reference a single point source from which everything exploded out of
    Until you come to that understanding/realisation, you will continue to search for a non existent centre of the universe

    Reread the posts by Phinds and Drakkith above


    Dave
     
  20. May 2, 2016 #19

    mathman

    User Avatar
    Science Advisor

    Distant galaxies are moving away from us at high speeds, due to expansion of the universe. Nearby galaxies may be moving toward us - Andromeda galaxy for example.
     
  21. May 2, 2016 #20

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    You have drawn inferences unsupported by facts. Galactic clusters, like individual galaxies, are also known to collide. This is possible because clusters have their own peculiar motion, also just like individual galaxies. Such collisions were more common in the early universe because the average density of the universe was greater than it is at present. For discussion see http://arxiv.org/abs/1502.01339, The merger rate of galaxies in the Illustris Simulation: a comparison with observations and semi-empirical models. As far as the OP question is concerned, welcome to the apparent center of the universe! Given that distant galaxies are receeding at velocities proportionate to their distance from earth. It is a fairly simple matter to deduce earth is located at the center of the observable universe. If that sounds like a highly improbable coincidence, welcome to the non-intuitive science of cosmology.
     
  22. May 2, 2016 #21
    Hi Dave. That must mean. If you rewind the tape.The universe does not contract to a "single" point of infinite mass etc from whence it came "you were told that the singularity DOESNT reference a single point". I can live with that but you cant have it both ways. I'm not searching for the centre of the universe. Are you saying Hubble was wrong and out dated now?
     
    Last edited: May 2, 2016
  23. May 2, 2016 #22
    Hi Chronos. Thanks again for your patience and an honest attempt to answer me. (no capitals of frustration)
    Mathman (Gold Member) say the universe is expanding. 'due to expansion of the universe"
    So if you play the tape back, the observable universe to us, would contract all the way back to our milky way. But because there is a limit to how far we can see, and we assume there is much much more we cant say what would happen?
    Am I getting close?
     
  24. May 2, 2016 #23

    davenn

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    that's correct
    Have you yet read the link at the bottom phinds' posts on balloon analogy ?
    be did suggest early in this thread that you do so

    but then you contradict that with this .....
    as that would still infer everything coming back to a fixed known point

    you topic title suggests you are ....
    so to use your words ... you cannot have it both ways :wink:


    Dave
     
  25. May 2, 2016 #24

    davenn

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    cant remember if these were sufficiently answered for you

    Not all galaxies are moving away from each other. There are clusters of, even super clusters of galaxies that are gravitationally bound

    our Milky Way galaxy belongs to a cluster called the Local Group
    https://en.wikipedia.org/wiki/Local_Group
    They are gravitationally bound and indeed some of those galaxies are moving closer to us. Eg M32, the Andromeda Galaxy

    But between any cluster of galaxies the distances are expanding



    Dave
     
  26. May 2, 2016 #25

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    Yes, you are on the right track. The observable universe appears to shrink as you rewind the movie. At some point in the distant past it would appear to be smaller than our own galaxy. Of course the universe is older than our galaxy so nothing paradoxical would occur. In fact neutral hydrogen, the stuff from which the first stars were formed, did not even exist until a few hundred thousand years after the big bang - the universe was simply too hot for atomic nuclei to capture and retain electrons any earlier than that. The observable universe, in theory, was already over 3 times the present size of our galaxy back when hydrogen atoms emerged. See here http://www.haystack.mit.edu/edu/pcr/Astrochemistry/3 - MATTER/nuclear synthesis.pdf, for discussion.
     
    Last edited: May 2, 2016
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted